IBM Confidential Field Engineering Education Student Self Study Course

IBM  Confidential Field Engineering Education Student Self Study Course
Field Engineering Education
Student Self-Study Course
IBM Confidential
Field Engineering Education
Student Self-Study Course
IBM Confidential
This document contains information of a proprietary nature. ALL
INFORMATION CONTAINED HEREIN SHALL BE KEPT IN CONFIDENCE. None of this information shall be divulged to persons
other than: IBM employees authorized by the nature of their
duties to receive such information or individuals or organizations
authorized by the Field Engineering Division in accordance
with existing policy regarding release of company information.
Introduction to Teleprocessing
This course is provided to acquaint Customer
Engineers with some of the important concepts of
Teleprocessing which differ from those of usual
Data Processing.
Address comments concerning the contents of this publication to:
ffiM Corporation, Field Engineering Education, Dept. 911, Poughkeepsie, N. Y. 12602
IBM Teleprocessing
Review Questions . . . . . .
Review Questions • . . . . . . .
Review Questions . . • . . • . •
Types of Channels • •
Signaling Methods
Frequency Spectrum
Carrier Systems . . •
Modulation. . . . . •
Grades of Channels
Broad Band Channels
Voice Grade Channels •
Subvoice Grade Channels.
Telegraph Grade Channels
Line Quality . . • • .
Data Sets • • • • • •
Reference Sources
Review Questions. . •
Message Switching . • . . . •
Manual Torn-tape Switching
Semi -automatic Switching
Automatic Switching. . . •
Circuit Switching. . • . . .
Selective Calling and Polling
Line Control .
Editing . .
Logging .
Error Control.
Terminal Operating Modes.
Review Questions. . • . . . •
SERD ES and Shift Registers
Transmit Operation
Receive Operation. . .
Shift Registers. .
Shift Register Shift Register Review Questions.
. . . • • •
. . . . .
Review Questions. . . . . • • . •
Bi-Polar Mode ••
Uni - Polar Mode
Current Mode
Review Questions .
Communications Common Carriers . . .
The Federal Communications Commission . •
State Utility Commission
The International Telecommunications Union
The Bell System. . • • • • • • • . . • • •
General Telephone and Electronics Corporation •
Independent Telephone Companies • . . •
Western Union. . • • • • • . . • . • •
American Cable and Radio Corporation.
RCA Communications, Inc. . . . • . • •
Privately Owned Communications Systems •
Review Questions . • • . • • . • • . •
Session 1
Session 2 •
Session 3 •
Session 4 •
Session 5 •
Session 6
Session 7 •
Session 8
Session 9
In this session, you should learn why the field of Teleprocessing exists, how IBM has
contributed to its growth, and how a typical system is configured.
Teleprocessing provides a means of obtaining timely, accurate reporting of information with reduced time, effort, and expense.
Since 1941, IBM developments have increased transmission rates of data from three
cards per minute to approximately 20,000 card equivalents per minute.
Future of Teleprocessing is as unlimited as human imagination.
Existing Telephone and Telegraph facilities feed information to computers.
The history of data communications can be
traced back to biblical times. The Book of Esther
relates that the delivery of orders from King
Ahasuerus to his provincial governors involved a
delay of eleven months. Later, history states that
the author Aeneas described a system, used during
war, in which a 5 x 5 matrix and a display of torches
were used to indicate coordinates of the letter desired. By the 15th century, a postal system had
been introduced in France by which the use of relay
teams speeded the communication of the written
form. Paul Revere's lantern signals and Reuter's
carrier pigeons were steps in the development of
communications systems which led finally to the
land and overseas telegraph. The telegraph, followed by the telephone and radio, has made possible
data communication as we know it today, and the
field of Teleprocessing.
With the increasing complexity of business, the
need of management for up-to-date information has
outgrown the facilities of normal postal services,
even air mail. The present need is for centralized,
immediately accessible information on diverse operations. Teleprocessing takes a giant step toward
fulfilling this need. The specific objectives of Teleprocessing are:
1. To provide rapid transfer of information from
the source to the data processing location.
2. To provide accurate and timely reports for
management information and decision making.
3. To accomplish the foregoing with reduced
effort, time, and expense.
IBM's first development in this field was the introduction, in 1941, of the IBM 057 and IBM 040. Figure 1 traces the necessary operations:
First, the cards were processed through the 057
and the data punched into paper tape. This involved
handling the cards twice. Next, the paper tape had
to be removed and transferred to a telegraphic type
reader for transmission. At the receiving location,
the paper tape was transferred from the telegraph
perforator to the 040 to generate punched cards.
The procedure required intervention at points A, B,
C, D, E, and F. When the cards were converted to
tape, the maximum rate of transmission was about
three cards per minute. Today, the IBM 063 and
IBM 047 perform corresponding functions. In this
publication, the term "cards per minute" re'furStO
80-column punched IBM cards.
To reduce the manual intervention required, the
IBM Card Data Transceiver was introduced in 1954.
This equipment allows several configurations (see
Figure 2): printing or nonprinting card reader
punches can be used on either telephone or telegraph
facilities. With the IBM 67 Telegraph Signal Unit,
three transmission rates are possible: 60, 75, or
100 words per minute., This corresponds approximately to 3, 4, or 5 cards per minute, respectively.
Transmission of 10 to 12 cards per minute is possible' using the IBM 068 Telephone Signal Unit, depending on whether the 066 or 065 punching unit is
employed. Note that intervention is now necessary
Card Reader
Paper Tape
Tape Reader
Paper Tape
Paper Tape
Card Punch
~-"7"'--=:--~ Reader II .2§
Signal Unit
Telegraph or
Telephone Lines
i :
Signal Unit
Reader 1 - - - - - ,
only at points A and B of Figure 2.
Recent additions to data terminal equipment include the IBM 357 Data Collection System, the IBM
1001 Data Transmission System, and the IBM 1013
Card Transmission Terminal. The 1013 features
transmission between terminals which operate in
constant synchronism with each other through the
use of an STR unit. (Synchronous TransmitterReceiver). With the development of the STR, we
can transmit data at speeds not previously possible.
We next consider the IBM 7701 and 7702 Magnetic
Tape Transmission Terminals utilizing the STR concept for data transmission. The 7702 can send and
receive data at 300 characters per second. Converting this to cards per minute, we can compare the
change in transmission rate with that of the transceiver. The following formula is used:
Cards per minute:
characters per second x 60 seconds per minute
80 characters (columns) per card
For the 7702, we obtain an answer of 225 cards per
minute - which is quite a change since the transceiver.
More recently, the IBM 7710 Data Communications Unit has been introduced. Also a member of
the STR family, the 7710 provides the capability of
connecting two computers, and transferring data
from the memory of one to the memory of the other
through communications facilities. Figure 3 shows
one method of connecting this unit for actual operation between two 1401's, and the corresponding
communications line facility.
In late 1964, the IBM 7711 Data Communications
Unit was introduced. This provided for connection
of a 729 or 7330 tape unit to a communications network. It made possible the transmission of data
over a communications facility at very high speed
by a method similar to that of the 7710, without
tying up the customer's computer system. The current maximum data rate is 28,800 characters per
With System/360, the requirements for Teleprocessing are multiplied tremendously. Complex
systems have enormous appetites for data and must
be fed to realize their full capabilities. For operation in this area, we have the 2701, 2702, 2703.
We shall examine the individual capabilities of System/360 working with these new devices. First,
consider the 2701, because it accommodates most of
the requirements previously outlined. The 2701
Data 14----~
14----~ Data
Telephone Lines
Data Adapter Unit has the ability, through the use of
appropriate adapters, to service data terminal devices operating in the telegraphic range of 60 words
per minute to high speed graphic display units operating at 1.1 million bytes per second. For example:
1. Telegraphic adapters exist that operate with
60, 66, 75, and 100 word per minute telegraph
terminals, such as those encountered on existing
2. Terminal adapters are available for operation with networks of IBM 1030, 1050, 1060 or 1070
terminals. These can operate over switchable telephone facilities with interfacing equipment provided
by the common carriers (communication companies),
or over the customer-owned or leased line facilities
with the IBM Line Adapter acting as an interfacing
device. Interface is the common boundary which
exists when lines pass from one unit to another, or
from one service to another. When the characteristics of a signal on one side of the interface is different from that required on the other side of the
interface, some kind of interface conversion device
must be used.
3. For high speed data transmission and compatibility with other members of the STR family, the
Synchronous Data Adapter (SDA) may be used with
the 2701 to provide data rates up to 28,800 characters per sec.ond.
4. The 2701 may also transfer data from a
System/360 to a graphic plotter or similar display
device, through the Parallel Data Adapter (PDA).
Data rates are limited only by the speed of the
processor (in the case of a high-speed Selector
channel, 1. 1 million bytes per second in groups of
2 to 16 bytes at a time). In a Teleprocessing envi'ronment, the 2701 can handle signals from a maximum of four telephone and/or telegraph lines; can
automatically dial the telephone number of the remote terminals, or operate with selective call or
automatic call telegraph terminals. Customers requiring interface with more than four telephone or
telegraph lines, at transmission rates less than 180
bits per second, may select the 2702 or 2703. The
2702 can control 31 channels or lines while the 2703
can control a maximum of 176 channels or lines.
Note that, if we were employing 1050 terminals on
this line, there could be a maximum of 26 terminals
on each of the 176 lines. Now we are dealing with a
practical application utilizing the maximum capability of a Teleprocessing system. More information
concerning the configuration possibilities of the 2703
appears later in this course.
Since the announcement of the 2701, 2702, 2703,
a new device called 2712 Remote Multiplexor has
been added to the line. A customer previously required up to 14 separate telegraph or 10 telephone
lines between the data processing center (DPC) and
the remote terminals. He can now combine the lowspeed networks for transmission on one higher speed
telephone channel. Figure 4 shows an example of
the configuration.
We can consider some more specialized applications of Teleprocessing. The IBM 7770 and 7772
provide voice answer back capability in response to
digital inquiries to a computer system. The IBM
2260 and 2848 are available to provide low cost,
quiet, efficient inquiry to computers, and visual
display of responses. This brings us up-to-date on
the history of Teleprocessing in IBM and leads us to
the future.
Accurate high-speed transmission of data between various locations is an extremely challenging
problem. Some immediately foreseeable applications .
1. Recording at point of sale with immediate
connection to a banking facility for account adjustment
2. Direct digital recording of remote data in a
computer system from any telephone
3. Time-controlled automatic meter reading
4. Optical scanning of cash register sales and/
or daily record tapes
5. Vehicle traffic control and air traffic control
(perhaps even space traffic control)
Without applications such as these, the need for
complex high-speed ultra reliable computer systems
diminishes. New applications will require a vast
amount of work in establishing world-wide compatibility standards of services and methods. Electronic
Industries Association (EIA) domestically and the
Comite' Consultatit International Telegraphique et
Telephonique (CCITT) in Europe have already done
a great amount of work in these areas. Continued
association and cooperation in these endeavors will
help eliminate confusion in this growing science and
provide a sound basis for future developments.
The future promises many new applications such
as data transmission for coordinating the space exploration effort. A trip to the moon requires many
days, but radio communication requires only about
one second. It can readily be seen that demands for
TP will expand fantastically. We have already seen
the use of telemetry methods employed for examining the surface of Mars and we realize that this is
only a beginning of a vast series of extra-terrestrial
applications. To talk about applications in the more
immediate future, we will have only to consider the
transmission facilities which have just been made
available for worldwide TP through the synchronous
communications satellites, such as Early Bird.
Remember that the primary aim of TP is more
effective business operations through outstanding
service to customers . Having examined the past,
Remote Multiplexor
High Speed Communication Line
2 4
:=f-}' To Terminals
29 } Communications Lines to Terminals
30 Ex; 1030, 1050,1060,1070,2741, Teletypewriters
Low Speed Line
Data Collection Terminals
Long Distance
40 Feet or Less
Data Communication
1--'2=--_ _ _ _ _ _ _ _ _ _ _ _ _--1. . Process Control Terminals
High Speed
(e.g., 2701 on another
System /360 CPU)
present, and future, we will look at the Teleprocessing application, and the various concepts and services required to implement it.
This completes Session 1 of this course. If you
can correctly answer all of the following review
questions, you may proceed with Session 2. If you
answer any questions incorrectly, you should review
the appropriate portions of the text before proceeding.
1. When did IBM enter the field of Teleprocessing?
2. What are the three objects of Teleprocessing?
cards per minute? (For the purposes of the question,
ignore any delays due to answer back or error procedures.)
cards per minute.
7. The four general types of remote terminals
through which a System/360 Computer may communicate, using an IBM 2701 Data Adapter Unit, are:
__________________ and _ _ _ _ _ _ _ _ __
3. What existing communications facility provided the connecting link between IBM Card Transceivers using type:
068 Signal Units? _ _ _ _ _ _ _ _ _ _ _ _ __
067 Signal Units?_ _ _ _ _ _ _ _ _ _ _ _ __
4. What are the three transmission rates in
Words p~r Minute when utilizing the IBM Card Data
Transceiver with an 067 Signal Unit? _ _ _ _ __
_____________ and _ _ _ _ _ _ _ ___
8. If you wanted a remote terminal from which
you could interrogate a computer, and receive
answers on a television-type display, which IBM
Teleprocessing Terminals would you select? ____
5. If a Word is equal to six characters, and
each IBM Card Data Transceiver Character is made
up of 8 possible bits, what would be the transmission
rates of question #4 in terms of Bits per Second?
10. Draw a diagram showing how a System/360
computer could be connected to five terminals of the
IBM 1050 type, where three of the terminals are in
the same building with the computer and two are located apprOximately 5 miles away. You are limited
in this problem to using communications facilities
that the customer already has installed and further,
each of the terminals must be able to communicate
with the others.
6. What is the approximate maxi?lum transmission rate of the IBM 7711 Data Communications Unit
handling magnetic tape if we express it in terms of
9. Which IBM Teleprocessing devices are capable of answering an inquiry with voice responses?
As a result of completing this session you should:
1. Become familiar with the important factors to be considered in Communication
System design.
2. Learn how to represent a system diagrammatically.
3. Learn how one type of customer utilizes these services.
4. Be able to define Teleprocessing.
Design factors: Purpose, number and size of messages, priority of messages,
accuracy and cost.
Network: System of devices capable of intercommunication according to certain
Teleprocessing provides our customer with a way to improve service to their customers and, at the same time, improve their own operating efficiency.
Airline Reservation System remote terminal hookup to Data Processing center.
Teleprocessing defined.
The history of rapid communications from
smoke signals to sattellites reflects the continual
growth in man's ability to convey information beyond the range of the human voice. In less than a
hundred years, the electrical communications industry has developed from the first manually-keyed
telegraph to complex television systems. Messages
have been sent and received over a million miles of
space, and television programs have been transmitted from the United States to Europe by way of a
communications satellite. With all of these technological advances, the basic considerations in a
modern communication system are not too different
from those in a primitive society using drums or
smoke signals to send messages between related
Some of the imnortant factors which must be
considered in communication system design are:
purpose of the communication system
number and size of the messages
priority of the messages
accuracy and cost of the system
The following description of a data communication system introduces some of the terms and illustrates the
scope of the material covered in succeeding sessions.
Before we discuss the individual pieces of equipment that make up the Teleprocessing data communications system, we should define "network". The
network describes the entire connection of the remote terminal eqUipment and the intermediate processing equipment and, finally, its connection to the
data processing center to perform all of the functions
of this system. The various remote devices and
intermediate devices are connected to the data processing center in the main office over telephone lines
or perhaps dial-up telephone facilities or telegraphic
facilities. Figure. 5 shows three basic types of
network. The network at A represents a single droptype - that is, there is only one remote terminal
working with one control station. In any network, we
have to define one location as the control station so
that it can set up the necessary controls for the
entire communications network. Note that in network
B, there are three terminals. At the extreme lefthand side, (the apex of the triangular arrangement)
Station 1 could be the control station. Each remote
terminal operates with the control station and is not
capable of directly communicating with the other
except through the control station. In Figure C, we
note that all three t~rminals or stations are connected together. Thus, terminal 1 can communicate
with terminal 2, 2 with 3, or 3 with 1, each independent of the other. Networks Band C are representative multidrop networks, since the control station
works with more than one other terminal.
The connecting lines between these terminals
represent the communication channel or circuit
. (also called lines). This channel or circuit can be
a wire, or a cable, or it can be a radio or microwave link. To review what we have learned about
In the figure at A, terminal 1 may communicate
with terminal 2. In the figure at B, terminal 1 can
communicate with 2 or with 3. However, if terminal
3 wishes to communicate with 2, it must first communicate with terminal 1. Terminal 1 may then relay the message to terminal 2. Here we have an
ideal example of "Message Switching". The message
must first be sent from 3 to 1, then rerouted from
1 to 2. In the Figure at C, we find that terminals 1,
2 and 3 may each communicate directly without requiring another station to perform any switching
function for them. The process by which the desired
terminal receives or transmits the message is
analogous to the concept of circuit switching operation. More about circuit switching and message
switching appears in later Sessions.
In summary, the four main factors to be considered in a communication system design are: purpose,
number and size of messages; priority or urgency
of messages; the accuracy of the system; and cost
of the system. The Airlines Reservation System
handles some of these factors in the following manner:
1. Improved customer service through:
(a) Reliable reservation control
(b) Advance hotel reservations for passengers
(c) Advance car rental arrangements at
(d) Special dietary instructions if necessary
(e) Arranging for wheelchairs, etc. to meet
arri ving passengers who may require
2. Improved airline efficiency thru:
(a) Timely maintenance scheduling
(b) Fewer unused seats on flying aircraft
(c) Accurate crew time-logging
(d) Advance weather information accurately
logged and constantly available
(e) Accurate control of arrival and departure
of aircraft
The concept of networks, as applied to the Airline
Reservation System, is shown in Figure 6.
The Airline Reservation System consists of a
data processing center (DPC). This DPC contains
computing equipment including disc files, drum
files, and tapes for both long and short...;.term storage
depending on the speed of access rate required. In
addition, there are special devices known as multiplexors to coordinate the operations of the duplicate
devices located in the DPC.
The DPC is connected to strategically located
terminal interchanges on high-speed telephone lines .
There may be as many as 30 terminal interchanges
on one line. (We will use the abbreviation TI to
represent a terminal interchange.) Through lower
speed channels, each of these terminal interchanges
may be connected to 30 "agents' sets". Agents' Sets
are the individual reservation terminals of the system. The agent sets cannot communicate directly
with each other but they may communicate through
the DPC under program control. The agent sets are
normally installed in locations such as travel bureaus,
ticket-selling agencies, or airports. The agent sets
send messages to the terminal interchange. The TI
stores these low-speed messages and then transmits
them in groups (soon after they are received) over
the high-speed telephone line for maximum efficiency
of transmission. Now, consider the matter of number and size of messages.
The agent set consists of equipment which can
send and receive prearranged fixed code sequence
messages as well as operator-controlled texts. The
relative sizes of these messages are:
1. The fixed-length messages -- these are normally 11 characters in length.
2. Variable-text messages -- approximately 100
character blocks. Additional blocks may be used
but they require special considerations either by the
TI or by the DPC.
The agent set can handle only one message at a
time. It can send or receive, but not simultaneously.
This is called a "half-duplex" operation. We will
cover the half-duplex concept again later. Remember that half-duplex means the ability to send or to
receive as long as we don't try to do both at the
same time.
TI = Terminal Interchange
AS = Agents Set Terminal
The TI can be operating with as many as 30 of
these agent sets. All these agent sets could conceivably send at t~e same time, but the terminal
interchange has the capability to so sample the incoming lines that it will never miss information
coming from the agent sets even if all transmit
simultaneously. The terminal interchange can store
up to 39 individual message segments, each message
segment consisting of as many as 100 characters.
These may be any combination of messages destined
for the DPC, or return messages from the DPC intended for agent sets.
The manner in which the terminal interchange
communicates with the data processing center is
variable: that is, it may either be based on a request from the DPC to transmit or it may be based
on a previously received "go-ahead" signal from
another' terminal interchange. This perhaps seems
a bit confusing; but if we consider that on any highspeed line there may be as many as 30 of these terminal interchanges, each serving 30 agent sets, it is
obvious that the most efficient way to handle this
operation is for each terminal interchange to say,
"Well, I'm done sending my traffic to the DPC, next
man down the line pick up the ball and carry it. "
This is the way it is done in some systems. Thus,
when a complete line of terminal interchanges has
sent all of its messages to the DPC, and finally, the
last one says, "Go ahead, DPC", the DPC lmows
that it's time to 'start the poll of all of the terminal
interchanges over again ("Poll" is the term meaning
"invitation to transmit" ). Only once does the data
processing center have to send out a specific command to transmit to a terminal interchange, and then
only to the first terminal interchange in the list.
From then on, as each terminal interchange finishes
sending its own blocks of traffic, it tells the next TI
down the line to send. This is the most efficient
way to handle this type operation.
Now let's examine the problem of urgency of
messages. As can be seen from the preceding paragraph, the urgency of messages is established only
by the DPC. The DPC can cause a change in the
sequence in which the terminal interchanges are permitted to communicate with it. Thus, we can alter
that sequential pattern of one terminal interchange
telling the next down line terminal interchange to
For example, consider Figure 6, communication
line H. On this communication line, we have terminal interchanges 1, 2, 3, 4 and 5. From terminal
interchange 3, note that we have agent sets connected.
The line that is shown (with the breaks) represents
the entire 30 agent sets that could be connected to
terminal interchange 3. Suppose while transmitting
from terminal interchange 1, the data processing
center in scanning its polling charts finds that on a
normal basis, at this time of day, terminal interchange 4 will be piling up a lot more messages than
it can possibly handle. The DPC can send a message
called a "change next interchange address" message
to terminal interchange 1 which will cause terminal
interchange 1 to tell terminal interchange 4 to go
ahead instead of 2. This method of changing the
polling sequence permits maximum utilization of
network facilities. The configuration of the network
can vary considerably from that shown in Figure 6.
The following particulars could vary, depending upon
the need for capacity and the demand for accuracy
of the particular system:
1. Number of lines
2. Number of terminal interchanges per line
3. Number of agent sets per line into the terminal interchanges
4. Complexity of the computing portion of the
Let's talk a little more about this problem of
accuracy and cost. To eliminate errors, many
checks are made as the data is handled. Some of
these are:
1. Each character sent from or received by the
agent set is parity checked
2. Each character received by the terminal
interchange is parity checked
3. Each message is checked for a specific count
agreement on transfer between the terminal interchange and the data processing center. Any disagreement in the count constitutes an error somewhere in the message.
Communication facilities are available to cover
the entire range from fast to slow • Microwave
channels can carry great amounts of information
quickly but are considerably more expensive than a
relatively slow, telegraph-rate, channel. Depending
upqn customer requirements, the Airline Reservation System agent set may use telegraph facilities
or some type of telephone channels. Between the
terminal interchange and DPC, transmission occurs
at about 3 milliseconds per character. This requires
higher speed telephone-type channels. In order
satisfactorily to establish a cost for the system, a
cus~omer m~st weigh the advantages of the system
against not having the system. The most efficient
configurations and the most economical communication channel consistent with required data rate and
accuracy are also important factors for consideration.
The completed network must be examined and considered relative to its purpose, the volume of work,
the required response times and, finally, in terms
of cost. While cost is important, it is extremely
unwise to attempt to plan a network based on meeting
a pre-established price. This -course will not deal
with specific costs of facilities or equipment but the
customer engineer should be aware that this is an
important consideration. Publications, covering
detail cost of required services, are available from
various common carrier companies, the Federal
Communications Commission, and some state utility
commissions. These are called "Tariffs," and
represent the standard prices charged for given
This completes the self-study portion of text for
Session 2. Additional information on any of the
initial concepts presented here, can be found in the
following publications:
1. Data Communications, Form 320-0903. This
book is a detailed review of the development and
application of Teleprocessing equipment in IBM.
2. Reference Manual IBM 9000 Series Airline
Reservation Systems Remote Equipment, Form
3. Teleprocessing Systems Summary, Form
A24-3090. This book will outline the basic characteristics of many of the Teleprocessing devices
marketed by IBM.
At this point, you have probably derived a definition of Teleprocessing. Teleprocessing as it concerns IBM designates IBM products used for transmitting and receiving data at separately located data
processing offices. Teleprocessing equipment consists of transmitting terminals and receiving terminals that utilize transmission facilities such as
telegraph lines, telephone lines, and microwave
radio signals.
The next section of this course consists of study
questions. for review to find out how much you learned
from what you read, and how well you understand the
principles presented.
If you can answer the following review questions
correctly, you may go on to Session 3. If you answer
any questions incorrectly, review the appropriate
sections of the text before proceeding.
1. The four main factors to be considered in the
design of a communication system are:
(a) _ _ _ _ _ _ _ _ _ _ _ _ _ __
(b) _ _ _ _ _ _ _ _ _ _ _ _ __
(c) _ _ _ _ _ _ _ _ _ _ _ _ _ __
(d) _ _ _ _ _ _ _ _ _ _ _ __
(a) Utilizing the principles of Figure 5,
draw a similar schematic representation of
the network consisting of a large Data Processing Center (DPC) connected to two smaller
DPCs (A & B). Each of the small DPCs is
connected with its own network of five remote
terminal devices. Each of the terminal devices on either DPC-A or DPC-B is capable
of communicating with any other terminal but
only through one or more of the DPCs.
3. When the terminals of DPC-B wish to communicate with each other, is Message Switching or
Circuit Switching utilized, as configured in question
#2 (a)? _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ ____
4. The terminal systems of question #2a would be
considered as
(a) Single drop
~) Multi -drop
5. A multiplexor coordinates the operation of
multiple devices utilizing shared facilities. (True,
6.· How many Agents sets could be serviced on
one high-speed telephone line if the maximum number
of terminal interchanges was employed?
70 List four areas in which the Airline Reservation System has made it possible for Airlines to
improve the service they render to their customers.
(a) _ _ _ _ _ _ _ _ _ _ _ _ _ _ __
(c) _ _ _ _ _ _ _ _ _ _ _ _ _ _ __
(d) _ _ _ _ _ _ _ _ _ _ _ __
8. List four ways in which the efficiency of the
Airline has been improved through the use of the
(a) _ _ _ _ _ _ _ _ _ _ _ _ _ __
(c) _ _ _ _ _ _ _ _ _ _ _ _ _ _ __
(d) _ _ _ _ _ _ _ _ _ _ _ _ __
(b) Repeat as above, except that the terminals
connected to DPC-B all communicate among
themselves without assistance of the DPC.
Show only the portion of the network that must
be reconfigured.
9. Can Agents sets communicate with each
other directly, i. e. without intervention by DPC?
10. Which governmental agencies regulate communications?
11. Define Teleprocessing, as you understand it.
This session presents the functions of data transmission and control through analogies
to normal data processing methods.
Keypunch and Summary Punch as examples of data transmission.
Data Flow in a typical Teleprocessing environment.
Serial versus Parallel transmission.
Interface defined.
Input/Output media available.
Message component sequence, including control signals.
The objective of Teleprocessing is the efficient,
accurate, and economical transmission of data from
one place to another. Teleprocessing requires
originating and terminating equipment, a communications facility, and equipment to convert data into
a form that may be handled by the communications
The simplest form of data transmission, familiar
to most customer engineers, is the keyboard of an
IBM keypunch. This device translates the mechanical keystroke of the operator into electrically coded
impulses to punch the required pattern into an IBM
card. Through mechanical means, the depression
of the key is converted into closures of certain contacts in the keyboard. These contacts provide electrical signals to be transmitted over the cable connecting the keyboard to the remainder of the machine.
Control signals are also sent to tell the punch drive
when all of the information has been transmitted to
the punch unit, and to perform the mechanical action
of punching the code into the card. A return acknowledgment signal unlocks the keyboard and permits the
next character to be keyed in. This is a form of
asynchronous transmission as it requires a stop and
a start signal to control the beginning and end of
each character handled.
Another form of data transmission is summary
punching. In this case, the carrier facility or
channel is the summary punch cable connecting the
accounting machine to the summary punch. The
accounting machine sends a signal at "total time" to
the summary punch to inform the punch that it wishes
to communicate. The summary punch starts to run
if all operational requirements are met, and it sends
a timed series of pulses to query the accounting
machine. The contents of the selected counters and
storage units are transferred to the summary punch
and punched into the summary cards. The summary
punch then returns an acknowledment signal, in this
case the "summary punch end" shot to the accounting
machine. Again, the accounting machine reverts to
processing data to make another record available to
the summary punch. During processing time, the
summary punch is idle. The initial call of the
accounting machine to the summary punch could be
called an inquiry; and it is a turn-around command.
This means that the accounting machine will not take
any further action until given permission to do so by
the return signal from the summary punch at summary punch end time. The signal from the summary
punch at summary punch end time is a turn-around
command from the summary punch to the accounting
machine, and the accounting machine again begins
processing. This same concept is used throughout
the field of data transmission and the terms most
frequently encountered have been introduced here.
In order to convert this operation to a true Teleprocessing operation, we might hook the previously
discuss~d summary punch to the accounting machine
by a mile-long cable. This wouldn't be very practical for several reasons. The most important reason
is that a cable a mile long has considerable resistance and the si.gnal to the punch magnets would decay
or deteriorate in quality sufficiently to cause a failure
to punch. This operation could be carried out if the
square wave impulses were converted to alternating
current impulses before they were sent; then amplified back to their original levels upon receipt by the
summary punch; finally, converted back to DC to
energize the interposer magnets. This conversion
process is modulation; it will be used extensively in
Teleprocessing, and covered in more detail later in
the course.
In order to use existing telephone and telegraph
networks for data transmission, data processing
equipment must prepare suitable input for network
terminals, or be connected to the network. In addition,
data processing codes and signals must be converted
to signals acceptable to the circuits or channels used.
The data flow shown in Figure 7 starts with input
media such as punched cards, a keyboard, punched
paper tape, or magnetic tape. Information enters
the system through an input device such as a card
reader; then goes to a terminal and data set before
actually entering the communications channel. The
point at which the data processing and communications equipment connect is called the interface. The
terminal changes the data to a form acceptable to
the modulator. The change or conversion may require changes in mode, format, speed, code structure, insertion or deletion of special control characters. The modulator converts the signals to the
form acceptable by the circuit or channel. At the
receiving station, a similar interface with a demodulator and a terminal reconverts the signal to a form
acceptable to the data processing equipment.
Figure 7 illustrates a data transmission system
and one method of signal conversion used in a telephone circuit. The components used are:.
1. Input equipment, and IBM 1401 System
2. The terminal - an IBM 1009 Data Transmission
3. The modulator - a common carrier company
data set
4. Circuit or channel - a communiqations line
5. Demodulator - a common carrier company
data set
6. Terminal - an IBM 1009 Data Transmission
7. Output equipment - an IBM 1401 System.
Note that the IBM 1009 is both a receiver and a
transmitter, capable also of code conversion. Each
character sent from one 1401 to another over this
network is changed in mode from a parallel bit configuration to serial form and changed back at the
receiving terminal.
We have just introduced two new terms "Serial"
and "Parallel". When we punch a card from a keypunch keyboard, the zone and numeric punches of
the character are all punched simultaneously into the
card. This constitutes a parallel transfer; that is,
Serial by Character, Serial by Bit
Serial by Character, Parallel by Bit
4/8 Fixed Count Code
Seri 01 by Character, Seri 01 by Bit
4/8 Fixed Count Code
Serial b Character, Parallel b Bit
4/8 Fixed Count Code
all of the bits or pieces malting up a character are
handled at one time. However, when we summary
punch a card using, for example, a 407 and a 514,
we punch the digits in the card in the order of 12
punches, 11 punches, 0 punches, etc., down through
the 9 numeric punch. If we punch just one column of
the card on a summary punching operation, we are
serially punching the character into the card; that is,
if the character represented is an A, we punch first
the 12 punch, skip the 11, skip the 0, another punch
at 1 time, and skip the remaining punching positions
2 through 9. Thus, scanning a storage unit in the
407 accounting machine where the letter A was
stored, we have converted the character from its
parallel format in the storage unit to one bit at a
time transmission over the summary punch cable to
the 514.
Another application of data handling is the magnetic tape unit in which characters are written one
at a time, in parallel format, across the width of the
magnetic tape. In this application, we have parallel
characters recorded serially; that is, one full character at a time. The manner in which we transfer
data to a tape drive is referred to as "serial-bycharacter, parallel-by-bit". The data processing
system in Figure 7 transfers information to the 1009
by this method. In the 1009, the characters received
from the data processing system in binary coded
decimal are converted to a fixed count 4 of 8 code.
For comparison of BCD with 4 of 8 codes, see Figure 7. In addition to converting the code, the 1009
takes each character, after it has been converted to
4 of 8 code, and instead of transmitting that character in one swat over the communication line, it transmits the character one bit at a time (like the A from
407 to 514). The data is still in a form unacceptable
to the communications facility. Thus, we require the
service of the data set between the communications
line and the 1009.
The data set will perform an interface conversion
function of changing the fixed-count 4 of 8 code bits
from the 1009, which are still in DC pulses, into a
form that the line facility will accept. The data set
is modulating the line signal in accordance with data
being presented by the 1009. The data appears on
the line in serial by character, serial by bit format
in 4 of 8 fixed-count code. As it is received by the
data set at the receiving end, it will be received
serial-by-bit, serial-by-character and delivered in
4 of 8 fixed-count code to the 1009 in DC pulses.
The 1009 will accumulate these bits one at a time
until it has a full character, and then transfer the
characters one at a time to the data processing
system. A conversion from serial-by-bit, serialby-character, to parallel-by-bit, serial-by-character has been effected. We have also translated the
character from 4 of 8 fixed-count code back to binarycoded decimal, which is the operating code of the
data processing system.
In Figure 7, we have considered the data processing system to be a 1401. This data processing
system could actually be anything from a data transceiver to a full System/360 operational complex.
Some of the available input/output choices are: cards,
magnetic tape, punched tape, printed copy, visual
display in lights or visual display of characters on
a television-type tube. In addition, data being handled could come from a disk or drum file of one
computer system to the main memory of another
The connection of the data processing system to
the communication equipment is called the "Interface". In the case of data transmission, the interface or demarcation strip provides a dividing line
between the responsibility of IBM and that of the
communication company. For this reason, each
manufacturer or supplier of communications equipment for attachment to business machines provides
a publication called "Interface Specifications".
This publication defines what signals the business
machine must provide for the communications equipment and describes the nature of signals which the
communications equipment will return to the business
In Figure 7, the interfaces that exist are from
the data proceSSing system to 1009, from the 1009
to the data set. On the receiving end, from the data
set to a 1009, and from the 1009 back to the data
processing system.
Besides handling data ("text" of the transmission),
this type of system sends and receives coordinating
signals. The most common of these are listed in the
sequence of their normal use:
1. Inquiry
The transmitting end checks the
receiving end to see if it is ready
to receive.
2. Turn-around Both ends switch their sending
and receiving functions. (Time
required may be up to 200 milliseconds. )
3. Acknowledge
By this control signal, the receiving end becomes a transmitter
signals the transmitting end
to proceed.
4. Turn -around is repeated.
Sending end begins transmission
5. Start of
with proper heading control,
followed by full text of that record.
6. End of Trans- This control signal follows the
mission or
last character of the record. It
End of
usually includes an LRC (Longitudinal Redundancy Check)
auuurnulaLiuIl uf Lhe LexL irnrneui-
7. Acknowledge
ately preceding.
An LRC is essentially an odd/
even count of the bits transmitted
in the preceding message. It
constitutes a turn-around command
and causes the sending end to await reply from the receiving end.
is repeated if the message has
been received without error and
the LRC agrees with that which
the receiving end has accumulated.
(Some acknowledgment signals
permit the sending end to ascertain whether the message was
successfully received or whether
it should be repeated.)
8. TUl'n-arUWld
is again repeated and sequence
continues from 5.
In handling data flow on this communication network,
we spoke of parallel-by-bit, serial-by-character
processing which was then converted to serial-bybit, serial-by-character. The process of converting data from parallel-by-bit to serial-by-bit is also
called "serialization". The reverse process on
receiving (i. e., receiving one bit at a time and converting it into a full character) is called "Deserialization". Having covered the requirements of the
terminal equipment, we turn to the communications
This completes the text for Session 3. If you can
answer all of the following review questions correctly,
you may go to Session 4. If you answer any questions
incorrecey, review the appropriate sections of the
text before proceeding.
10. List 6 choices of input and output media
available for this operation.
When a character is transferred from the
KeypWlch keyboard to the punching Wlit (serial/parallel) mode transmission is employed. (Underline
choice. )
2. Summary punching is an example of (serial/
parallel) transmission. (Underline choice.)
3. Briefly describe what is meant by a "turnaroWld command".
(a) _ _ _ _ _ _ _ _ _ _ _ _ _ __
(b) _ _ _ _ _ _ _ _ _ _ _ _ __
(c) _ _ _ _ _ _ _ _ _ _ _ _ _ __
(d) _ _ _ _ _ _ _ _ _ _ _ _ __
(e) _ _ _ _ _ _ _ _ _ _ _ _ _ _ __
What is modulation, and why is it used?
11. The function of an interface or demarcation
strip is: _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ ____
12. To what would you refer if you wanted complete information regarding signals exchanged between a data terminal and a common carrier data
set? _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ __
13. Converting data from serial-by-character,
parallel-by-bit to serial-by-character, serial-bybit is called _ _ _ _ _ _ _ _ _ _ _ __
5. Data transfer from the IBM 1401 in the text
figure to the IBM 1009 is in serial-by-character.
________________ bilform.
6. Data transfer from the IBM 1009 to the Data
Set is in serial-by-character, _ _ _ _ _ _ _ _ ___
7. The signal on the line is _ _ _ _ _ _ _ __
by the data stream from the IBM 1009 to the Data
Set (when transmitting).
8. On the receiving end, the Data Set _ _ _ __
______ the line signal and provides _ _ _ __
_ _ _ _ _ _ _ bit, serial-by-character input to the
IBM 1009.
9. Between the receiving IBM 1009 and the IBM
1401, the received characters are transferred
serially in
bit form and, at
the same time, are translated from
code to
14. The process of receiving characters serialby-bit and then transferring them in parallel-by-bit
form is called _ _ _ _ _ _ _ _ _ _ __
15. Why is an LRC exchanged between the sending
and receiving terminal?
16. What control signal usually indicates the beginning of the Text portion of a message? _ _ _ __
17. What control signal is sent by the transmitting
terminal to the receiving terminal to check whether
or not it is ready to receive? _ _ _ _ _ _ _ _ __
18. What reply is expected by the transmitting
terminal of question 17 ? _ _ _ _ _ _ _ _ _ _ ____
19. List 3 control signals which are considered
as turn-around commands.
(a) _ _ _ _ _ _ _ _ _ _ _ _ _ ____
(b) _ _ _ _ _ _ _ _ _ _ _ _ _ _ __
(c)_ _ _ _ _ _ _ _ _ _ _ _ _ _ __
This section covers the kinds of Communication facilities available for data communication, the equipment necessary to provide these services, and their combined
characteristics and problems.
Types of channels
Signaling Methods
Morse Code
Baudot Code
Binary Coded Decimal (BCD)
4 of 8 Fixed Count Code
American Standard Code for Information Interchange (ASCII)
Frequency Spectrum, Carrier Systems and Channelization
Types of Modulation
Amplitude Modulation
Frequency Modulation
Phase Modulation
Grades of Channels
Broad Band
Voice Grade
Subvoice Grade
Telegraph Grade
Line Quality
Phase Delay and Distortion
Echo Suppressors, Turn-Around, Conditioning, Compensators, Loading Coils
Measurement of Decibel Levels
Data Set as an interface device
Table 1. BCD and 4 of 8 Code
1 2 4 8 R
4 8
8 R
1 2
N 1 2
4 8 C A
8 C
N 1
2 4
2 4
N 1
2 4
1 2 4
1 2 4
2 4
1 2 4
8 R
1 2
1 2
N 1
N 1
2 4
1 2 4
N 1 2
8 C A
4 8 R
Blank (Note 2)
8 C
8 C A
4 8 C
4 8 C A B
4 8
4 8
1 2
1 2
4 8
4 8
4 8
8 R
4 8 R
N 1 2 4
4 8
4 8
1. This character is transmitted as the substitute character
2. An :lll-..ero 7-bit character is illegal, even when the tape unit is operating in even parity, because an attempt to write it on tape
causes a tape echo check.
2 4 8 C
2 4 8 C A
4 8
4 8
2 4 8
R 0
8 C A
8 C A
1 2 4 8
1 2 4 8
4 8 C A
1 2 4 8 C
2 4
4 8 R
4 8 R
8 R
2 4 8
N 1
1 2
N 1 2
8 C A
2 4
1 2 4
2 4 8 R
2 4
R 0
: (Note 1) ..
N 1 2 4 8 C A
l\ 0
8 R
8 R 0
2 4 8
2 4 8
SM -tt+WS-VTM";-
4 8
4 8
4 8
4 8 R 0
o )
8 C
X N 1
N 1
8 C
X N 1 2
1 2
1 2
N 1
8 R
N 1
1 2
N 1 2
8 R
A channel, circuit, or line is a path for electrical transmission between two or more terminals.
The channel may be a group of wires or a special
part of the radio frequency spectrum. The channel
carries information from one place to another. Although "circuit" is used conversationally as a synonym for channel or line, technically, a circuit can
contain several communications channels.
requires a certain amount of turn-around time. In
most applications, this is in the vicinity of 200 milliseconds. In preliminary planning for a network,
consideration must be given to the time required for
answer-back signals that can occur before, during,
and after each message as a response from thereceiving station, indicating a ready-to-receive status.
In some instances, the time required for the answerback can affect the throughput of a data transmission
The terms Simplex, Half-Duplex and Full-Duplex
represent the types of circuits. The names indicate
only the directional c~pabi1ity of the channel or circuit. The grades of channels, however, are Broad
Band, Voice Grade, Sub-Voice Grade, and Telegraph. Circuits are graded on their basic line speed
(transmission rate) expressed in characters per
second, bits per second or words per minute. Since
these different grades of circuits use different types
of signaling, an understanding of code structures is
necessary to evaluate their potential. The following
section introduces some of the more common codes
and signaling techniques now in use.
The three basic channel types are Simplex, HalfDuplex, and Full-Duplex. (See Figure 8.)
A Simplex circuit can carry information in only
one direction. It is Simplex because, in most cases,
the type of sending or receiving units cause this
A Half-Duplex circuit can carry information in
either direction but only in one direction at a time.
A Duplex circuit or Full-Duplex circuit can carry
information in both directions at the same time.
A network can consist of any combination of these,
according to the application requirements. In a halfduplex telephone circuit, reversing the flow of data
Simplex Circuit (SPX)
Half Duplex Circuit (HDX)
Full Duplex Circuit
Morse or international code is made up of dots and
dashes. For example, IBM, in Morse code, is two
dots, a pause, a dash, and three dots, pause, two
dashes. Although this code is written as dots and
dashes, the actual transmission consists of long and
short signals with pauses between characters. The
use of long and short intervals, intermixed with
pauses, indicates that time is an element of transmission. Compare the transmission of the code for
V, (see Figure 9) that is, three dots and a dash,
with the actual periods of signal. The presence or
absence of a signal indicates the character configuration. This code was developed for manual telegraph
signaling, and the experienced operator could differentiate between letters because of the length of the
no-signal or pause condition. Early mechanized
telegraph equipment could not handle Morse code
because the characters varied in length between
pauses. A five-bit code (called the Baudot code, for
its inventor) was developed and is still used (see
Figure 10).
The most common form of the Baudot code in
telegraphic use requires 7.42 bits or units of time
per character. Only five of these units are used to
carry information. A bit, in this instance, is the
fixed unit of time in which a signal is present or
absent. One bit is used as a start signal and 1. 42
bits as a stop signal (replacing the pause used between letters in the Morse code). The stop signal
shown in this example is 1. 42 bit times. The presence of a bit or signal on the line is a: Mark; the
absence of a bit or signal is a Space. Baudot code
signals consists of Marks and Spaces. The start bit,
corresponding to the pause between characters in
Morse code, is always a space. The long stop bit is
always a mark and the line is in a mark condition
when not being used. Thus, the start and stop bits
of the character perform also the functions of synchronizing the equipment at both ends of the communication facility.
Perforated Tape
Line Signals
Character Sent
1 2
• •
• •••
•• •
•••• •
•• •
• ••
• •• •
--- ---
Spacing Impulses
Marking Impulses
••• ••
. . . .......... .
3. •• ••
No Signal:
• •••
• __•______• __~
••• •
• ••••
!:iAB U:9124 !:iADA,MS
3 4 5
The following anecdote describes the origin of the
terms "mark" and "space". At each end of the circuit
on the old Morse code telegraph, there was a telegraph key to switch the line current OFF and ON,
and an associated electromagnetic type "sounder".
(See Figure 11.) When the line was in the idle state,
full current flowed and the electromagnetic sounder
armature was attracted to the pole piece. Copying
the message consisted of listening to the spacing
between clicks of the armature striking the pole
piece of the sounder. A short gap between clicks
was a dot and a long gap between clicks was a dash.
According to the story, the telegraph operator wanted
to sneak out for a cup of coffee without missing any
of the transmission coming to him. He attached a
pencil to the armature of the sounder and then rigged
up a reel of paper strips pasted together to form a
long piece of narr.ow tape. He ran the paper past
the pencil and wound it up on another reel under control of an old clock works. The result was that, so
long as the current flowed in the line, the pencil
marked on the paper tape; whenever the line was
opened by keying, the pencil moved away from the
paper tape and the tape merely spaced. Therefore,
the line was either marking or spacing. The terms
have remained to the present day.
Early electromechanical telegraph sending units,
used to transmit Baudot code, were attached to a
three-row keyboard resembling that of a typewriter.
Five contacts (one for each bit in the code) were
arranged in a circle. Each time a key was depressed,
certain of the five contacts were activated, tested by
a rotary wiper arm, and then reset. Figure 12,
titled Transmission Modes, shows this mode of
#1 Sounder
#2 Sounder
Units of Time
Serial Start - Stop
Serial Synchronous
-- ..
-- - ..
-- - --
Second Channel
Third Channel
Fourth Channe I
Fifth Channel
WiOiiiI = Mark or Current Flowing in Telegraph line
transmission under a classification mown as Serial
Start-Stop (also called asynchronous). For example,
the letter A, when depressed, activated only the
first and second contacts of the five. As the rotary
wiper started testing the five contacts, it sent an
automatic no signal or space indicating the start of
the character, then marks from the first two contacts, spaces from the remaining three, and finally,
a long mark for the end of character or stop signal.
Conceivably, if the sending and receiving term.inals
were controlled by the same timing pulse, the start
and stop bits could be eliminated. Characters would
be detected on the basis of every five bit times. The
serial start-stop Baudot code requires 7.42 units of
time for each character of 7.42 bits. Without start
and stop bits, however, the character requires only
five units of time for the five actual intelligence bits.
Transmission without start and stop signals is termed
In synchronous signaling, a special pattern of
bits is sent periodically to keep the terminals operating in unison. In most IBM applications, these are
called idle characters. They are generated automatically and sent as required by the system.
In addition to the serial start-stop and synchronous methods of transmission, the figure also shows
how the same characters can be sent over five channels simultaneoulsy, allowing one channel for each
bit in the code structure. This is called Parallel
Transmission. It is much the same as the keyboard
of the key punch transmitting data to the punching
In summary, a network may include Simplex,
Half-Duplex, or Duplex channels. The transmission
mode may be Serial Asynchronous, Serial Synchronous, or Parallel. In addition, depending on the
rate of transmittion necessary, it may require a
Broad Band, Voice Grade, Sub-voice Grade, or
Telegraph Grade channel.
The basic method of transmission in telegraphy is the
use of the key, a switch or a rotor to interrupt periodically the flow of direct current (dc). Various~
other methods have been developed which are more
suitable for transmission over long distances. Figure 13 is the schematic representation of an electromagnetic wave. Radio waves (which are electromagnetic waves), electromagnetic waves in a wire
conductor, and sound waves share the common characteristic of oscillating perpendicular to the direction of movement of the wave. In the figure, the
perpendicular oscillation is shown to have occurred
three times.
r----- h
The extend of the oscillation represents the relative energy level and is called the Amplitude; each
complete oscillation is called a cycle. Thus, the
figure indicates three complete cycles. The number
of times that one of these cycles occurs per second
is referred to as the frequency of the wave. Frequency is usually expressed in cycles, kilocycles,
or megacycles (abbreviated CPS, KC, MC). A
newly adopted term called Herzians (Hz) corresponds to cycles. The overall frequency spectrum
for electrical communication ranges well above
30,000 megacycles. Different parts of the frequency
spectrum are used for various types of communication because the characteristics and behavior of the
signals change with the frequency.
The lower costs of communications today have been
made possible by the efforts of communications
engineers in using the frequency spectrum to full
advantage by employing Carrier Systems. These
systems carry many conversations or messages on
a single circuit. The human voice has a frequency
range of approximately 50 to 16,000 cycles per
second. If modern telephone systems were allowed
to use this entire frequency range for each conversation, only very few conversations per channel
would be possible at anyone time; and the cost per
call would be high. Communications engineers have
found that a frequency range or band approximately
three kilocycles wide is sufficient to retain most of
the individual characteristics of the human voice
during transmis sion. Since many telephone circuits
have a range or bandwidth of 150 kilocycles, at 3 kcs
per channel we can have 50 possible voice channels
in this range. The problem, of course, was to device a method of carrying voice signals at different
frequencies on the same line. Through a method
mown as multiplexing, each three kilocycle band is
assigned an individual sub-carrier frequency to be
transmitted by the carrier facility. Thus, in much
the same way that we can send many radio frequency
carriers through the air at one time, we can send
multiple carriers over a wire at one time. The only
restriction is that the frequencies used must endure
transmission without excessive loss. On the transmitting end of the carrier facility, modulators are
necessary for each of the sub-carriers used. On
the receiving end, demodulator circuits are necessary to extract the original messages. (See Figure
14. )
Consider an imaginary one-wire communication
channel capable of carrying frequencies from 10,000
to 30,000 cps. The human voice with a range of 50
to 16,000 cps is used to control the flow of current
in a telephone hand-set. Filtering circuits within
the telephone hand-set permit only frequencies from
200 to 3, 000 cps to pass. This signal enters the
modulator and controls the operation of an oscillator.
The oscillator's frequency is permitted to vary in
proportion to the frequency of the applied signal.
Thus, if the oscillator'S center frequency is 15,000
cps, we have 15,000 cycles plus 200'to 3,000 cycles,
and 15,000 cps minus 200 to 3,000 cycles. We obtain an upper side band from 15,000 to 18,000 cycles,
and a lower side band from 15,000 'cycles down to
12,000 cycles. The upper and lower side bands both
contain the same waveform or signal and either can
be used for the actual transmission.
At this point, filters are used to eliminate or
suppress the carrier and, in this example, the lower
side band. The upper side band alone with a frequency
of 15,000 to 18,000 cps (which is a direct function of
.the original voice signal) is routed to the telephone
wires or cables. At the receiving terminal, the
voice signal enters the demodulator and filter. The
recovered voice signals, reduced from 15,000 18,000 cps to the audible level of 200 - 3,000 cps,
are forwarded to the receiving telephone hand-set.
To allow buffering, we have also provided to this
same channel the spectrum up to 19 kilocycles to
prevent interference from one channel to another.
The modulators and demodulators, along with the
fixed carrier frequencies, act to raise and lower the
voice frequency level. By using different oscillators,
generating different carrier frequencies, the same
technique can be used for bands A, C, and D in Figure 15 to provide four simplex voice channels on a
single wire. Band B shows the upper side band of
three kilocycles as previously described. In a more
complex carrier system, more channels can be
The three-kilocycle band, used in the preceding
illustration, can be further subdivided into bands of
150 to 200 cycles each. These are not wide enough
for the human voice but could be used for the transmission of information in code. For data transmission, a code structure based on the presence or
absence of a bit or Signal can be indicated by changes
in the waveform of the side band frequency transmitted. Figure 16 illustrates some modulation
methods and shows the relationship between the
binary-coded data, the presence or absence of a
signal, and the representations of the various waveforms produced for serial transmission. The binarycoded data is indicated by a 1 or a 0 corresponding
to the bit configuration. Thus, the presence of a
bit is aI, the absence of a bit is a 0, corresponding
also to a Mark or a Space respectively. The example of Amplitude Modulation indicates a drop in the
amplitude for a no-bit conditiqn. Frequency Modulation changes the frequency of the carrier to indicate a no bit. Phase Modulation is not limited to the
illustrated 180 0 change but may be accomplished by
a 90 0 or other change according to the design of the
terminals and the quality of the channel. For Phase
Modulation, the IBM 3977 Modem utilizes a 180 0
change. Various common carrier data sets employ
four-phase Modulation using shift increments of 45 0 •
30 kc
Band D 25 - 29 kc
/I,.qI'j,,'V-A,<-,U-'p-'-pe_r_Si_de_b_an_d_ _
'(....tlt', Carrier Signal
'v.,ft., Lower Sideband
15 kc
Band D
25 - 29 kc
15 - 18 kc
Filter -
12 - 15 kc
Band C
20 - 24 kc
Band B
15 - 19 kc
Band C 20 - 24 kc
Band B 15-19kc
Band A 10 - 14 kc
Band A
10-14 kc
Binary Coded Data:
requires a band width of about 4,000 cycles. On the
carrier system that we looked at previously, this
would eliminate our buffer channel to prevent interference. Technological advances have made it possible to pass 2, 000 bits per second over a three
kilocycle band width. The number of bits per second
over a given band width of channel is a function of
the type of modulation used.
The grades of channels we will look at are Broad
Band, Voice, Sub-Voice, and Telegraph. This is in'
descending order of bit handling capacity. These
grades do not imply strict limitations. There are
general categories of high, medium, and low speed
facilities that are specifically described in tariffs
filed with the Federal Communications Commission
or equivalent State utility Commissions.
No Signal:
Frequency Shift
Broad Band Channels
The agreement between the communications common
carrier and the subscribers is basically an arrangement for the use of a certain type and grade of communications channel. The channels are classified
by type, i. e., Half-Duplex or Full-Duplex, and by
grade, i. e., the capacity of the channel to carry
information in bits per second (or a similar grading
factor). The physical circuit or technique used,
whether a voice frequency circuit, a wire carrier
channel, or a portion of a microwave band, depends
on the facilities available in each geographic location. The subscriber is not necessarily concerned
with the fact that a voice grade channel uses a carrier frequency of 15 kilocycles or 500 kilocycles but
only that the available band width is capable of carrying the required volume of information with the specified degree of accuracy.
The line speed of a channel, measured in bits per
second, is direotly related to the band width of the
channel. It is important to keep this in mind because a high bit rate per second requires a wider
channel (more cycles per second). Anything occurring in the channel which would tend to limit the
transmission band width of that channel will, of necessity, limit its transmission rate also. The communications companies have found that from a practical point of view, considering cost and performance,
the band width should be about twice as wide as the
number of bits per second passing through the channel. This means that to handle 2, 000 bits per second
Broad band channels, via microwave or radio relay
communications systems, use the super-high frequencies above 3,000 megacycles. Radio waves of
these frequencies tend to travel in a straight line
requiring repeater stations with dish -like antennae
on high buildings or towers every 20 to 35 miles.
Carrier, modulation, and channelizing techniques
are used. The speed of transmission is limited only
by the band width available. Some idea of the cost
of a broad band channel can be gained by noting that
one type of channel capable of carrying 5100 characters (40. 8k bits) per second costs about $15 per
mile per month. This is similar to the common
carrier Telepak service. The IBM 7710 or 7711
Data Communication Unit provides point-to-point
communication over a Broad Band channel at speeds
up to 230,400 bits per second (28,800 characters
per second). If the customer's application cannot
justify the utilization of Broad Band channels, the
same equipment can be operated over a Voice Grade
channel at lower rates. The 7710 and 7711 are compatible with the 1009 Data Transmission Unit, the
1013 Card Transmission Terminal and the 7702
Magnetic Tape Transmission Terminal.
Voice Grade Channels
Voice grade channels have a band width of approximately three kc and, when used for voice communication, can carry only one conversation. In data
communications, it is possible to handle 2,400 bits
per second with this type of a line facility. Higher
speeds are possible but are dependent on the commercial availability of suitable channels and terminal equipment. The common carrier can channelize
the voice grade line into smaller bands (of 150 to
200 cycles) each suitable for Subvoice or Telegraph
communications: that is, for up to 200 (approximately) bits per second of transmission. The co:::t
of a leased Voice Grade line used for data is determined by the length and type of channel. Dialed
calls for data transmission are usually charged at
the same rate as ordinary voice telephone calls. A
telephone and data set are required at each terminal.
A telephone connection is established between two
terminals either by direct dialing or through an
operator. Each terminal attendant then presses a
data key on his phone to connect the data set to the
line, and disconnect the telephone. Pressing the
key establishes a direct line for data transmission
between the two terminals and prevents use of the
telephone. The completion of this operation is called
Channel Establishment. Sometimes it is referred to
as the "hand- shake routine" or other similar terminology. The IBM equipment, encountered on this
type of channel, may range from a 1001 to a complete
System/360 utilizing IBM 2701 or IBM 2702 through
data sets to the line.
Grade Channel s
Subvoice grade channels have a lower speed than
Voice Grade channels. There are no specific limits
but Subvoice grade channels are usually considered
to be those below 600 bits per second transmission
rate. The following are examples of utilizing Subvoice Grade channels.
IBM 1030 Data Collection System - An Application
The IBM 1030 has a transmission rate of 60 characters per second. The 1030 is a multi-application
data collection system that provides two-way communications between a centralized IBM 1440 or 1460
Data Processing System and remote locations in a
plant utilizing an IBM 1448 Transmission Control
Unit. With the appropriate adapter, this system can
be linked to a 1410, 7010, or a fully complemented
System/360 through a 2701 or 2702.
The IBM 1050 Data Communications System is
the general purpose data terminal for on-line transmission or off-line preparation of data at rates up to
14.8 characters per second. Input and Output devices include the 1052 Printer-Keyboard, the 1053
Printer, the 1054 Paper Tape Reader, the 1055 Card
Punch; all are controlled through the 1051 Control
Unit. Each terminal can communicate with another
1050 system, an IBM 1440, 1401, 1410, 1460, 7010,
or to the 2701 or 2702 of a System/360 installation.
The IBM 1060 Data Communications System is
designed for banking applications. The IBM 1070
system is designed for process control applications.
Either system may be connected also to System/360
through the 2701 or 2702 with the appropriate adapter
Telegraph Grade Channels
The line speeds of these circuits range approximately from 45 to 75 bits per second. The circuits
normally employ asynchronous mode of transmission;
that is, both start and stop bits are required to synchronize the two ends of the operation. The telegraph
was the first effective means of cross-country electrical communications. One of the characteristics of
early telegraph lines was the use of regenerative
repeater stations along the circuit. As mentioned
before, the quality of the DC signal decays on a normal line as we go along that line, due to resistance
and other characteristics of the line.
To maintain the quality of communication, mechanical repeaters were necessary to regenerate a
distorted or a decayed signal (one whose quality was
no longer satisfactory for operating a teleprinter)
into a good, clean signal. This repeater samples
the weak, distorted signals and then generates a
completely new, undistorted signal to forward along
the line. Regenerative repeaters, however, are
code-sensitive and may restrict transmission to the
code for which the repeater was designed.
Some electronic type repeaters have recently
been introduced which are not code sensitive. The
IBM 65 and 66 Data Transceivers are units used in
conjunction with IBM 067 Signal Unit to send and
receive punched card data over telegraph grade circuits, at 60, 75, and 100 words per minute to provide the capability of sending approximately 3, 4, or
5 cards per minute.
In discussing line quality, some of the terms that we
encounter are: resistance, impedance, conductance,
and attenuation. In this introductory course, however, only a few of the communications problems
will be covered to illustrate techniques used to provide faster and more accurate communications.
Based on theory, electrical communications
travel at the speed of light. In practice, however,
wires and cables have characteristics that tend to
reduce this speed. The frequency of transmission
is a direct factor in determining how effective these
characteristics are in changing the propagation rate
of the signal. Some factors that influence the efficiency of long distance wires and cables are: the
distance involved, the ability of copper or aluminum
wire to carry the signal, interference from other
circuits (crosstalk), and electrical characteristics
which cause distortion or loss of signal.
Before tubes and amplifier circuits were developed, the length of a telephone line was limited to
relatively short distances. Amplifiers increased
the signal strength and the useful length of a line but
required additional feedback in the amplifiers (for
this particular application, called repeaters) to
prevent howling noises. Their function was to build
up the line signal and prevent loss of signal along
the telephone line.
A problem in long-distance voice lines is a succession of echoes of diminishing volume that is disturbing to the person speaking. To solve this problem, an echo suppressor was developed. The echo
suppressor takes effect just after speaking starts,
and drops out just after speaking stops. The slight
delay is unnoticeable in a conversation and the echo
problem is elimated. The delay required for the
echo suppressor to drop out is known as "Drop-out
Time" or "Turn-around Time" and must be taken
into consideration when using a Voice Grade line for
data transmission. If a terminal begins to transmit
too soon after receivin~, data bits could be lost.
Devices designed specifically to compress the
volume range of the human voice for economical
transmission and to prevent crosstalk and noise are
called Companders. Compander is a contraction of
the words "compressor" and "expander". Effectively"
soft sounds are amplified to a higher power level,
and loud sounds are amplified to a lesser degree.
The pr.ocedure is reversed by expansion at the receiving end. The common carrier normally removes
companders from circuits used for data transmission.
It is possible for a common carrier to provide
lines of especially high quality at additional cost.
These specially conditioned lines permit data transmission at lower error rates or higher rates of
transmission than would otherwise be possible. One
disadvantage of conditioned lines is that alternate
routes may not be readily available should they be
required. Usually, loading coils are inserted into
the line at proper intervals. These are inductive
loads to compensate for the capacity of the line.
They will, in some cases, cause unwanted distortion
of the data signals. Loading coils are sometimes
combined with resistance and capacity to correct
delay or attenuation characteristics of the line.
When used in this manner, they are referred to as
The signal levels on telephone lines are measured
in a unit called the "decibel", abbreviated db. In
telephone m:> rk, specifically where the line impedances are based on 600 ohms, the db changes to dbm
(decibel milliwatt). The ·reference level for this
measurement is 0 dbm equals 1 milliwatt dissipated
in a load of 600 ohms. The formula for determining
db IS: dbm = 10 Log 10 P-;cl
If we use 0 db as the reference, the reference
power is .001 watts, or 1 milliwatt, and we can
measure relative to other signals either above or
below this level and obtain + or - db readings.
The common carrier personnel talk about power
levels and signal levels on telephone lines in terms
of decibels. Your readings, however, using an IBM
Line Adapter, and injecting a signal on a leased-line
facility, will be in peak-to-peak volts from your
The reference, 0 dbm, is equal to 1 milliwatt in
a load of 600 ohms. This would correspond to a
voltage level of .775 volts RMS or 2.2 volts peak-topeak. Figure 17 provides a graph for rapid conversion of db levels from +10 to -50 db in terms of peakto-peak voltage readings that you obtain from your
In summary, we are interested in attenuation of
a line as measured in a unit called the decibel; we
have explained companders, echo suppressors, compensators, and repeaters. Having covered some of
the concepts of types of channels, grades of channels,
the quality of a particular channel, channelization
methods, and the kind of equipment used on lines,
we know that a telephone line consists of a lot more
than just two pieces of wire strung across the country, and we are ready to look at data sets.
Data sets are mown also as subsets or modems.
The word modem is a contraction of modulator-demodulator. Data sets perform the necessary interface conversion between data processing equipment
and communications facilities. They are used at
each termination of the communications channel at
its interface with the data processing equipment.
The basic function of a data set is modulation and
demodulation for data transmission. Following is
an examination of typical data set specifications:
1. .Equipment: - Transmitter Receiver - It is
not implied that the device is an original input or
output device but that it functions both as a modulator and demodulator. One is required at each ter-minal in a network.
2. Transmission: Binary Serial
3. Input-Output: Binary - Conforms to EIA
RS-232-A. This designation indicates that a binary
code representation, meeting the current recommended standards of the Electronic Industries Association, must be used by the data processing equipment with the appropriate hardware adapter. (For
availability of documentation on RS-232-A, see references at end of session.)
4. Modulation: FM - Frequency modulation is
used. Frequency modulation in telephone work can
be either straight FM (e. g., frequency shifting) or
it can be phase modulation.
5. Bit rate: 150 to 1200 bits per second - The
bit per second range over which the data set can
i=E =f=
- ::r:-:=:==._
. =i •
--5<lJ 0.6
~ 0.5
6. Synchronization: None provided by this type
data set. Synchronization is referred to as the
"synch" or "clocking". The clock is not a simple
mechanical or electromechanical device but an electronic component that provides a constant pulse rate.
The clock is used by the receiver to control the
sampling of received data and keep in step with the
data flow. Some high-speed data sets (above 1200
bits per second) utilize either internal or external
7. Operation: 2-Wire Half-Duplex - Indicates
that the data set and channel can carry information
in both directions but not simultaneously.
8. Unattended Answer: Yes - Indicates that an
inCOming call can be connected without human intervention.
Additional information given may include power
Certain basic lines are necessary to control the
operation of a data set with a data terminal. (See
Figure 18).
* Rev Data
De - Mod
IJ From line
Carr Det
* Snd Data
* Req Snd
To Line
* Clr Snd
Reference Sources
Fr Gnd
= Basic
Lin es
At the transmitting terminal, Request to Send
will activate the data set. When the data set is
ready for the terminal to begin sending, it will return a Clear to Send line to the data terminal equipment. Beginning with the conditioning of the Clear
to Send line, a Send Data line may be activated from
the data terminal equipment to the data set presenting the bits for transmission. This is the minimum
configuration required for transmitting data. For
receiving data, we must have a Received Data line.
In addition to these basic lines, other data sets
may use Serial Clock Transmit, Serial Clock Receive. These clockings may be either internally
generated by the data set or provided from an external source such as the data terminal. A Data Carrier Detect line will indicate to the data set that a
complete circuit, or channel, has been established
with the remote end. You will learn more about the
additional lines when you study particular machines
utilizing data sets requiring them.
The Send Data line to the data set is normally
conditioned by the output of the serializing device in
the data terminal. Similarly, when receiving, the
Received Data line provides the input to the des erializing device of the receiving data terminal. Some
problems that will concern you in this regard are:
1. The data set has accompanying interface
specifications delineating what must be delivered on
the Send Data line, the Request to Send, and simi~arly what the data set will deliver on the Clear to
Send line and the Received Data line.
2. Some data sets are equipped with automatic
gain control (A GC), while others are equipped with
only manual gain control, which requires setting by
the common carrier personnel for the levels of signals encountered on the communications line.
3. It is important that the customer engineer
understands whose responsibility each of these lines
is, the line characteristics, and the permissible
variations from established criteria in the interface
A self-study course, consisting of an audio tape and
work books, is offered on the basic STR Synchronous
Transmitter-Receiver. Your field manager may
consult the Field Education Guide to determine ordering procedures for this course.
A special self-study course is also available for
the UC STR (the Universal Code STR) which is
attachable to a multitude of processors. Your field
manager may refer to the Field Education Guide for
ordering procedures.
For more detailed information regarding the
actual configuration of the equipment used on telephone and telegraph lines, you are referred to
"Principles of Electricity Applied to Telephone and
Telegraph Work" published by the American Telephone and Telegraph Company.
For more information regarding the Electronic
Industries Association Standards, a form on Recommended Standards 232A is available from the engineering department of the Electronic Industries Association. This publication describes the interface between data processing terminal equipment and data
communication equipment.
Many Original Equipment Manufacturers' Information (OEMI) manuals are available within IBM which
pertain to the attachment of IBM equipment to other.
devices. Examples of these are the OEMI manuals
on the IBM 2701, and the IBM 7701, 02, 10, and 1I.
For more information on tele-typewriter fundamentals, there is a booklet called "Tele-typewriter
Fundamentals Handbook" written by William D. Rexroad and available from the Computer Design Publishing Company.
This completes the study portion of Session 4.
You will now find a group of study questions provided
so that you may evaluate whether you have obtained
the principal points from this session. If you do not
answer all of the questions correctly, you should
review this session before proceeding.
1. A path provided for electrical communications
is referred to variously as a
___________ or a _ _ _ _ _ _ _ __
This path can be provided only by wires.
(a) True
(b) False
3. After each of the following channel types,
place the letter corresponding to the correct definition from the right hand listing.
a. Capable of simultaneous
transmission and reception.
Half-Duplex _ _ __ b. Can operate in only one
c. Can send or receive,
but only one or the other
at any given time.
4. The four grades of channels are:
(a) _ _ _ _ _ _ _ _ _ __
11. Carrier systems use a method lmown as
Frequency Division Multiplexing to channelize the
frequency spectrum they occupy.
(a) True
(b) False
12. Both sidebands of a signal must be transmitted to accomplish satisfactory communication.
(a) True
(0) False
13. Define Modulation in your own words as
applied to Data Sets.
14. Why are 4 kc channels provided when voice
signals require only a bandpass of approximately
200-3000 Hz after filtering?
15. Identify the type of Modulation being presented
in each of the following figures:
(b)------------------(c) _ _ _ _ _ _ _ _ _ __
(d) ___________________
5. The code developed for use with printing
telegraphic systems is called
consists of ___ data bits plus a _ _ _ _ _ __
bit and a 1. 42 unit
6. Cross out the undesired terms in the following chart to make each horizontal line of entries
Logical State
If this referred to a telegraphic transmission, the
line would be at fullcurrent condition for a (SPACE/
MARK). (Underline your choice.)
7. In your own words, describe the difference
between Asynchronous and Synchronous signaling.
8. In your own words, explain the difference
between Serial-Bit and Parallel-Bit transmission.
9. What is the name of the new term which has
been adopted to replace the term "cycles per second" ?
10. A voice channel is how wide in cycles (approximately) ?
(a) 1000
(b) 2000
(c) 3000
(d) 4000
(e) 5000
16. The agency of the U. S. Federal Government
which regulates communications is the
17. What characteristic of data transmission is
primarily limited by the bandwidth (in Hz) of the
channel? _____________________
18. What minimum grade of channel would be
required to transmit data at each of the following
bit rates?
(a) 45 bits per second _ _ _ _ _ _ _ __
(b) 135 bits per second - - - - - - - - (c) 1800 bits per second __________
(d) 200 to 600 bits per second - - - - - _
19. Refer to the Line Voltage vs. DBM chart
provided in the text and give the peak-to-peak voltages that would be displayed on an oscilloscope for
a 1 kc signal at each of the following levels on a 600
ohm telephone leased line.
(a) 0 dbm
(b) -6 dbm _ _ __
(c) -8 dbm _ _ _ _ _ _ _ _ _ _ __
20. Why are "Repeaters" used on:
(a) Telegraph lines: ___________
21. The device provided in long distance telephone lines to eliminate the annoyance of hearing
your own voice coming back to you is called an
_ _ _ _ _ _ _ _ _ _ _ _ " This device injects
a problem of Turn-Around time into design of a
communications system. What is meant by TurnAroundtime? ______________________________
How may Turn-Around time affect the throughput of
a system ? ___________________
24. Loading Coils are placed in a telephone line
to compensate for the _ _ _ _ _ _ _ _ _ _ _ __
capacity of the line.
25. When capacitive, resistive, and/or inductive
components are added to a telephone line to improve
its characteristics from the standpoint of attenuation and/or phase distortion, the components thus
added are referred to as ___________
26. The reference of decibels as used in telephone work is _ _ _ _ watts dissipated in a load of
______ ohms.
27. What is the advantage of a "Conditioned"
telephone line over an unconditioned line? _ _ __
22. What two undesirable characteristics of telephone lines are eliminated (or at least improved)
through the use of Companders?
(1) _ _ _ _ _ _ _ _ _ _ _ _ __
(2) _ _ _ _ _ _ _ _ _ _ _ _ __
23. A compander provides (little/great) amplification of weak signals and (little/great) amplification
of strong signals, on the sending end which constitutes (Compression/Expansion) of the signal level on
the line. (Underline your choices.)
28. The Data Set converts the output of a Business Machine (Data Terminal) to a form which will
be accepted by the _ _ _ _ _ _ _ _ _ _ _ _ __
when sending.
29. The two principal functions of a Data Set are
___________ and __________________
You should understand the following concepts after completing of this session:
Message routing and control including line access, formating, record keeping, error
control, priorities and terminal operating modes.
Message Types - Single Address, Multiple, Group, Broadcast.
Message Switching - store and forward.
Semi -automatic
Circuit Switching - immediate line routing.
Selective Calling and Polling - to overcome "Contention" problem.
Addressing is direction to receive.
Polling is direction to send.
Intercept of messages for reroute to active terminal.
Editing of messages for sequence control and record keeping.
Examination of messages for priority, to determine status in "queue. "
Logging of traffic, Journal or record of transmissions.
Error Control, answerbacks.
Terminals in Control Mode and Text Mode.
The requirements for control of a data communication system will be explained with reference to
Figure 19.
The block labeled "Communications Control"
must analyze each message to be sent or received
to determine its proper routing. This information
is usually contained in the heading of -each message.
For messages to be sent, there will normally be a
"destination code" which represents the unique address of some remote data terminal.
The Control Center of a network may handle
several types of message.
1. Single address message - intended for transmission to only one remote terminal.
2. Multiple Address Message - requires transmission to more than one remote terminal.
3. Group Address Message - requires transmission to a predetermined group of remote terminals.
4. Broadcast Message - requires transmission
to all remote terminals.
Messages may be sent as they are readied by
the Control Center or they may be saved until a required number has been accumulated, or until a predetermined interval of time elapses. This is called
"Queueing." Queueing permits more effective utilization of communications channels.
The function of "routing" must be performed in
any data communication system, but Queueing is
used with Message Switching Systems. Other types
of systems include: Circuit Switching, and Selective
Calling and Polling.
Mes'sage Switching is used by the Control Center to
transfer messages from one channel or circuit to
another. In Figure 10, a message from terminal 8
to terminal 2 would require that the Control Center
completely receive the message from terminal 8 and
then retransmit it to terminal 2 on a different channel. This method of switching is also referred to as
"store and forward." In telegraph-oriented systems,
three versions of this are employed:
1. Manual Torn-Tape Switching
2. Semiautomatic Switching
3. Automatic Switching
When two or more terminals are operating on the
same circuit; all terminals will receive all messages whether intended for them or not. This problem can be overcome through "Selective Calling."
This means that only the called terminal will copy
the message. If more than one terminal tries to
send at the same time, a condition called "Contention" arises. That is, multiple terminals are contending for the facilities of the Circuit or Channel.
This problem can be eliminated by "polling" which
is a controlled invitation to send.
Manual Torn -Tape Switching
In a data communications system using this form of
switching, a message is received in punched paper
tape at the Control Center. The Origin and Destination codes are examined and the tape is then transferred by the operator to a tape reader for transmission on the proper outgoing line.
Semi -automatic Switching
In data communications systems using this form of
switching, an operator checks the destination code
preceding each paper tape message and, by pressing
the proper keys on a control unit, causes the message to be relayed to the proper destination as it is
received by the Control Center.
Automatic Switching
In systems utilizing this form of switching, an incoming message is perforated in paper tape and
automatically routed through a tape reader to analyze
the destination address. Circuits are then conditioned to assign the message to the proper outgoing line
as it passes through the reader.
Circuit Switching (also called line switching) does
not require the receipt and retransmission of messages. The Control Center provides a direct connection between the terminals desiring to communicate under the Control of an operator or automatic
switching equipment. One common example of Circuit Switching is the use of the dial telephone. In
this application, dialing the required digits sets
automatic equipment in operation to connect your
line directly with the line of the called party. Compare this with the Message Switching counterpart of
sending a telegram which must be received, printed,
and then delivered to the addressee.
Some systems have automatic mechanical polling
devices which will sequentially permit each terminal
to send. This function can also be performed through
programming by a computer. Each terminal as it is
polled is expected to answer. If the terminal does
not have any message to send, it will answer with a
"negative" response. If the terminal has messages
(also called traffic) to send, it will respond to the
poll by sending its queued messages. Failure of a
terminal to respond to a poll, indicates that it is inoperative or nonexistent. The programmer and the
Computer Control Center may elect to try again and/
or delete the terminal from the polling table or, he
may leave the terminal in the list and, after waiting
a predetermined length of time, go on polling the
rest of the terminals.
In a message control network, each communications
line to the control center is shared by many terminals. Since only one terminal can use a line at a
given time, terminals will contend for the use of the
line, unless some form of supervision is imposed.
The control of the lines and terminals is a function
of the control center.
Polling is the systematic interrogation of terminals in the network by the control center. Each terminal has a unique address, and can be started and
stopped by the control center. A table of terminal
addresses is used by the center as a polling sequence.
When a terminal is polled, a signal is sent back to
the control center indicating the status of the terminal. This response, known as an answerback, indicates that the terminal is ready to send, ready to
receive, or not ready to send or receive. The control center, in response to the answerback, accepts
messages that are ready, sends messages that have
been waiting at the center, and proceeds to poll the
next terminal.
With terminal and line control responsibility, the
control center can restrict or stop polling when
necessary. For example, if during the normal operational day a momentary burst of traffic exceeds the
handling and queueing capacity of the system, the
control center recognizes this overload and restricts
polling to limit the number of incoming messages.
This procedure allows the total systems traffic to be
processed, although some delays will be encountered
during this period because of the system's "flattening" of the traffic peaks.
The concepts to be derived here are:
1. Polling invites a terminal to send.
2. Addressing commands a terminal to receive.
2. Hetention of priority addresses in core storage - In this method, a priority message is placed
in its normal queue position, but the sequence number and address location of the message are retained
in core storage.
3. Transmission prior to logging - This method
is based upon recognizing that a message is priority
type. The message is transmitted to its destination
immediately and logged later.
4. Priority queues - In this method, special
queues are established for priority traffic.
In some cases, messages may be intercepted by the
control center. This means they are prevented from
reaching their desired destination. This may occur
under program control for the following reasons:
1. Message contains errors.
2. Message has incorrect format or heading.
3. Addressed terminal is inoperative. Control
center reroutes to an alternate terminal at a nearby
4. Due to passing from one time zone to another,
the addressed terminal is not yet in operation. The
control center will hold the message queued until the
proper terminal comes on line.
The control center may edit incoming messages
prior to reroute, such as:
1. Add control center sequence number, date
and time to the header.
2. Receive from an IBM terminal device and
change heading for required address to print out
messages on a telegraphic printer.
The control center can examine header information
in messages to determine priority or initial or retransmis sion.
There are several methods for handling priority
1. Flagging - Priority traffic can be flagged
during the header analysis and before the placement
of the message in the queue. On testing the queue
for traffic to be transmitted, the queue can be scanned for priority traffic to be sent before normal
Some systems utilize long and short term "Journals"
for retaining messages handled. Retrieval may
require special identification "Keys" to prove
authorization for access to the Journal.
Long term Journals are usually Magnetic Tape;
intermediate term on disk files; short term in memory, data cell, or drum files. This method helps in
determining where an error entered a message and
in repeating messages.
Error Control
Normally, procedures will be provided for receiving
terminals to answer back to the sending terminal
acknowledging correct or incorrect receipt of a
message. If the answer-back indicates incorrect
receipt, procedures usually exist to provide for retransmis sion.
Terminal Operating Modes
There are normally two modes of operation:
1. Control mode
2. Text mode
Control mode is in operation for polling and addressing while text mode indicates the data handling
portion of the terminal operating procedures. Some
terminal devices also revert to control mode for
acknowledgment sequences between messages. The
division between control and text mode usually
occurs when addressing is ended.
This completes the text for Session 5. Answer
the following study questions. If you get them all
correct, you have conquered the main points of the
section. If you get any questions wrong, you should
review the necessary material before proceding to
the next session.
1. In your own words, define the term "Queueing."
2. In your own words, compare the concepts of
Message Switching and Circuit Switching.
3. If more than one termi.nal tries to send on
the same line at the same time, what term is applied
to this situation?
4. What is the difference in terminal operation
when being Polled from terminal operation when
being Addressed?
5. If a terminal has traffic for the Control
Center, what constitutes a positive answer to a Poll ?
8. If a terminal yields a negative answerback to
a Poll, the Control Center should drop that terminal
from its Polling List.
(a) True
(b) False
9. The Control Center must always follow the
same sequence of polling terminals on a line. SpecifyWHY.
-(a) True
(b) False
10. If the terminal for which a message is intended happens to be inoperative, there is no way to
send the message to its destination. Specify WHY.
(a) True
(b) False
11. In a Message Switching system, the Control
Center can add other information to a message before it is forwarded to the required addressee.
(a) True
(b) False
12. The Control Center of a Message Switching
system must send all messages it has received for
terminals in the same sequence in which they were
received. Specify WHY.
(a) True
(b) False
13. What four methods may be used for processing priority messages in a Message Switching system?
(a) _ _ _ _ _ _ _ _ _ _ _ _ _ _ __
(b)-------------------------(c) _ _ _ _ _ _ _ _ _ _ _ _ _ __
(d) _ _ _ _ _ _ _ _ _ _ _ _ __
6. If the Control Center has Polled a given terminal twice in succession, what should it do?
(a) Continue trying until an answer is received.
(b) Skip that terminal and proceed to process
the remainder of the Polling List. .
(c) Stop and wait for operator intervention.
7. The terminals on a given line must be polled
and/ or addressed sequentrally by their physical locations on the line.
(a) True
(b) False
14. Why is a Journal or Log maintained?
15. Anyone can access all of the messages in the
Journal. Specify WHY.
(a) True
(b) False
16. Describe the difference between Control Mode
and Text Mode.
In this session you will learn in more detail the methods of serializing and deserializing
data for transmission and receiving, respectively.
SERDES is a Contraction of Serialize/Deserialize.
Use of "Storage Elements," "Store Control" blocks.
Line Control "Storage Element" and "Bit Timer."
Register is a group of Storage Elements.
In transmit, characters are converted from Bit Parallel to Bit Serial.
In receive, characters are converted from Bit Serial to Bit Parallel.
Characters are TRANSLATED from BCD to 4 of 8 Code on transmit operation.
Characters are TRANSLATED from 4 of 8 Code to BCD on receive operation.
Shift Register moves character up through it while presenting the character one
bit at a time to the line instead of leaving the character stationary and sampling it
one bit at a time to the line as in SERDES.
"OR Control" accepts either input ON to activate one leg of a Store Control block.
Line over a designated bit means "NOT" or logical zero condition of that bit.
(Example: N)
This section deals with two methods of serializing
and deserializing data, and describes how a data set
or modem responds to data bits.
SERDES is a contraction of "Serialize/Deserialize."
A SERDES device is capable of loading a character
bit-parallel, then dismantling the character and
presenting it one bit at a time to the data set for
transmission. This is Serializing. On the receiving end, the SERDES will accumulate bits, one at a
time, until a complete character is assembled.
This is Deserialize. When a complete character
has been assembled, it can be transferred bit-parallel to other circuitry.
Figures 20 and 21 show the machine circuitry
necessary to perform these functions.
First, examine the notations we are using. The
letter "T" represents a storage element. This element can be a relay, a magnetic core system, an
electronic latch, or an electronic trigger. The
device can be signaled to go into either of its two
states and will stay that way until signaled to change.
Thus, we require a line to turn the device on, and
one to turn it off. The output line from its upper
half is then considered plus (+) if the device is on,
and negative (-) if it is off. The store control block
merely indicates that a plus (+) condition is necessary on both of its inputs (left hand side) in order to
condition its output (right side) plus (+) to perform a
turn on function to a Storage Element (T) or present
a plus (+) level to the data set interface. The blocks
labeled "Delay" serve only to provide a difference in
time between the input of the block going plus and the
output going plus. This enables us to do certain operations at points within a single bit-time period.
The bit times provide a source of sequential pulses
of plus polarity and properly spaced to time the
operation. (Comparable to the distributor of an
automobile ignition system or to a digit selector of
an accounting machine when the digit selector is
wired as a digit emitter.)
Storage Element
+ Off
Bit Parallel
Input BCD
4 of 8
Serial Bits to Modem
Send Data
Figure 29
- Space
(4 of 8 Code)
4 of 8 Char = A
1,2,4,8, R, 0, x, N
Line Over bit Indicates
No-Bit Condition
1-- - -
Bit 8
Bit 1
1 '
End of Character SERDES Reset
Gate New Character Control
Delay -2
Transmit Operation
Sequentially, the operations to be performed are:
(Refer to Figures 20 and 21)
1. Reset storage elements (entire group is
called a Register).
2. Translate BCD character to 4 of 8 code and
place it in the register.
3. Continue running the Bit Timer which will
sequentially sample the storage elements of the register and transfer the information to the Send Data
line of the Data Set or Modem. Note: If the "T" is
ON, at the time the Bit Timer samples the associated
AND, a + or Mark will be delivered. If the "T" is
OFF when sampled, a no-bit condition exists and a
- or Space will be delivered for that bit interval to
the Send Data line.
4. Slightly after the 8th bit (N) of the character
is sampled to the Send Data line, Delay 1 will be
satisfied and the SERD ES register will be reset
5. Later, at the end of Bit 8 time, Delay 2 will
be satisfied and a new character will be translated
and loaded, ready for transmission.
Bit Time
Reset Reg
~I -
Set Reg
r== I -
line Ctl
(T) F -
Send Data
The SERDES Register may also be referred to as a
Transmitting Data Register (D. R.) or an Assembly/
Disassembly Register (A/D Register).
Another device utilized for serializing and deserializing data is the shift register. It accomplishes the
same logical functions as the SERDES.
In explaining Shift Register operational sequences,
Figures 24 and 25 will be referenced.
First, additional conventions of operation must be
established. The Store Control Block requires both
inputs to be plus to produce an ON output; but, if the
upper input leg is negative when the oscillator pulse
is generated, then the OFF output line will be conditioned. The OR control is slightly different. If
either of the input legs is plus, the output will be plUS.
In the Shift Register, since it is designed for
higher transmission rates, we have replaced the
emitter with an oscillator to provide Bit Timing to
the circuit. To control where we are in the processing of a character, the counter circuit provides
the necessary Bit Time identification.
Shift Register - Transmit (See Figures 24 and 25.)
Receive Operation (See Figures 22 and 23.)
All data arriving at the terminal from the Data Set or
Modem appear bit-serial on the Received Data line.
These data are applied to all Store Control Blocks at
the same time. Sequentially, the receiving operation progresses as follows:
1. At the end of Bit Time 8, the entire SERD ES
Register is reset (cleared).
2. As the Bit Timer advances to 1, if the Received Data line is in a Mark State, the "T" 1 will
be turned on.
3. For each additional Bit Time, 2 through 8,
the appropriate store control (+ AND) will be tested
and its associated "T" turned ON, if the Received
Data Line is Marking, or will remain OFF if the
line is spacing.
4. Mter time has been allowed for the 8th bit
(N) to be stored, Delay 1 times out and the entire
character, which has been assembled by deserialization, is gated through the translator and presented
to the machine circuitry as BCD.
5. At the end of the 8th Bit interval, the SERDES
Register is reset and we are ready to receive another
The sequence of operation follows:
1. By Bit Time 1, a new character has been
gated through the translator and loaded bit-parallel
into the shift register.
2. The first pulse from the "pulse maker"
performs the following:
a. N bit moves up to "T" X, because TN was
off and no plus condition was available to the
Store Control block for "T" X. When the
pulse arrives on the lower leg of the Store
Control block, "T" X turns OFF; however,
during this same period, "T" X was ON, and
therefore the store control block of "T" 0 is
conditioned ON when the pulse is made and
."1''' 0 stays ON, but now "T" X contains the
N bit and "T" 0 the X bit of the "A" character in 4 of 8 code.
b. In a similar manner, the 0 bit moves to
the "T" R, the R bit moves to "T" 8, the 8
bit moves to "T" 4, the 4 bit moves to "T" 2.
the 2 bit moves to "T" 1 and the 1 bit enters
the line control T to condition the send data
line to the Modem at Mark level (+).
3. For each succeeding pulse, the bits in the
shift register advance one position until Bit Time 8
when the N bit is delivered to the Line Control "T"
and another new character is loaded for transmission.
Figure 29
(Receiving Letter A = 1,
in 4 of 8 Code)
(Serial Bit Input)
Received Data
/Tom Modem + Mark
(4 of 8 Code) - Space
2, 4, 8,
R, 0,
x, N
- Off
+ Control
Bit 1
Bit 3
Bit 2
Bit 4
Bit 5
Bit 7
Bit 8
End of. Char
Dump Control
-----2--"7 Reset SERDES
(Ready to Receive Again)
Bit 6
Bit Time
.M...-I -
Reset Reg
L f-----
Rec Data
Shift Register - Receive (See Figures 26 and 27.)
The operational sequence for receive follows:
1. At Bit Time 1, the Receive Data line is at
Mark (+). When the pulse is generated, the 1 bit
from the line is set into "T" N and all of the other
Storage Elements are reset OFF since their corresponding Store Control blocks are not conditioned
2. At Bit Time 2, the 2" bit from the line is set
in "T" N and the 1 hit moves up to "T" X.
3. For each succeeding Bit Time, the Receive
Data line condition is entered in "T" N and all other
bits in the shift register are shifted up one position
until at Bit Time 8; the 1 bit arrives in "T" 1 and a
complete character is identified by the Counter.
4. The assembled character in 4 of 8 code is
then gated bit-parallel through the translator to
other machine circuits in BCD in Bit-parallel mode.
5. Accumulation of next character begins.
The specific timings and circuitry of the Shift Register, as employed in Teleprocessing equipment, will
vary somewhat from this conceptual shift register
due to requirements of the particular components
from which it is constructed as well as to the actual
speed at which it must operate.
This completes the text for Session 6. If you
can answer all of the review questions correctly, you
may go on to Session 7. If you answer any questions
incorrectly, review the appropriate sections of the
text before proceeding.
4 of 8 Output
BCD Input
Bit Parallel
Count 7+ Time
Gate Translator
Oscillator (Timing)
n..JU1..J ~
11 121 3 I I 8
(Local or from Modem)
A = 1, 2,
4, 8,
R, 0,
x, N in 4 of 8 Code
l 8
Bit Time
(Caunter Va lue}
r-L rL M- iL rt.- ILM- ~MIL-lL- L L L M- IL-tL- L L
Send Data
Line Ctl
I Mark
Bit Parallel
Received Data
+ Mark, - Space
( 4 of 8 Input
Bit Serial)
Gate Translator Out
Synch (Local or from Modem)
1/2 Bit Out of Step with
Transmit Osc.
Letter A = I, 2, 4, 8, R,
0, X, N in 4 of 8 Code
Rec Data
1. What does SERD ES stand for? _ _ _ _ __
2. The primary function of SERDES when transmitting is to convert characters from _ _ _ _ __
bit to
bit format.
3. The primary function of SERDES when receiving is to convert characters from _ _- - - - - _
bit to
bit format.
4. If a Store Control block has only one of its
input legs ON, the output will be (ON/OFF). (Underline choice.)
5. If a Storage Element is turned ON when a
character is loaded for transmission, at what Bit
Time will it be turned OFF ? _ _ _ _ _ _ _ _ __
6. Briefly describe why the Line Control Storage
Element is necessary. _ _ _ _ _ _ _ _ _ _ _ __
7. The Translator used in the example in the
text converts
code to. _ _ _ _ _ __
code on transmit operation.
8. Why is Delay 1 provided in transmit? _ __
13. How does the Shift Register know it has transmitted a whole character and that it is time to load
a new one? _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ __
14. In which Storage Element is the "X" bit of
the character "A" (In 4 of 8 code) at Bit Time 3 on
Transmit? _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ ___
15. At Bit Time 3, the Send Data line will be at
(Mark/Space) and represents the _ _ _ _ _ _ __
bit of the "A" character in transmit operation.
16. At Bit Time 3, in receive operation, the "1"
bit of the "A" character is in the _ _ _ _ _ _ __
Storage Element and the Received Data line is at
(Mark/Space) representing the
bit of
the "A" character.
17. How does the Shift Register recognize that a
complete character has been assembled and is ready
for Bit Parallel transfer through the Translator to
the rest of the machine? _ _ _ _ _ _ _ _ _ _ __
18. Briefly describe the difference in the way
the SERDES and the Shift Register accomplish Serialization on Transmit.
9. Why is Delay 2 provided in transmit? _ __
~9. Briefly describe the difference in the way
the SERDES and the Shift Register accomplish Deserialization on receive.
10. The line is a (Mark/Space) at Bit Time 7.
(Underline choice. )
11. Shortly after the beginning of Bit Time _ _ ,
the completely assembled (deserialized) character is
transferred in
bit form through
to the remainder of the machine circuits on receiving operation.
12. If both input legs are ON or only one input leg
is ON, of an OR Control block, the output will be
(ON/OFF). (Underline your choice.)
This session will provide you with an understanding of IBM's Modem equipment (one
type) and how it can provide the interface function between a Serialize/Deserialize device
and the communications line for transmit and receive. The student should also become
aware of some of the equivalent components (R, L, C) of a transmission line and their
effect on the transmission of data via the line.
IBM Line Adapter, IBM Modem are similar terms.
Line Adapter converts serial data bits from square wave to two different frequencies
representing Mark and Space which can be handled by the communications channel.
Line consists of distributed Inductance, Capacitance and Resistance.
Line components attenuate the two frequencies used by different amounts. Causes
amplitude loss of Space frequency.
A Delay occurs in the propagation of the signal from the sending modem to the receiving modem because of line characteristics.
Line Distortion of the sine wave signals of the Mark and Space frequencies may cause
extraneous bit pick-up and resultant errors at the terminal.
In previous sessions, we discussed the required
function of a Data Set or Modem. We have established the minimum control lines for interfacing the
Data Set with the Data Terminal and also those for
the connection of the Data Set to the Communications
facility. The IBM line adapter (Modem) is a device
performing the functions of a Data Set but it is supplied by IBM instead of the Common Carrier.
In the preceding section, the operation of
SERDES and Shift Registers has been explained.
Now, examine how we can hook up a complete data
communication link between two terminals. (See
Figure 28.) The Data Terminal at A develops data
for transmission to Terminal B. The SERDES of
Terminal A will deliver serialized data to Modem 1
for conversion to a signal capable of being transmitted over the line facility (C). The form in this
case is two different Audio frequency sine wave signals. The low frequency signal represents a Mark
and the higher frequency signal represents a Space.
In Figure 29 Compare Line "Send Data" with line AA
and note that AA is a sample of the output of the
Sending Modem as it would appear on an Oscilloscope. Note also that, at this point, we have an input signal to the Modem which is of plus polarity to
represent a Space (and cause emission of the higher
frequency signal by the Modem) and a negative polarity signal to represent a Mark (and cause the Modem
to emit its lower frequency signal). These are the
actual signal levels used in the IBM 1050.
The communication facility (telephone line or
channel) has been presented in "lumped-constant"
form. A telephone line comprises DC Resistance,
as well as Inductive and Capacitive reactances.
These characteristics cause a condition called
Line Facility
CD = Send Data from SERDES to MODEM
(]) =
Recd Data from MODEM to SERDES
.on in which all signal frequencies do not
vith the same efficiency on the line. In our
the higher frequency signal is attenuated;
~, a lower amplitude of signal for the higher
frelJ.uency than that developed for the low when observed with an oscilloscope at point BB and shown
in line BB of the Chart. Note also the time delay
involved to propagate the signal from Terminal A to
Terminal B as shown by the arrows X. The amount
of delay and the attenuation of the signal are functions of the telephone line. The Received Data line
represents the data signal which will be delivered by
Modem 2 to Data Terminal B.
In line AA of Figure 29, note section C-C' and
examine this in detail in Figure 30. The wave shapes
at A, B, and C are oscilloscope representations of
the signal on line during a Mark period. (A Space
interval could have been used also). The line A
shows an Ideal wave shape but B is still acceptable.
If a signal such as pictured in line C is received,
errors will result. See 1 of line Band C in Figure
30. In line B, the signal levels off and then continues to rise - no actual change of polarity (direction)
but in line C at point 1, the signal actually changes
direction. This change of direction is due to distortion usually created by the line facility or due to
improper Termination of the line in the Modem. It
causes extraneous bits to be detected. The foregoing will briefly introduce some of the new problems
you will have to work with and analyze in maintaining
Teleprocessing equipment.
This completes the text for Session 7. If you
can answer the following review questions correctly,
you may go on to Session 8. If you answer any
Detail "C - C"
questions incorrectly, review the appropriate sections of the text before proceeding.
Received Data - Mark
Send Data - Mark
Equivalent af Telephane line
Bit Periads
Send Data
~----. .------~+EO
Rec Dala
1. When used with customer owned or leased
lines, the IBM Line Adapter performs the same
function as a common carrier _ _ _ _ _ _ _ __
2. The bits and no bits of the data stream produced by the SERDES on transmit are presented to
the communications line as two different _ _ _ __
4. The loss in amplitude of the Space frequency
as compared to the Mark frequency is partly due to
_ _ _ _ _ _ _ _ distortion of the line.
5. If the signal shown in Figure 30, line C is
presented to the receiving modem (line adapter), __
_ _ _ _ _ _ _ _ _ _ _ _will probably result.
6. As delivered to the SERDES on receive, the
signals on the Received Data line are _ _ _ _ _ __
polarity for Space and
polarity for Mark.
3. The low frequency of the Modem corresponds
to a
condition of the signal from the
This session will familiarize you with the modes of Data Set control as far as input and
output signals between the data set and the data terminal are concerned.
Bi-Polar-signals relative to Signal Ground.
Uni-Polar-Single Polarity relative to Signal Ground.
Current-Change in current instead of voltage.
Basically, there are three methods of presenting
data to, and receiving data from a Modem:
1. Bi -polar mode
2. Uni-polar mode
3. Current mode
Bi - Polar Mode
Bi-polar mode indicates that the data lines between
the Terminal and the Modem swing plus and minus
symmetrically with respect to a zero reference.
Example: ±E relative to 0 Volts for IBM Modem.
Uni -Polar Mode
This mode indicates that the data line swings from
zero reference to a single voltage level (either plus
or minus polarity) to represent Marks and Spaces.
Example: 0 Volts (signal ground) could be a Space
while +6 Volts could represent a Mark.
Current Mode
ill current mode data delivery, the presence of
Marks and Spaces is indicated by a change in current
level exchanged between the data terminal and the
Modem. Example: (1) 0 current equals Space, 62
Milliamperes equals a Mark, (2) 20 Milliamperes
equals a Space, 5 :Milliamperes equals a Mark.
Various types of Data Sets will use different
versions of these basic modes.
This completes your text material on Implementation which has included SERDES, Shift Registers,
IBM Line Adapter and Data Control of Modems.
This material represents the practical applications
of the concepts developed in the preceding sections
and is intended to familiarize you with some of the
different concepts you will be working with in maintaining Teleprocessing equipment.
Teleprocessing is a field of unlimited development and capable of presenting an unending challenge
to the Customer Engineer who wants to stay abreast
of technological developments of space age communication.
This completes the text portion of Session 8.
Answer the review questions and proceed to Session
9. The following review questions are provided so
that you may determine for yourself whether or not
you have learned the principles presented in this
section. If you give any wrong answers, you should
review the necessary sub-topics.
1. A signal input or output which swings positive and negative around a reference is considered
mode of operation.
2. A signal which swings in only one direction
relative to reference is considered a _ _ _ _ __
_ _ mode operation.
3. If the signal condition representing a Mark
is denoted as 62.5 milliamperes of current and a
Space is 0 milliamperes, then
is indicated.
This session is provided to familiarize you with some of the available facilities for
communications, by whom they are provided, and the agencies that control these
Common Carriers defined.
The Federal Communications Commission controls domestic interstate and international communications.
State Utilities Commissions control intrastate communications.
International Telecommunications Union, of which CCITT and CCIR are portions,
provides international efforts aimed at standardization and coordination.
Common Carriers and their services:
Bell System
General Telephone and Electronics
Independent Telephone Companies
Western Union
American Cable and Radio Corporation.
RCA Communications, Inc.,
Privately Owned Communications Systems
Communications common carriers are companies
which furnish communications services to the public.
They are regulated by the Federal Communications
Commission or appropriate state agencies. Their
services include communications facilities for VOice,
data, facsimile and printed messages, as well as
appropriate communications channels for television,
telemetry and telephoto. In the United States approximately 2, 800 companies are recognized as communications common carriers. These companies
can provide complete communications services to
the subscriber including channels, modulating equipment, and the necessary terminating arrangements.
Alternatively, the customer may lease only the
channels and provide purchased or leased terminals
equipment for data communications.
The services mentioned in this section are presented as a survey of some of the communications
facilities available today. The representatives of
the communications companies should be consulted
in order to obtain current information on rates,
availability of service, billing arrangements, channels and related devices pertinent to data transmission.
The Federal Communications Commission, an independent federal agency, regulates interstate and
internatiorial communications originating in the
United States, such as radio, telephone, telegraph,
facsimile, telephoto and other broadcast program
transmissions by wire, cable or radiO.
The Communications Act, establishing the Federal Communications Commission, requires that
every subject common carrier furnish service at
reasonable charges upon reasonable request. No
carrier may construct, acquire or operate interstate or foreign facilities originating in the United
States without the approval of the commission.
Under the provisions of the Communications Act
every common carrier must file with the commission schedules showing all charges, practices,
classifications and regulations for interstate communications services offered to the public. These
schedules, lmown as tariffs, are normally filed at
least 30 days before their terms become effective.
The tariffs form a significant part of the machinery
by which the commission enforces the duties and
prohibitions imposed on the common carriers for
the benefit of the public. They are also the basic
contract between the common carrier and the user.
Telephone and telegraph companies are not required
to file tariffs of their own if they concur in the tariffs
filed by other common carriers. Due to local conditions, however, tariffs may vary from company to
One example of a tariff with a high degree of
concurrence among a large number of telephone
companies is tariff #245 filed with the Federal Communications Commission by the American Telephone
and Telegraph Company. This tariff covers the V-H
Measuring Plan. It establishes the basis for determining the cost of a telephone call by providing a
uniform means of calculating the distance between
the calling station and the receiving station. In the
V -H Measuring Plan, each area in the United States
and Canada is assigned a mathematical coordinate
on a vertical. (V) and a horizontal (H) basis. This
divides the entire area into a series of small
squares, each having an equivalent of latitude and
longitude. The distance for billing purposes in airline miles between any two points can then be calculated from these coordinates.
Intrastate common carrier communications service
is subject to regulation by state commissions similar in function to the Federal Communications
The International Telecommunications Union is an
administrative international organization responsible
for the allocation, registration and utilization of the
radio frequency spectrum. The main objective of
the union is to maintain and develop international
cooperation for the improvement and rational use of
communications. The union undertakes studies and
issues recommendations for the benefit of all members, as well as participating in the Technical
Assistance Programs of the United Nations. Within
the ITU, the Consultive Committee on International
Telephone and Telegraph (CCITT) and the Consultive
Committee on International Radio (CCIR) are actively
engaged in the standardization and coordination of
international worldwide communications facilities.
The parent company of the Bell System is the American Telephone and Telegraph Company, incorporated in 1885, which owns all or part of the stock of
23 associated operating telephone companies and the
Western Electric Company. Together with Western
Electric, AT&T owns the Bell Telephone Laboratories. Western Electric manufactures, purchases,
distributes and installs equipment to fit the needs of
the Bell System. The Bell Telephone Laboratories
carryon centralized research for the Bell System.
The people of the Bell Telephone System design,
build, operate and maintain facilities for approximately 67 million telephones in 48 of the states and
the District of Columbia. Bell System lines connect
with telephone systems in all 50 states of this country and throughout the world.
The Bell System provides a wide range of products and services to fit specific communications
needs, from the basic telephone instrument to DATAPHONE* Service which enables business machines
to communicate via the telephone network in the
same way people do. The technological contributions of the Bell System Include the invention of the
transistor and the solar battery, as well as the designing and building of the Telstar satellites.
Touch-Tone calling which will replace the rotary
dial with push buttons, and completely electronic
telephone call switching are being developed to improve the service of the Bell System to its customers.
Some of the communications services of the Bell
System are Wide Area Telephone Service (WA TS),
Telpak, Teletypewriter Exchange Service (TWX) and
WA TS. -- Wide Area Telephone Service is arranged
for subscribers who make many outgoing long distance calls to many points. Monthly charges are
based on the size of the area in which the calls are
placed, not on the number or length of calls. Under
the WATS arrangement, the United States is divided
into six zones. The subscriber is billed a flat rate
according to the zones to be called on a full-time or
measured-time basis. This can be an advantageous
arrangement for data transmission.
TELPAK. -- Telpak service is a pricing arrangement that makes available wide band communications
channels of various sizes suitable for large-volume
point-to-point transmission of data, voice, teletypewriter, facsimile, or other services for a flat-rate
regardless of usage. Telpak can be used as a single
large channel or a group of smaller channels. For
*Trademark of American Telephone and Telegraph Company
example, a Telpak "A" channel may be divided into
12 voice grade channels.
Teletypewriter Exchange Service. -- Teletypewriter
Exchange Service (TWX) provides direct dial pointto-point connections using input/output equipment
such as page printers, keyboards, paper tape readers
and paper tape punches.
Western Electric equipment will be used in most instances where GT&E does not manufacture equivalent equipment. GT&E equipment will be compatible
with the equivalent Bell System equipment, where
required, to allow direct interconnection of services.
Special arrangements will be available when required
to meet individual requirements.
DATA-PHONE Service. -- DATA-PHONE Service
provides for the transmission of data between a variety of business machines, using regular local or
long distance telephone networks, or WATS lines.
The cost to the customer is the same as an ordinary
telephone call in addition to a monthly rate for the
DATA-PHONE data set.
Other Services. -- In addition to the familiar dial
telephone services, voice grade and telegraph grade
lines can be leased for the exclusive use of the subscriber.
General Telephone and Electronics Corporation is a
highly diversified communications and manufacturing
enterprise whose operations throughout the United
States and abroad are known as the "General System". GT&E provides communications services
ranging from telephone service for the home and
office to highly complex voice and data systems for
industry and national defense. The company has
approximately 5-1/2 million telephones in more
than 30 domestic telephone operation subsidiaries
and three international subsidiaries located in
British Columbia, the Dominican Republic and Haiti.
Automatic Electric Company, a subsidiary of
GT&E, produces communications equipment for the
independent telephone industry. Lenkurt Electric
Company, Incorporated, another subsidiary, manufacturers data sets, microwave radio, and carrier
multiplexing equipment for commercial and military
The General System has had broad and extensive
experience in the data communications field. Over
the years, it has provided a wide variety of services
making use of channels ranging from narrow band
telegraph up to four-megacycle video channels. Its
services include teletypewriter services in its operating territories and direct dial TWX operations.
It is the General System's intention to offer,
under tariffs, all data services of the common carriers including wide band Telpak channels. The terminal and hardware equipment utilized in providing
data communications will be developed and manufactured by GT&E subsidiaries where practicable.
The General System and balance of the 2,800 telephone companies operating 13 million telephones in
the United States provide services that interconnect
with the Bell System and each other. Over 150 independent telephone companies have more than
10,000 subscribers, and another 100 have more than
5,000 subscribers.
The United States Independent Telephone Association (USITA), with headquarters in Washington,
D. C., represents many of the independent telephone
companies. It provides guidance to its members and
coordinates their practices through committees.
"Telephony Magazine", published weekly by the
Telephony Publishing Corporation of Chicago, and
"Telephone Engineer and Management", published
semi-monthly by the Telephone Engineering Publishing Corporation of Chicago, are important vehicles
for distributing information about the telephone industry. Both companies publish annual directories
indicating the corporate structure and pertinent
statistics about telephone companies in the United
The Western Union Telegraph Company, incorporated in 1851, has played a vital role in the development of communications in the United States.
The company furnishes communications services
by wire and microwave radio throughout the United
States and by ocean cable between the United States
and foreign countries. It provides the only national
telegraph message service. It also furnishes custombuilt private wire systems and facsimile systems on
a leased basis and Telex, a direct-dial teleprinter
Western Union leases more than 2,000 private
wire systems of varying sizes and speeds for industry and government. It recently placed in service
for the Department of Defense the world's largest
and most advanced digitar data network with a capacity of 7,000,000 punched cards, or the equivalent
of 100,000,000 words daily.
Western Union recently expanded its offerings
of leased wire facilities to include a full range of
voice, alternate voice-record, Telpak and voicedata services. It also leases circuits and equipment
to speed communications by facsimile.
Scheduled for operation by Western Union in 1964
is a new coast-to-coast microwave system which will
be capable of accommodating all modern forms of
communication at high speeds and in large volume ..
It will be used to transmit vOice, facsimile, and
high-speed data as well as public message and private wire services.
American Cable & Radio Corporation (AC&R), a
subsidiary of International Telephone and Telegraph,
owns and operates an international telegraph system
with both cable and radio facilities. AC&R was
formed when the operations of four IT&T affiliates,
All America Cables and Radio, The Commerical
Cable Company, Mackay Radio and Telegraph Company, and Sociedad Anonima Radio Argentina
(Buenos Aires) were consolidated. The company
later acquired control of Globe Wireless, Incorporated.
AC&R, in addition to its operating centers located in New York, Washington, and San Francisco,
maintains and staffs more than 110 overseas traffic
offices, as well as seven marine radio stations. In
the three "gateway" cities of New York, Washington,
and San Francisco, the public deals with AC&R directly. Customers outside these cities use AC&R
international facilities through the intermediate telegraph or teleprinter services of the domestic
carriers, Western Union and the TWX Bell System.
This applies to telegraph and Telex service, as well
as to leased channel operation.
AC&R's communications network now includes
more than 500 cable and radio channels, extending
across the Atlantic to Europe, down through the
Caribbean, Central and South American areas, and
out to the Far East. Through the cable network, its
own radio facilities, and by connection with foreign
cable and telegraph companies, the IT&T/AC&R
Communications System not only connects the United
States with practically all countries of the world but
also furnishes communications between countries
outside the United States.
This worldwide network provides customers
with regular message service, press-cast service
radiotelephone service, Telex, Datatelex and leased
channel service.
AC&R technical activity in the communications
field is demonstrated by its installation of the first
automatic Telex switching centers in the United
States. AC&R Telex customers may call direct to
suhscribers in London or Paris, for example, without any operator intervention whatsoever.
The Radio Corporation of America was created in
1919 to provide international communications facilities. Ten years later RCA organized a wholly owned
subsidiary company, RCA Communications, Inc.,
to concentrate on the further development of international radio communications. In recent years RCA
has augmented its global radio facilities with coaxial
cable channels to Europe, Puerto Rico, Hawaii,
Bermuda, and Jamaica. Since 1962 the company has
significantly added to its cable facilities across the
Atlantic and has acquired extensive facilities throughout the Pacific. RCA also plans to expand its cable
facilities to South and Central America in the very
near future.
Today RCA's global communications network
comprises almost 1000 radio and coaxial cable channels providing telegraph service to the entire world,
Telex service to 105 countries, and Radiophoto
service to 53 foreign terminals. In addition, RCA
operates the terminals of 14 radiotelephone circuits
in the Pacific area and provides two-way Program
Transmission Service for broadcasters to almost
any point on the globe. The company also maintains
facilities for communication with ocean-going vessels and ships plying inland waterways.
RCA Communications furnishes private leasedchannel communication services to all parts of the
world for commercial and governmental customers.
More than 210 such channels for teletypewriter,
telephone, facsimile, and data communications are
in use. New facilities are being designed to meet
the needs of proposed transoceanic data processing
systems. In New York a wholly automated telegraph
terminal will electronically route, process, and
transmit telegrams. RCA now provides automatic
Telex service between the United States and Belgium,
Japan, the Netherlands, Switzerland, United Kingdom,
and West Germany. Subscribers to the RCA Telex
network simply dial the number of their overseas
correspondent after dialing an area code as in direct
distance dialing in the United States. Tll Telex,
however, the calls are on a teleprinter. These international connections are now established, subscriber to subscriber, within seconds. Datatelex
service provides for data communications between
the United States and Great Britain on a call-up
basis. RCA has recently extended Datatelex service
to Hawaii ~nd plans to make it available to other
overseas points when the demand arises.
The federal government and some industries purchase, maintain and operate some of their communications facilities. Examples of these private systems are telegraph and telephone systems owned by
railroads and pipeline companies operating in remote areas. Licenses can be obtained to operate
private radio and microwave systems under certain
circum stance s.
1. Define Communications Common Carrier.
6. What do the following abbreviations stand for?
(a) WATS _ _ _ _ _ _ _ _ _ _ _ __
(b) TWX _ _ _ _ _ _ _ _ _ _ _ _ _ __
7. What kind of service is Telex?
2. Schedules showing charges, practices, regulations and other particulars for interstate communications services must be filed with and approved
bythe ___________________________________
3. If the communications service is limited
specifically to intrastate service, the regulating
agency becomes the _______________
4. The schedules referred to in question 2 are
known as _ _ _ _ _ _ _ _ _ _ ____
8. How many voice channels can be multiplexed
on one Telpak "A" service? _______________
9. Photolex provides a means for ____________
transmis sion.
This completes the text of Introduction to Teleprocessing. Contact your course Administrator for
the final examination. Your grade on the examination
will be your grade for the course. No reference
material is to be used and you will have 1. 0 hour to
complete the examination.
5. The CCITT and the CCIR work to provide
_______________ and __________________
of world wide communications systems.
AC/DC Ringing - A method of telephone ringing
which utilizes current having both alternating and direct current components: ac to
operate a ringer; dc to aid the action of a
relay which stops the ringing when the called
party answers.
ACK - Aclmowledge.
ACU (Automatic Calling Unit) - Unit capable of establishing a circuit connection upon input of
a digital calling sequence from an attached
Address - In communication usage, the coded representation of the destination of a message.
Addressee - The intended recipient of a message.
Addressing - This is the means whereby the multiplexor or control station selects the unit to
which it is going to send a message.
ADU - Automatic Dialing Unit.
Aerial Cable - A cable connected to a pole or similar overhead structure.
Aggregate - The transmitted carrier signal which
consists of the 12 single-sidebands being
sent over the transmission circuit.
Alphanumerics - Characters which may be either
letters of the alphabet or numbers.
Ambient Noise - Acoustic noise existing in a room
or other location.
Amplifier - A device which receives energy at a low
level and sends it out at a high level in identical or nearly identical form. See Repeater.
Amplitude - The size or magnitude of a voltage or
current wave form.
Amplitude Distortion - See Distortion Amplitude.
Amplitude Modulation (AM) - Variation of a carrier
frequency's amplitude in accordance with an
information signal.
Analog - Information varying in amplitude, frequency or both as opposed to information expressed by discrete levels.
Annunciator - A visual signaling device, operated by
relays, which indicates conditions of associated circuits.
Answerback - The response of a data set or terminal
to remote control Signals.
Answer Lamp - Telephone switchboard lamp which
lights when an answer cord is plugged into a
line jack, goes out when the called telephone
answers, and lights when the call is completed.
Area, Exchanges - Areas set up for administrative
reasons for telephone service covered by a
single rate basis. Usually a single city or
large division of town or village.
Area, Operating - Separation of most of the telephone company's work into a large geographical unit. An area functions as a complete
telephone company up to the executive level.
ARQ - An automatic system which provides error
correction by utilizing a constant ratio code
and a closed loop to request retransmission
of mutilated characters as indicated by receipt of nonconstant ratio characters.
Artificial Load - See Dummy Load.
ASA Code - A standardized code set recommended
by the American Standards Association for
industry-wide use in transmission of information.
ASCII - American Standard Code for Information Interchange, usually pronounced "Askee". An
8-level code for data transfer proposed to
achieve compatibility between data devices.
ASR - Automatic Send/Receive - A teleprinter unit
with keyboard, printer, paper tape reader
and paper tape punch, which allows tape to be
produced and edited off line for automatic
A-Switchboard - A switchboard in a local central
office, arranged primarily for receiving
local orders for telephone connections and
for completing these orders, either at the
same switchboard or by way of trunks to
other switching equipment.
Asynchronous - (Start/Stop) Transmission - Transmission in which each information character
is individually synchronized by the use of
start and stop pulses.
Attended Operation - In data set applications, individuals are required at both stations to establish the call and transfer the data sets from
talk mode to data mode. As opposed to Unattended Operation.
Attenuation - Decrease in magnitude of current, voltage, or power of a signal in transmission
between points. May be expressed in decibels
or nepers.
Attenuation Equalizer - See Equalizer.
Attenuator - See Pad.
Audio - Frequencies that can be heard by the human
ear (usually 50 cycles to 16,000 cycles per
second) •
Autodin - (Automatic Digital Network) - The datahandling portion of the military communications system. (See COMLOGNET).
Babble - The aggregate cross talk from a large
number of interfering channels.
Balanced-Balanced to Ground - When applied to a
cable pair or two wire line it means that the
impedance to ground as measured from one
side of the line is equal to the impedance to
ground as measured from the other side of
the line.
Band - A range of frequency between two defined
Bandwidth-Band Pass - The difference, expressed
in the number of cycles per second, between
the two limiting frequencies of a band.
Bank - An aggregation of similar devices (e. g. ,
transformers, lamps, etc.) connected together and used in co-operation. In automatic switching, a bank is 'an assemblage of
fixed contacts over which to establish electric connections.
Baseband - The frequency band which defines an information signal, e. g., the baseband signal
is the information vehicle which modulates a
carrier wave.
Base Frequency - In any wave, that frequency which
is considered to be the most important. In
a driven system it would in general be the
driving frequency while in most periodic
waves it would correspond to the fundamental
Base Group - Term used to designate a number of
carrier channels combined to form a channel
bank which will be further modulated to a
final frequency band.
Baud - A unit of signaling speed used in data transmission. It may contain several binary decisions but most commonly is equal to one
bit per second (bps).
Baudot Code - The conventional5-level teletypewriter
code consisting of a start impulse and five
character impulses, all of equal length, and
a stop impulse whose length is 1. 42 times
that of the start impulse. Also !mown. as the
7.42 unit code.
Bel - See Decibels.
Bellfast - A teletypewriter switching system leased
from AT&T.
Bell Idles - Special Code sent during periqds of nonactivity between data sets (10001000).
Bias Distortion - See Distortion, Bias.
Bid - An attempt to occupy a line in order to pass
Bipolar - See Polar Circuit.
Bit - Contraction of binary digit, the smallest unit
of information. It has two possible states:
1 or O.
Bit Rate - The speed at which bits are transmitted,
usually expressed in bits per second (bauds).
Bit Stream - Referring to a binary signal without
regard to groupings by character.
Black Box - A generic term used to describe an unspecified device which performs a special
function or in which !mown inputs produce
!mown outputs in a fixed relationship.
Blank - 1. A machine character to denote the presence of no information rather than the absence of information. It is composed of all
spacing pulses between the start and stop
2. In paper tape, it references a feed hole
without intelligence holes.
Blank Coil - Tape (for perforation) with only the feed
holes punched.
Blank Deleter - Device which eliminates the receiving of blanks in perforated paper tape.
Blind - (Unblind) - The ability of a device to receive
selective information of a transmission and
be "blinded" or non-receptive to unwanted
data such as item description, price, polling
and addressing characters, etc.
Book Message - A message to be sent to two or more
terminals and is of such a content that the
originator feels that no destination needs to
be informed of any other destination.
BPS - Bits per second.
Break - To break, in a communication circuit, is
when the receiving operator or listening subscriber interrupts the sending operator or
talking subscriber and takes control of the
Bridge Tap - An unterminated length of line attached
somewhere along the system. Bridge taps
are undesirable.
Broadband - High speed communication channel ha ving a bandwidth greater than voice grade
Broadcast - Simultaneous transmission and reception of a message to all receiving locations
on a communications service.
Buffer - A system component which permits a change
of speed, voltage, or interface characteristic; frequently presumed to include storage
Bug - A semi-automatic telegraph sending key in
which movement of a lever to one side produces a series of correctly spaced dots and
movement to the other side produces a single
Buried Cable - A cable installed under the surface of
the ground in such a manner that it cannot be
removed without dif:lturbing the soil.
Bus - A conductor, or group of conductors, in a
switch gear assembly which serves as a
common connection for two or more circuits.
Bussback - Term denoting the common carrier's
action of tying the output portion of a circuit
back into the input portion of a circuit.
Bustback - A Western Union term. Similar in nature
to Bussback.
Busy Test - In telephony, a test made to find out
whether certain facilities which may be desired, such as a subscriber line or trunk,
are available for use.
Cable - Assembly of one or more conductors within
an enveloping protective sheath, so constructed as to permit the use of conductors
separately or in groups.
Cable Branch - Cable which leaves a main cable to
reach some secondary point.
Cable Coaxial - A cable consisting of one conductor,
usually a small copper tube or wire, within
and insulated from another conductor of
larger diameter, usually copper tubing or
copper braid.
Cable Distribution - Branch off a feeder cable.
Cable Feeder - Principal cable from a central
Calling, Selective - The ability of transmitting
station to direct a call to 'one or more specifically designated stations.
Camp-On - A method of holding a call for a line that
is in use and of signaling when it becomes
free. (Also called Clamp-On).
CARR - Carrier.
Carrier - A high-frequency current that can be
modulated by voice or signaling impulses.
Carrier, Communications Common - A company
which furnishes communications services to
the general public, and which is regulated by
appropriate state or federal agencies.
Carrier System - A means of conveying a number of
channels over a single path by modulating
each channel on a different carrier frequency
and demodulating at the receiving point to
restore the signals to their original form.
Some typical Carrier Systems are:
10 FD
3 FD
J (Telephone)
12 FD
o (Telephone)
16 FD
4 Wire,
4 Wire,
4 Wire,
2 Wire,
12 FD
N (Telephone
or Program)
12 FD
2-19 GA Cable
Pairs (NonLoaded)
Toll or Exchange Cables
ON (Telephone)
20-24 FD
12-16 FD
TD2(Micro600 FD
L1(Tele600 FD
Paired Coax
phone or
L3(Tele1860 FD
Paired Coax
phone or
CAX (Community Automatic Exchange) - A small
dial office serving a community.
CDC - Call directing code - character directing
code - An identifying call, usually two letters,
which is transmitted to an outlying telegraph
receiver and automatically turns its printer
on (selective calling).
Central Office - A switching center of the telephone
company where all telephone lines terminate
for a certain exchange area. See Exchange,
Central Office.
Centrex - Central office type telephone equipment
serving subscribers at one location on a
PABX basis. The system allows direct inward dialing, direct distance dialing, console
switchboards, etc.
CHAD - That piece of paper removed in punching a
hole in perforated tape.
Chadless Tape - Perforated tape with the Chad partially attached.
Chaining - A system of storing records in which
each record belongs to a list or group of records and has a linking field for tracing the
Channel - A path for electrical transmission between
two or more stations or channel terminations
in telephone or telegraph company offices,
furnished by wire, radio or a combination of
both; also called circuit.
Channel,. Analog - A channel on which the information transmitted can take any value between
the limits defined by the channel. Voice
channels are analog channels.
Channel, Four Wire - A two-way circuit using two
paths so arranged that the communication
currents are transmitted in one direction only
on one path and in the other direction on the
other path. The transmission path mayor
may not employ four wires. (See Channel,
Full Duplex, Simultaneous Transmission,
and Equivalent 4-Wire System.)
Channel, Full Duplex - A circuit capable of transmitting and receiving in both directions at
the same time. Usually req:l.ires 2 subscriber loops. (See Channel, Four Wire,
Simultaneous Transmission, Non-Simultaneous Transmission.)
Channel, Half Duplex - A circuit capable of transmitting and receiving signals but in only one
direction at a time. Usually requires 1 subscriber loop. (See Channel, Two Wire.)
Channel, Simplex - A circuit capable of transmitting
or receiving in one direction only. The circuit is not capable of being reversed.
Channel, Two Wire - A two-way circuit for transmission in either direction, but not simultaneously. (See Channel, Half Duplex.)
Channel, Voice Grade - A channel that permits
transmission of speech. Usually will pass a
frequency range of approximately 300 to
3000 cps. In data applications, phase distortion limits the data speeds to 1200 bps or
less. Suitably conditioned lines, with phase
equalization, may handle 1600 to 2400 bits
per second.
Channelizing - The process of dividing one circuit
into several channels.
CHAR - Character.
Character - T~e actual or coded representation of a
digit, letter or special symbol.
Character Density - A measure of the number of
characters recorded per unit of length or
Character Emitter - An electromechanical device
which emits a timed pulse or group of pulses
in some code.
Character Printer - A printer in which only a single
character is composed and determined within
the device prior to printing.
Character Reader - An input device which reads
printed characters directly from a document.
Character Recognition - The act of reading, identifying and encoding a printed character by
optical or other means.
Character Set - A list of characters acceptable for
coding to a specific computer or input - output device.
Characteristic Distortion - See Distortion, Characteristic.
Characteristic Impedance (Zo)·- The ratio of voltage
to the current at every point along a transmission line on which there are no standing
Circuit - A physical, metallic connection between
two points, also called a channel.
Circuit Grade - The grades of circuits are broad
band, voice, sub-voice, and telegraph. Circuits are graded on the basic line speed expressed in characters per second, bits per
second or words per minute.
Circuit, Multidrop - A circuit interconnecting several locations that makes information transmitted over the circuit available at alliocations simultaneously.
Circuit Noise Level - The ratio of the circuit noise
to some arbitrary amount chosen as a reference. This ratio is normally indicated in
decibels above the reference noise, dbrn, or
in adjusted decibels, dba, which signifies a
noise meter reading adjusted to represent the
interfering effect under specified conditions.
See dba explanation under the db heading.
Circuit Switch - A communications switching system which completes a circuit from sender
to receiver at the time of transmission (as
opposed to a message switch).
C L - Control Leader.
Clamp-On - A method of holding a call for a line
that is in use and of signaling when it becomes
free. (Also called Camp-On.)
CLK - Clock.
COAM Equipment - Customer communication owned
and maintained equipment connected to communication company lines but not owned by
the communication company. The IBM Terminal is a good example (even though not
owned by the customer).
Coaxial Cable - See Cable, Coaxial.
COD - Change of Direction.
Code - A system of symbols and rules for use in
representing information.
Code Conversion - A process for changing the bit
groupings for characters in one code into the
corresponding character bit groupings for a
second code.
Code Holes - The information holes in perforated
tape, as opposed to the feed or other holes.
COMLOGNET - Abbreviation for Air Force Combat
Logistic Network. See Autodin.
Common Carrier - A company that furnishes communication services to the general public and
which is regulated by appropriate state or
federal agencies.
Communication - The process of transferring information from one point, person or equipment
to another.
Communication, Data - The transmission of data
from one point to another.
Compandor - Equipment that compresses the outgoing speech volume range and expands the
incoming speech volume range on a long distance telephone circuit.
Composite - To composite a pair of wires used for
voice communication is to apply "compositing" equipment to the pair. It is a means of
getting two dc telegraph channels in addition to the voice channel on a single pair of
Compressor - Electrical device which compresses
the volume range of a signal.
Comprise Net (Network) - Network, used in conjunction with a hybrid coil to balance a subscriber's
loop, that is adjusted for an average loop
length or an average subscriber's set or both,
to secure compromise (not precision) isolation between the two directional paths of the
Constant Ratio Code - A code in which all characters
are represented by combinations having a
fixed ratio of ones.
Contention - 1. A condition on a multidrop communication channel when two or more locations
try to transmit at the same time.
2. Unregulated bidding for a line by
multiple users.
Continuous Loading - The practice of wrapping a
cable conductor with a permalloy tape or
wire. Seldom done because of high cost.
Control Character - A character used to cause
functions such as line feed, carriage return,
etc., to occur.
Control Switching Points - Collectively, the class 1,
2, 3 offices (Regional, Sectional, and Primary Centers) needed for nationwide dialing.
The greatest number of long distance toll
links required for any subscriber-to-subscriber connection is 7. The average number
of links required is 3 or 4.
Converter - A device capable of converting impulses
from one mode to another, such as analog to
digital or parallel to serial.
CPS - Abbreviation for "characters per second" and
"cycles per second".
Crossbar - Switching system using mechanisms
called crossbar switches, consisting of rectangular fields of contact springs operated in
co-ordination by horizontal and vertical
Crossbar No.1, the earliest type, is a local
telephone switching system for use in metropolitan areas where there is a high percentage of interoffice calls.
Crossbar No.5, the most recent type, was
designed primarily to serve the residential
sections of large cities and the fringe areas
around these cities.
Crossbar Exchange - An exchange in which switching
is done by crossbar apparatus.
Cross Fire - Interference between telegraph circuits
that cause the faulty operation of the relays
in a telephone circuit.
Crosstalk - Interference from adjacent communication channels.
Crypto - Cryptographic - Usually pertains to military equipment used for the transmission of
classified documents.
CT - Control Trailer.
CTS - Clear to send (return from data set in response to request to send).
Customer Station Equipment - Telephone company
equipment located on the customer's premises. Such as a telephone instrument or data
Cutting Through - See Switching Through.
Data, Analog - A physical representation of information such that the representation bears an
exact relationship to the original information.
The electrical signals on a telephone channel
are analog data representation of the original
Data Collection - The act of bringing data from one
or more points to a central point. May be
in-plant or out-plant.
Data Digital - Information represented by a code
consisting of a sequence of discrete elements.
Data Gathering - See Data Collection.
Data Link - D/L - Electronic equipment to perform
automatic transmission of digital information.
Data Origination - The translation of information
from its original form into a machine readable form or directly into electrical
Data Phone - Both a service mark and a trade mark
of the Bell System. As the service mark it
indicates the use of the Bell System message
network for the transmission of data. As a
trade mark it identifies the branch of data
sets designed and manufactured for DATA
PHONE service.
Data Set - A modulation/demodulation device designed to provide compatibility between, input/
output equipment and communication facilities. It is also referred to as line adapter,
modem and subset. Some manufacturers of
data sets are:
American Telephone & Telegraph
General System
Collins Radio
Lenkurt Electric
Western Union
Classes of Data Sets:
Data sets can be classified according to
type (analogue or digital), speed in bits/
second, characters/second or bandwidth in
kilocycles, synchronous or non-synchronous
and transmission mode (serial or parallel).
These are defined in the following list of
Low - Below 200 bits per second on a
voice channel or more narrow band facilities. In general, use of voice channels
is always preferable where distances are
short, or where this bandwidth permits
the simplified design of the data set.
Medium - 200 to about 2000 bits per second on voice channels. New data sets
under development will probably extend
this to about 2400 bits per second.
High - Above 2400 bits per second on
broad band facilities such as:
48 kc
240 kc
1000 kc (or 1/2 TD
2 Channel)
Video Channel
NOTE: The bandwidths shown above
are carrier system bandwidths and
the customer signals (baseband) must
be limited to more narrow spectrums,
due to filter characteristics and pilot
Non-Synchronous - A non-synchronous
data set will accept any rate of input up
to its designed limit and will deliver at
the same rate on output.
Synchronous - Synchronous data sets will
accept only one specified rate of input and
will deliver only at that rate on output.
Analogue - Signals which may assume'
any amplitude for any period within a
generally restricted range and are of a
continuous rather than pulsed nature.
(Speech waves are analogue waves. )
Digital - Signals made up of pulses of
discrete durations, amplitudes and periods. (Teletypewriter, etc.)
Transmission Mode:
Serial - Where each pulse is sent separately, one after the other. The data
speeds are usually referred to as bits per
Parallel - Where all the pulses comprising
a character are sent simultaneously or in
parallel. Here, the data speeds are
usually referred to as characters per
second. (Bits per second = characters
per second times bits per character.)
Data Set Clocking - This term identifies the service
where the data set (common carrier) supplies
the time base oscillator for the bit rate of
transmission. This, is also referred to by
IBM as "external clocking" and by the common
carrier as "internal clocking". To avoid
confusion use "Data Set Clocking" or "Business Machine Clocking".
DB - See Decibel.
DB Meter - A meter having a scale calibrated to
read directly in decibel values at a reference
level that must be specified (usually 1 milliwatt equals zero db). Used in audio-frequency amplifier circuits of broadcast stations, public-address systems, and receiver
output circuits to indicate volume level.
D Code - Control characters are sometimes identified by this term. They have the configuration, 1 and R and X and not 0 or 0 and not X
and not N.
D=l. R. (OX + XO). N
DCT - Dibit Clock Transmit (data set interface).
Dead Line - A telephone circuit disconnected from a
central office.
Decibel - 1/10 Bel - The unit used to express power
ratio: db = 10 log P 1/P 2 , where PI and P 2
are the two powers being compared; for example, 3 db down (-3 db) means a 50 percent
loss of power. Decibel is = 1/10 bel or
0.1151 neper. It requires a known refe:cence
level, and impedance. For example; db
usually in telephone work means dbm where
o db is 1 milliwatt in 600 ohm. Another
common value is db6m where the reference
of 0 db is 6 milliwatts in 500 ohm. This
value is used principally in audio amplifier
work where 500 ohm lines are used. Dbv is
used in measuring the open-circuit output of
devices such as microphones where the reference is 0 db = 1 volt with an infinite impedance. Dba, decibels adjusted, this is
used as a base for noise measurements. It
references 85 dbm at 1000 cycles. Dba replaces the obsoleted value dbrn. Some other
references which have been used are 10,
12.5 and 50 milliwatts with impedances of
52, 75, 100, 125, 150, 250, 300 ohm.
Therefore, unless the reference and impedance are known, the term db is of little or
no value except to get an idea of the relative
gain or loss in a circuit between points.
Decineper - 1/10 neper.
Dedicated Service - See Leased Channel and Private
Delay Distortion - See Distortion, Delay.
Delay Equalizer - A corrective network which is designed to make the phase delay or envelope
delay of a circuit or system substantially
constant over a desired frequency range.
See Equalizer.
Demarcation Strip - Usually refers to a terminal
board acting as an interface between the
business machine and the common carrier.
Demodulation - The process of retrieving an original
signal from a modulated carrier wave. This
technique is used to make communication
signals compatible with business machines
DFT - Diagnostic Function Test - A program to test
over-all system reliability.
Dial Exchange - An exchange where all subscribers
can originate their calls by dialing.
Dialing, Direct Distance - An exchange service
which enables a telephone user to select
subscribers outside the user's local area.
Dial Pulse - An interruption in the DC loop of a
calling telephone. It is produced by the
brealting and malting of the dial pulse contacts of a calling telephone caused by the
dialing of a digit. The loop current is interrupted as many times as the digit units
Dial-Up - The service whereby a dial telephone can
be used to initiate and effect a station-tostation telephone call.
Dibit - A group of two binary bits; in 4-phase modu1ation' each possible dibit is encoded as one
of four unique carrier phase shifts.
Digital Data - Information represented by a code
consisting of a series of bits.
Digital Subset - See Data Set.
Direct Distance Dialing - DDD - Service which enables the caller to dial toll calls without operator intervention.
Director - A type of crossbar equipment used- to
make cross-office selection and connection
from an input to an output in accordance with
CDC's. (Telegraph applications.)
Direct-Point Repeater - A telegraph operator in
which the receiving relay controlled by the
signals received over a line repeats corresponding signals directly into another line or
lines without the interposition of any other
repeating or transmitting apparatus.
Disconnect (Release) - To disengage the apparatus
used in a telephone connection and to restore
it to its condition when not in use.
Disconnect Signal - A signal transmitted from one
end of a subscriber line or trunk to indicate
at the other end that the established connection should be released.
Display Unit - Device which provides a visual representation of data.
Distortion - Lack of similarity between a signal received over a transmission line and the signal put on the line at the transmitting end.
Usually measured in percent.
Amplitude - (Frequency - Harmonic) - Distortion caused by unequal attenuation or
amplification of different frequencies by
a transmission line, filter, or other circuit.
Bias - In teletypewriter applications, this is
defined as marks or spaces consistently
too long or too short. If an alternate
mark space test pattern were transmitted
with an equal amount of time between
transitions, the received signal will probably have a percentage of marking or
spacing bias. This may be caused by the
characteristics of the terminal, data set,
or by an unequal rate of transmission.
When the mark pulses appear lengthened,
the space pulses will be shortened. This
condition is called MARKING or POSITIVE
Bias. When the mark pulses appear
shortened, the space pulses will be
lengthened. This condition is called
Characteristic - The normal and predictable
distortion of data bits produced by characteristics of a given circuit at a particular transmission speed.
Delay - (Envelope Delay, Phase Delay) Distortion of a signal resulting from unequal propagation time for different frequencies over a transmission line,
through a filter, or other circuit.
End - In start-stop teletypewriter signals,
the shifting of the end of all marlting
pulses from their proper positions in relation to the beginning of the start pulse.
A marlting end distortion denotes a
lengthening of the marlting bit. A spacing
end distortion denotes a lengthening of
the spacing bit (mark bit shortened).
Fortuitous - A type of distortion which results in the signals being intermittently
shortened or lengthened. This distortion
is entirely random in nature and can be
caused by battery fluctuations, hits on
the line, power induction, etc. See
Frequency - See Amplitude Distortion.
Harmonic - See Amplitude Distortion.
Jitter - Peak Jitter - Random variation in bit
length due to noise, modulation process,
etc. This results in a varying signal
duration caused by the non-constant
switching time in the transition of a signal
level. Jitter for the most part cannot be
adjusted. It becomes more frequent at
the higher rates of transmission. Also
mown as the combined effect of characteristic and fortuitous distortion.
Marking - See Bias Distortion.
Negative - See Bias Distortion.
Positive - See Bias Distortion.
Spacing - See Bias Distortion.
Teletypewriter Signal - In start-stop teletypewriter signals, the shifting of the
transition points of the signal pulses from
their proper positions relative to the beginning of the start pulse. The magnitude
of the distortion is expressed in percent
of a perfect unit pulse length. See Bias,
End, Fortuitous Distortion.
Distributing Frame - A structure for terminating
permanent wires of a central office, private
branch exchange, or private exchange and for
permitting the easy change of connections between them by means of cross-connecting
DLT - Data Loop Transceiver - Western Union's
low-speed data set.
Dot Cycles Per Second - One dot cycle is a mark
and 'space. The speed of transmission of a
system in dot cycles per second is numerically equal to half the speed of transmission
expressed in bits per second or bauds.
DR - Data Register.
Drop Relay - A relay actuated by a ringing current
and used to call on operator's attention to a
subscriber's line.
Drop, Subscriber's - The line from a cable termination to a subscriber's location.
Dry Contact - Term indicating a contact closure
only. Used where one vendor controls by
contact closure another vendor's voltage/
current loop.
Dry Reed Contact - Term usually used to differentiate from mercury wetted type.
DS - Data Set.
Dummy Load - A device in which the output power
can be absorbed, used for simulating conditions of operation for test purposes. In
modem applications, a resistor, 8. 2K, which
may be substituted on the line for a modem.
A dummy load counts as one modern.
Duplex - Term used to indicate that a channel or system can be used for communication in two
directions. See Half Duplex and Full Duplex
Duplex System - Having two distinct and separate
machine facilities each of which is capable of
assuming the proceSSing function while the
other assumes a standby.status. Usually,
both systems are identical in nature. Duplex':"
ing is utilized for greater reliablilty.
EAX - Electronic Automatic Exchange - A high
speed semi-automatic switching office now
undergOing development by General Telephone.
Echo Check - A check which transmits received information back to the source and compares
it with the original.
Echo Suppressor - A line device used to prevent
energy being reflected back (echoed) to the
transmitter. It attenuates the transmission
path in one direction while signals are being
passed in the other direction.
Effective Speed - Speed (less than rated) which can
be sustained over a significant period of time
and which reflects slowing effects of control
codes, timing codes, error detection, retransmission, tabbing, etc.
EIA Interface - A set of signal characteristics (time
duration, voltage and current) specified by
the Electronic Industries Assn. for business
machine/data set connections.
Eight Level - A telegraph code which utilizes eight
impulses, in addition to the start and stop
impulses, for describing a character.
End Distortion - See Distortion, End.
End Office - Class 5 office - local telephone exchange where subscriber's loop terminates See Exchange, Central.
End of Message - The specific set of characters
which indicates the termination of a message.
End to End Test - Dynamic test - a Bell System test
utilizing the 900 series test equipment. Requires Bell maintenance personnel at each
end of the circuit.
Energy Level Diagram - A line drawing that shows
increases and decreases of electrical power
as current intensities rise and fall along a
channel of signal communications.
EOI - End of Idles.
EOM - End of Message.
EOT - End of Transmission.
EOTR - End of Transmittal Record.
Equalization - The process of reducing frequency
and/ or phase distortion of a circuit by the
introduction of networks to compensate for
the difference in attenuation and/or time delay at the various frequencies in the transmission band.
Equalizer - Any combination (usually adjustable) of
coils, capacitors, and/or resistors inserted
in transmission or amplifier circuit to improve its frequency response.
Equivalent 4-Wire System - A term applied to a
carrier on repeater system using only one
pair of wires but different frequencies for
each direction of transmission.
ERR - Error.
Error Rate - A measure of quality of circuit or
equipment; the number of erroneous bits or
character in a sample, frequently taken per
100,000 char.
ESS - Electronic Switching System - Bell Systems
new electronic switching center utilizing
high speed computers.
Exchange - Exchange, Central Office - An office in a
telephone system that provides service to the
general public where orders fo.r, or signals,
controlling telephone connections are received
and connections established. See End Office.
Exchange Classes of Class 1 - See Regional Center
Class 2 - See Sectional Center
Class 3 - See Primary Center
Class 4 - See Toll Center
Class 5 - See End Office
Exchange, Dial - An exchange where all subscribers
originate their calls by dialing.
Exchange, Manual - An exchange where calls are
completed by an operator.
Exchange, Private Automatic (PAX) - A dial exchange
that provides private telephone service to an
organization and that does not allow calls to
be transmitted to or from the public telephone
Exchange, Private Automatic Branch (PABX) - A
private automatic exchange that provides for
the transmission of calls to and from the
public telephone network.
Exchange, Private Branch (PBX) - A manual or dial
exchange, connected to the public telephone
network often located on a customer's premises and operated by his employees.
Exchange, Service - A service permitting interconnection of any two customers' telephones
through switching equipment.
Exchange, Toll - A toll office where customer's calls
are connected to toll circuits or toll circuits
are interconnected. An exchange where toll
or long-distance connections are made.
Exchange, Trunk - An exchange primarily devoted to
handling trunk calls.
Expander - A transducer which for a given amplitude
range of input voltages produces a larger
range of output voltages. One important type
of expander employs the envelope of speech
signals to expand their volume. range.
Extension - 1. Additional telephone set on the same
line but at a different location other than the
main station.
2. PBX station.
Extension, PBX, Outside - PBX station on premises
separated from the PBX. Sometimes called
Long Distance Locals.
Extension Station - A telephone station associated
with a main station through connection to the
same subscriber line and having the same
call number designation as the associated
main station.
External Clocking - This term is used by IBM to
identify the service where the data set (common carrier) supplies the time base oscillator for the bit rate of transmission. See
Data Set Clocking - the preferred term.
Facsimile (FAX) - A system for the transmission of
fixed images. Type A facsimile is a system
of facsimile communication in which images
are built up of lines or dots of constant intensity. Type B facsimile (telephotography,
photoradio, etc.) is a system of facsimile
communication in which images are built up
of lines or dots of varying intensity.
Facsimile-Signal Level - An expression of the maximum signal power or voltage created by the
scanning of the subject copy as measured at
any point in a facsimile system. According
to whether the system employs positive or
negative modulation, this will correspond to
picture white or black, respectively. It may
be expressed in decibels with respect to
some standard value such as 1 milliwatt or 1
Far-End Cross Talk - Cross talk which travels
along the disturbed circuit in the direction in
which the signals travel in that circuit. To
determine the far-end cross talk between
two pairs, 1 and 2, signals are transmitted
on pair 1 at station A, and the cross talk
level is measured on pair 2 at station B. See
Near-End Cross Talk.
FCC - Federal Communications Commission - A
board of seven commissioners appointed by
the President under the Communications Act
of 1934, having the power to regulate all
electrical communication systems originating in the United States.
FD or FDX - Full Duplex.
FDM - Frequency Division Multiplex.
Feed Holes - Holes punched in a tape to enable it to
be driven by a sprocket wheel.
Feed Pitch - The distance between the centers of
feed holes.
Fieldata Code - A standardized military data transmission code, 7-data plus 1-parity bits.
Figure Shift - A control character in the Baudot
code after which characters are interpreted
as belonging to the grouping containing numerics, punctu.ation and special symbols
(upper case).
FINAC - A leased automatic teletypewriter system
provided by AT&T.
Five Level - A telegraph code which utilizes five
impulses, in addition to the start and stop
impulses for describing a character.
Foreign Exchange Service - A service which connects a customer's telephone to a telephone
company central office normally not serving
the customer's location (such as tie lines).
Fortuitous Distortion - See Distortion, Fortuitous.
Four-Row Keyboard - Used for origination of 8-level
codes eliminating the need for figures/letters
case shifts. As opposed to three row.
Four-Wire Circuit - See Channel, Four-Wire.
Four-Wire, Half Duplex - IBM term only, does not
refer to communication line services.
Four-Wire Repeater - Repeater which provides for
transmission in opposite directions on two
transmission paths.
Four-Wire Terminating Set - Hybrid arrangement by
which 4-wire circuits are terminated on a
2-wire basis for interconnection with 2-wire
Fox Message - A standard mesflage which is usen
for testing telegraph circuits and machines
because it includes all the alphanumerics as
well as most of the function characters.
Frame, Type A - Distributing frame carrying on one
side (horizontal) all outside lines, and on the
other side (vertical) the terminations of the
central office equipment and protective devices for them.
Frame, Type B (Main) - Distributing frame carrying on one side (vertical) all outside lines and
protective devices for those lines and on the
other (horizontal) all connections of the outside lines toward the central office equipment.
Framing - The process of selecting the bit groupings representing one or more characters
from a continuous stream of bits.
Framing Bits - Non-information carrying bits used
to make possible the separation of characters
in a bit stream.
Free Routing - That method of traffic handling
wherein messages are forwarded toward
their destination over any available channel,
normally the most direct without depending
on a predetermined routing doctrine.
Frequency Division Multiplex - See Multiplexing.
Frequency Modulation - Variation of a carrier frequency in accordance with an information
Frequency-Shift Keying (FSK) - Frequency-Shift
Transmission - A system of automatic code
transmission and reception that shifts the
the carrier frequency back and forth between
two distinct frequencies to designate mark
and space, instead of keying the carrier on
and off •.
FTS - Federal Telecommunications System.
Full Duplex - See Channel, Full Duplex.
Full Speed - Referring to transmission of data in
teleprinter systems at the full rated speed
of the equipment.
Fully Perforated Tape - Perforated paper tape in
which the perforations are complete. That
is, the punch makes a complete hole in the
tape (as opposed to chadless tape where the
hole is not completely punched out).
Function Key - A term associated with specific keys
on a teletypewriter (for example, "CR",
"LF", "LTRS", "FIGS", etc.) which, when
operated, cause the teletypewriter to perform mechanical functions so that a message
may be received in proper form.
GM - Group Mark.
Group - In telegraphic usage - a word.
Group Mark - Any indicator to signal the end of a
word or other unit of data.
Half Duplex - See Channel, Half Duplex.
Half Speed - Referring to transmission of data in
teleprinter systems at half the rated speed
of the associated equipment.
Hand Shaking - Exchange of predetermined codes
and signals when a connection is established
between two data terminals.
Hard Copy - A machine printed document, such as
a message, order or invoice.
Harmonic Distortion - See Distortion, Amplitude.
Harmonic Telephone Ringer - A telephone ringer
which responds only to alternating current
within a very narrow frequency band .. A
number of such ringers, each responding to
a different frequency, are used in one type
of selective ringing.
HD or HDX - Half Duplex.
Header - The initial characters of a mes,sage designating addressee, routing, time of origination, etc.
Hits - Hit on the Line - Momentary line disturbances
which could result in mutilation of characters being transmitted.
Hollerith Code - The 12-unit code used in conventional punched tabulating machine cards.
Home Loop - Data path, allowing off-line use of
terminal components.
HSSTR - High Speed Synchronous Transmitter Receiver.
Hybrid Coil - Four-winding coil wound and connected
so that incoming and outgoing currents in a
2-wire path are separated and kept from interfering with each other.
ICA - Industrial Communications Association.
Idles - Bit Code 1 8 R 0 set when no other transmission is called for by the STR.
Incoming Trunk - Trunk coming into a central office.
Indicator, Routing - A group of letters, engineered
and assigned to identity a station within a
teletypewriter network.
Induction Coil - Coupling transformer.
Information Bit - One of those bits which are used to
specify the characters of a given code group
(opposed to framing bits).
In-Line Processing - Processing in order of receipt,
without prior grouping or sorting, of transactions.
In-Plant System - A system whose parts, including
remote terminals, are all situated in one
building or localized area.
INQ - Inquiry.
Intercepting - Routing of a call placed to a disconnected or nonexisting telephone number, to a
operator or to a machine answering device.
Intercepting, Operator - Answering an intercepted
call by an operator who asks, "What number
did you call, please?" and is then given instructions on completing the call.
Intercepting Trunk - A trunk to which a call for a
vacant number or changed number or a line
out of order is connected fo:c action by an
Intercommunicating System - A privately owned
system without a switchboard capable of twoway communication. It is normally a single
unit, building or plant. Stations mayor may
not be equipped for originating a call, but
they can answer any call.
Inter-Exchange Channel - Channel connecting two
different metropolitan areas.
Interface - A common boundary - for example,
physical connection between two systems or
two devices.
Intermediate Distributing Frame (IDF) - A distributing frame in a local central office, the primary purpose of which is to cross-connect
the subscriber line multiple to the subscriber
line circuit. In a private exchange, the intermediate distributing frame is for similar
Internal Clocking - This term is used by IBM to
identify the service where the business machine supplies the time base oscillator for
the bit rate of transmission. See Non Data
Set Clocldng - the preferred term.
International Telecommunication Union (ITU) - A
civil international organization established to
provide standardized communication procedures and practices including frequency allocation and radio regulations on a world-wide
Interoffice Trunk - A direct trunk between local central offices in the same exchange.
Interpreting - Printing the symbol or meaning of
holes in the same paper tape or cards in
which they are punched.
Intertoll Dialing - Dialing over intertoll trunks.
Intertoll Trunk - A trunk between toll switchboards
in different offices.
Inverse Neutral Telegraph Transmission - That
form of transmission in which marlting signals are zero current intervals and spacing
signals are current pulses of either polarity.
Jitter - See Distortion, Jitter.
Joint Use - The simultaneous use of a pole, line, or
plant. Facilities by two or more kinds of
Junction Pole (S-Pole) - A pole at the end of a transposition section of an open-wire line or the
pole common to two adjacent transposition
Key Click - Those components of telegraphic radiation that are set up as transients by the
opening or closing of the signaling key but
are not essential for communication.
Key-Click Filter - A filter that attenuates the surges
produced each time the keying circuit of a
transmitter is opened or closed by the key.
Key Pulse - System of signaling where numbered
keys are depressed instead of using a dial.
Keying - The forming of signals, such as those employed in telegraph transmission, by the
modulation of a direct current or other
carrier between discrete' values of some
. characteristic.
Keying Chirps - Sounds accompanying code signals
when the transmitter is unstable and shifts
slightly in frequency each time the sending
key is closed.
Keying Wave - The emission that takes place in
telegraphic communication while the active
portions of the code characters are being
transmitted. Also called Marking Wave.
KSR - Keyboard Send/Receive - A teleprinter unit
with keyboard and printer.
Lag (In Telegraph System) - The time elapsing between the operation of the transmitting device and the response of the receiving device.
Landline - A telegraph or telephone line passing
over land, as opposed to submarine cables.
LD - Long Distance.
LD Line - A telephone line or channel between two
central offices in different exchanges.
LD Office - A central office primarily arranged for
terminating LD lines, switching trunks, recording trunks, and recording-completing
trunks and for their interconnection with each
LD Switching Center - An LD office that serves as a
switching point between local exchanges and
the long distance telephone network.
LD Switching Trunk - A trunk extending from an LD
office to a local central office for connecting
LD lines to user lines.
Leased Channel - A point-to-point channel reserved
for sole use of a single leasing customer.
See Private Line Circuit.
Letters Shift - A control character in the Baudot
code after which all characters are interpreted as belonging to the group containing
letters (lower case).
Level Compensator - An automatic gain control device used in the receiving equipment of a
telegraph circuit.
Line - See Circuit.
Line Adapter - See Data Set.
Linear Distortion - Amplitude ·distortion wherein
the output signal envelope is not proportional
to the input signal envelope but no alien frequencies are involved.
Line Hit - An electrical interference causing the
introduction of spurious signals on a circuit.
Line, In-House - A privately owned or leased line
that does not cross a public right of way.
Line Level - The signal level in decibels at a particular position on a transmission line.
Line Loop - See Local Loop .
Line Loop Resistance - The metallic resistance of
the line wires that extend from an individual
telephone set to the dial central office.
Often called loop resistance.
Line Noise - Noise originating in a transmission
Line Printer - A printer in which all characters
across an entire line of type are printed in
one printing cycle.
Line Relay - A relay which is controlled over a
subscriber line or trunk line.
Line Speed - The maximum rate at which signals
may be transmitted over a given channel;
usually in baud or bits/sec.
Line Stretcher - An impedance matching device for
coaxial transmission lines.
Link - 1. In dial central offices, a complete talking
2. Any combination of switches to accomplish
that path, such as a linefinder and connector
tied together.
Link Group - Those links that employ the same
multiplex terminal equipments.
Load - 1. A resistor or other impedance which can
replace some circuit element temporarily or
permanently removed.
2. The power that a machine or apparatus
3. The device used to absorb power and convert it into the desired useful form.
Load and Go - A computer operation and compiling
technique in which the pseudo language is
directly converted to machine language and
the program run without an output machine language program being created.
Loaded Lines - (Loaded Cable) - A transmission line
or cable pair which has had inductance (load
coils) added in series with each conductor,
usually at definite spaced intervals. Common
load coil spacings are:
3000 feet
4500 feet
6000 feet
M 9000 feet
Loaded cable pairs are usually designated by
an alphanumeric code as follows: Two digits
representing the wire gauge, a letter denoting the load coil spacing and two or three
digits representing the load coil inductance
in millihenries.
For example:
Load Coil
Wire Gauge
Load Coil Inductance in
Local Battery Talking-Common Battery Signal Telephone Set - A local battery telephone set in
which current for signaling by the telephone
station is supplied from a centralized directcurrent power source.
Local Channel - In private line service, the term
"local channel" refers to that portion of a
complete circuit between the toll office (interexchange channel terminal) and the customer's
premises. See Local Loop.
Local Exchange - An exchange serving a comparatively small group of customers, not exceeding 10, 000 in number; 10, 000 is the basic
unit of the A T&T switching" system. The
term is quite often used to refer to the building which houses the central office equipment used to provide the local exchange service.
Local Loop - A channel connecting the subscriber's
equipment to the line terminating equipment
in the central office exchange. Usually metallic circuit (either 2-wire or 4-wire).
Local Service Area - That area within which are
located the stations which a customer may
call at rates in accordance with the local
Lock-Out - In a telephone circuit controlled by two
voice-operated devices, the inability of one
or both subscribers to get through, either
because of excessive local circuit noise or
continuous speech from either or both subscribers.
Long Line - A T&T term for circuits between exchanges.
Loopback Test - A test, if available within the data
set, under control of a Bell data test center
where signals are looped from the test center
through the data set and back to the test center for measurement.
Loop Jack Switchboard - A patch panel with rows of
vertical jacks. (Maximum capacity of 90
channels). Each vertical jack strip consists
of two looping jacks, a set jack, and a miscellaneous jack.
Loop Test - A method of testing employed to locate
a fault in the insulation of a conductor when
the conductor can be arranged to form part
of a closed circuit or loop.
LRC - Longitudinal Redundancy Check.
LTRS - "Letters" shift.
Lumped Loading - The action of inserting uniformly
spaced inductance coils along the line, as
opposed to continuous loading. See Loaded
Main Distributing Frame (MDF) - A distributing
frame, on one part of which terminate the
permanent outside lines entering the central
office building and on another part of which
terminate the subscriber line multiple cabling, trunk multiple cabling, etc., used for
associating any outside line with any desired
terminal in such a multiple or with any other
outside line. It usually carries the central
office protective devices and functions as a
test point between line and office. In a private exchange the main distributing frame is
for similar purposes.
Main Station - A telephone station with a distinct
call number designation, directly connected
to a central office.
Margin Control - See Range Finder.
Mark - The presence of a bit or signal on the line.
In telegraph applications it is an impulse
which, in a neutral circuit, causes the loop
to be closed; or in a polar circuit, causes
the loop current to flow in a direction opposite to that for a space impulse.
Mark-Hold - The normal no-traffic line condition
whereby a steady mark is transmitted. May
be a customer selectable option.
Marking Bias - Bias distortion which lengthens the
marking impulse by advancing the space-tomark transition. See Distortion, Bias.
Marking End Distortion - End distortion which
lengthens the marldng impulse by delaying
the mark-to-space transition. See Distortion, End.
Marking Wave - In telegraphic communication, the
emission which takes place while the active
portions of the code characters are being
transmitted. Also called Keying Wave.
Mark-to-Space Transition - The transition, or
switching, from a marking impulse to a
spacing impulse. Also may denote a type of
bias distortion.
Master Station - A terminal having selection control
of all other terminals on a multipoint circuit.
MBR - Memory Buffer Register.
Mercury Wetted Relay - Uses mercury as relay contact closure substance.
Message - A sequence of words or symbols which is
complete in itself; typically consists of header, text and EO M.
Message, Broadcast - A message sent to all terminals ona network.
Message Circuit - A long distance telephone circuit
used in furnishing regular long distance or
toll service to the general public. The term
is used to differentiate these circuits from
circuits used for private line service.
Message, Multiple Address - A message to be delivered to more than one destination.
Message, Single Address - A message to be delivered
to only one destination.
Message, Switching - A system'in which messages
between stations are routed through a central
point having a storage media. Messages are
stored and forwarded when the appropriate
outgoing circuit becomes available.
Microwave - An electromagnetic wave in the superhigh frequency radio spectrum ranging from
1, 000 to 300, 000 megacycles per second.
Miscellaneous Intercept - The act of intercepting
single-address messages containing a nonvalid CDC and/or the action of intercepting
multiple-address messages without a proper
multiple-address code. See Willful Intercept.
Mod/Demod - Abbreviated form for modulating and
demodulating units. See Data Set.
Modem - Contraction of modulator - demodulator.
See Data Set.
Modulation - The process by which some characteristic of one wave is varied in accordance with
another wave.
Multidrop-Multipoint - So named because several
terminals are connected to a single line.
These terminals mayor may not have the
ability to communicate with each other. The
prime application for this circuit configuration would be a low traffic circuit. A savings
in line cost can be realized when using the
multidrop configuration.
Multiplexing - Multiplexing is any means whatsoever
whereby two or more signals (or the output
of two or more system elements) share the
same physical transmission facility, circuit
element or system component. Multiplexing
can be subdivided into two categories using
method as a criterion.
A. Time Division Multiplexing or TimeSharing. This method can be applied to
sharing of a transmission line or sharing
of a centrally located computer by several
input/output units or sharing of anyone
system component among several other
components. Time multiplexing is generally accomplished by switching techniques.
In this method, the various signals or system components take turns in having exclusive use of the transmission line or common system component. Examples - pulse
code modulation systems wherein the instantaneous amplitudes of several signals
are sampled and codes corresponding to
these amplitudes transmitted in turn, telemetering schemes wherein meter readings
from many different locations are "read"
one at a time by a central recording device.
Time division multiplexing may be effected
either of two ways:
1. Sequential Servicing - The input/output
Units are served in a fixed sequence.
The two examples of time sharing given
above are of this type.
2. Demand Servicing - The input/output
units are served on a demand basis. A
dial telephone exchange uses this type of
servicing. Th~ common switching equipment is connected to a subscriber's line
on demand, the demand being ,indicated
when the subscriber lifts the handset off
the "hook".
B. Frequency Division Multiplexing - This
method is generally applied in getting several signals over one transmission line.
rt is accomplished by having the various
signals modulate different carrier frequencies using mixing and filtering techniques. In this method all signals may be
"ON" simultaneously. Examples - two or
more IBM transceiver chanr..els on one line,
radio broadcasting, telephone carrier
Multipoint - Private line arrangement having more
than two terminals connected by the same
Near-End Cross Talk - Cross talk which is propagated in a disturbed channel in the direction
opposite to the direction of propagation of the
current in the disturbing channel. The terminal of the disturbed channel at which the
near-end cross talk is present is ordinarily
near or coincides with the energized terminal of the disturbing channel. See Far- End.
Neper - A unit of the same nature as the decibel but
differing from it in magnitude. One neper is
equivalent to 8.686 decibels.
Network - 1. Communication term: An organization
of stations capable of intercommunication but
not necessarily on the same channel.
2. Engineering term: Two or more interrelated circuits.
Network, Balancing - Lumped circuit elements (inductances, capacitances, and resistances)
connected so as to simulate the impedance of
a uniform cable or open -wire circuit over a
band of frequencies.
Network, Equalizing - Network connected to a transmission circuit to correct, control, or alter
the response of the circuit in a desired way;
particularly to equalize the response over a
frequency range. See Equalization.
Network, Leased Line or Private Wire - A series of
points interconnected by communications
channels, and reserved for the exclusive use
of one customer.
Network, Private Telegraph - A series of points interconnected by telegraph or telephone channels and reserved for the exclusive use of
one customer.
Network, Private Telephone - A series of points
interconnected by leased voice-grade telephone lines, with a customer operated exchange.
Neutral Circuit - A telegraph circuit in which current flows in only one direction. The circuit
is closed during the marking condition and
open during the spacing condition. Also
called Unipolar.
Noise - Random electrical signals introduced by
circuit components or natural disturbances
which tend to degrade the performance of a
communications channel.
Noise Killer - An electric network inserted in a
telegraph circuit, usually at the sending end,
for the purpose of reducing interference with
other communication circuits.
Nominal (Rated) Speed - Maximum speed or data rate
of terminal which makes no allowance for necessary delaying functions as checking, tabbing, etc.
Non Data Set Clocking - This term identifies the
service where the business machine supplies
the time base oscillator for the bit rate of
transmission. This is also referred to by
IBM as Internal Clocking and by the common
carrier as External Clocking. To avoid confusion use Non Data Set Clocking or Data
Set Clocking.
Non Loaded Lines - (Non-Loaded Cable Pairs) Cable pai rs or transmission lines with no
added inductive loading. As opposed to
Loaded Lines.
Non Simultaneous Transmission - A communication
device capable of transmitting on one pair of
a Full Duplex Circuit, but not capable of
receiving on the other pair at the same time.
Off-Hook - Unit activated.
Off-Line - In the Teleprocessing Systems sense,
off-line describes that kind of system in
which human operations are required between
the original recording functions and the ultimate data processing function. This includes conversion operations as well as the
necessary loading and unloading operations
incident to the use of point-to-point or datagathering systems. In the telegraph sense,
off-line means not in the loop. Paper tapes
frequently are punched "off-line" and then
transmitted using a paper tape transmitter.
On-Demand System - A system from which timely
information or service is available on request.
One-Way Trunk - A trunk between central exchanges
where traffic can originate on only one end.
On-Hook - Unit de-activated (hung-up).
On-Line - In the Teleprocessing Systems sense, an
on-line system eliminates the need for human
intervention between source recording and the
ultimate processing by a computer. In the
telegraph sense, it means in the loop.
Open-Circuit Jack - A jack that normally leaves its
circuit open. The circuit can be closed only
through a circuit connected to the plug that is
inserted in the jack.
Open Wire - A conductor separately supported above
the surface of the ground, and separately
supported on insulators.
Open-Wire Line - A pole line whose conductors are
principally in the form of open wire.
OPM - Operations Per Minute - Equivalent to characters per minute when control functions are
Options - Usually refers to the strapping options
(features) available within data sets. These
options are wired in during installation.
Some typical options are:
Originate - Data set always is in originate
frequency mode.
Answer - Data set always is in answer
frequency mode.
Remote - Business machine controls the data
set's operation in either the originate or
answer frequency mode.
Mark Hold - Quiescent condition of the line is
is in a mark condition.
Space Hold - Quiescent condition of the line is
in a space condition.
Orientation (Teletypewriter) - An adjustment of the
time the receiving apparatus starts selection,
with respect to the start signal. See Range
Out-of-Service Jack - Jack associated with a test
jack which removes the circuit from service
when a shorted plug is inserted.
Out-Plant System - A data transmission system
consisting of one or more centrally located
terminals and one or more terminals located
at some distance.
PABX - Private Automatic Branch Exchange.
Pad - Device which introduces transmission loss
into a circuit. It may be inserted to introduce loss or to match impedances.
Pad Switching - Transmission loss pad automatically
cut in and out of a toll circuit for different
desired operating conditions.
Page Printer - A printer printing characters one at
a time in page format.
Paper Tape Reader - A device which senses and
translates the holes in perforated tape into
internal form.
Parallel-By-Bit - or Parallel Transmission System where the bits of each character
occur simultaneously. Implies as many lines
or wires as there are bits in a character.
Parity Check - Addition of non-information bits to
data, making the number of ones in a grouping of bits either always even or always odd.
This permits detection of bit groupings which
contain single errors. It may be applied to
characters, blocks, or any convenient bit
PAX - Private Automatic Exchange.
PBX - Private Branch Exchange.
Peak Jitter - See Distortion, Jitter.
Perforated - A more suitable word to use in combination with paper tape than the word
punched, e. g., perforated tape includes
punched paper tape.
Perforator - A paper tape punch which is controlled
manually. It is used for punching tape offline.
PH - Phase.
Phantom Circuit - A superimposed circuit derived
from two suitably arranged pairs of wires
called side circuits, with each pair of wires
being a circuit in itself and at the same time
acting as one conductor of the phantom circuit.
Phase Distortion - See Distortion, Delay.
Phase Modulation - Control of the phase, or timing,
of a carrier signal by an information signal.
Plant (As Used by Bell System) - The local facility
of Bell System consisting of the switching
equipment, local service department, and
central office personnel.
Plantman - See Plant.
Point-to- Point Transmission - Transmission of data
directly between two points without the use of
any intermediate terminal or computer.
Polar Circuit - In telegraph applications a circuit in
which current flows in one direction on a
marking impulse and in the opposite direction
during a spacing impulse. Also called
Polar Relay - A relay containing a permanent magnet
that centers the armature. The direction of
movement of the armature is governed by the
direction of current flow.
Poll - A flexible, systematic method, centrally controlled, for permitting stations on a multipoint circuit to transmit without contending
for the line.
Polling - Orderly selection, one at a time, of multiple terminals to allow transmission of traffic.
The action whereby the multiplexor or control
station says effectively to the terminal, "Do
you have anything for me?"
Power Level (dbm) - The ratio of the power at that
point to some arbitrary amount of power
chosen as a reference. This ratio is usually
expressed either in decibels referred to 1
milliwatt, abbreviated dbm, or in decibels
referred to 1 watt, abbreviated dbw. See
Primary Center - A control center connecting toll
centers together - a class 3 office. Can also
serve as a toll center for its local end offices.
Primary Jack - Terminate line conductors in a long
lines testboard in one direction and central
office equipment in the other.
Priority Indicators - Groups of characters which
indicate the relative urgency of a message.
Private Automatic Branch
} See listing
Exchange - P ABX
Private Automatic Exchange - PAX
Private Branch Exchange - PBX
Private Lme Circuit - A circuit assigned to one
particular customer for his exclusive use.
Tie line.
Private Line Service - Communication service used
exclusively by one particular customer.
Also, the whole process of providing private
line circuits.
Processing, Batch - A method of processing in which
a number of similar input items are accumulated and grouped.
Processing, In-Line - A method of processing in
which individual input transactions are completely processed and all pertinent records
are updated without previously having been
Pulse Modulation - Transmission of information by
modulation of a pulsed, or intermittent,
carrier; pulse width, count, phase, and/or
amplitude may be the varied characteristic.
Quad - A structural unit employed in cable, consisting of four separately insulated conductors
twisted together.
Quarter-Speed - Refers to transmission of data in
teleprinter systems at one fourth the rated
speed of the associated equipment.
Queue - 1. A waiting line generated by the time
required to move through channels, e. g. ,
traffic at a toll booth, a line at a ticket window.
2. To arrange in or form a queue.
Rain Barrel Effect - Sound noted on an over compensated line.
Range Finder - An adjustable mechanism on a teletypewriter receiver which allows the receiver distributor face to be moved through an arc
corresponding to the length of a unit segment.
It is adjusted normally for best results under
operating line conditions. See Receiving
Rate Center - A point of measurement from which
the telephone company bases its mileage
Rated Speed - See Nominal Speed.
RCD - Record.
RCL - Receive Control Leader.
RDC - Receive Data Condition.
Real-Time Operation - Concurrent operations for
data processing (computing) and physical
processing in such a way that the results of
the computing operations are available whenever needed by the physical processing operations, and vice versa.
Receiving- End Cross Fire - The cross fire in a
telegraph channel introduced from one or
more adjacent channels at the terminal end
remote from the transmitter.
Receiving Margin - In telegraph applications, the
usable range over which the range finder may
be adjusted. The normal range for a properly adjusted machine is approximately 75
points on a 120 point scale. Sometimes referred to as range or operating range.
Record - A group of related facts or fields of information treated as a unit.
Recording Trunk - A trunk from a local central
office or private branch exchange to a long
distance office, which is used only for communication between operators.
Redundancy - In the transmission of information,
redundancy is the fraction of the gross information content of a message which can be
eliminated without loss of essential information. Numerically, it is one minus the ratio
of the net information content to the gross
information content, expressed in percent.
Redundancy Check - A check which uses extra bits.
Reference Level - The level used as a starting point
when designating the value of an alternating
quantity or a change in the quantity by means
of decibel units. For sound loudness, the
reference level is usually the threshold of
hearing. For communication receivers, the
commonly used level is 60 microwatts. A
common reference in electronics is 1 milliwatt and power is stated as decibels above or
below 1 milliwatt (dbm). See Decibel.
Reference Noise - The magnitude of circuit noise that
will produce a circuit noise meter reading
equal to that produced by 10 micromicrowatts of electric power at 1, 000 cycles per
Reference Volume - That magnitude of a complex
electric wave,' such as that corresponding to
speech or music, which gives a reading of
zero vu on a standard volume indicator. The
sensitivity of the volume indicator is adjusted
so that reference volume or zero vu is read
when the instrument is connected across a
600-ohm resistance to which there is delivered
a powe-r of 1 milliwatt at 1,000 cycles per
Regenerative Repeaters - See Repeaters, Regenerative.
Regional Center - A control center connecting sectional centers together. Every pair of regional centers in the United States has a
direct circuit group between them.
Relative Transmission Level - The ratio of the testtone power at that point to the test-tone
power at some point in the system chosen as
a reference point. The ratio shall be expressed in db. The transmission level at the
transmitting switchboard is frequently taken
as the zero level reference point. See Zero
Transmission Level Reference Point.
Relay Center - Synonymous with message switching
Repeaters - A combination of apparatus for the
reception and transmission of either l"";way
or 2-way communication signals and delivering corresponding singals which are either
amplified or reshaped or both. A repeater
for I-way communication is termed a "I-way
repeater" and for 2-way communication
impulses a "2-way repeater". Normally a
repeater is part of an echo suppressor.
Some types of repeaters are:
Type 22 - Maximum 18 db gain: triode
operating with all equalization and balancing equipment included.
Type 44 - Maximum 42 db gain: similar
to Type 22 but used on four-wire circuits.
Type E - 8 to 10 db gain: a negative
impedance repeater or converter which is
line connected through a single, doublewound transformer.
Type V - Maximum 25 db gain: this is the
most universal type of repeater in use.
It is independent of the hybrid filters,
equalizers, and line equipment which are
associated with the line. This permits
complete interchangeability.
Repeater, Regenerative - Also called "Regens".
Normally a repeater utiliz ed in telegraph
applications. Its function is to retime and
repower (retransmit) the received signal
impulses. These repeaters are speed and
code sensitive and are intended for use with
standard telegraph codes.
Repeater, Single Line - A telegraph repeater utilizing a pair of cross-coupled polar relays
which are inserted in series with a circuit
to repower the signal.
Repeating Coil - A 1 to 1 ratio audio-frequency
transformer for transferring energy from
one electrical circuit to another and to permit,
in wire communication work, the formation
of simplex and phantom circuits.
Reperforator - A paper tape punch which is controlled electrically. It is used for punching
tape on-line. Reperforators, in some cases,
may be used as perforators for manually
punching tape, as well as for producing a
tape from on-line traffic.
Reperforator /Transmitter - Trade names used by
AT&T to indicate a teletypewriter receiver transmitter consisting of a reperforator and
a tape distributor, each of which is independent of the other. It is used as a relaying
device and is especially suitable for transforming the incoming speed to a different
outgoing speed.
Retard Coil - Choke coil.
Reverse Battery Supervision - A form of supervision
in which supervisory signals are furnished
from the terminating end of the originating
end by reversing the direction of current
flow over the trunk.
Ring - 1. A ring-shaped contacting part, usually
placed immediately in back of the tip.
2. The negative or battery side of a telephone line.
Ringdown - Signaling methods used to operate
receiver's ringer.
Ringing - The production of an audible or visible
signal at a station or switchboard by means
of an alternating or pulsating current.
Ringing Tone - Tone sent to a caller to indicate that
that called line is being signaled.
Ring Side - That conductor of a circuit which is
associated with the ring of a plug or the ring
spring of a jack.
NOTE: By extension, it is common practice
to designate by these terms the conductors
having similar functions or arrangements
in circuits where plugs or jacks may not
be involved.
RM - Record Mark.
RO - Receive Only - A receive only page printer.
ROPP - Receive Only Page Printer - A teleprinter
unit with prjnter only for stations which do
not generate messages.
Rotor - Phonetic term for ROTR.
ROTR - Receive Only Typing Reperforator - A paper
tape punch which also types the received
character on the edge of the tape.
Routing - The assignment of the communications
path by which a message or telephone call
will reach its destination.
Routing, Alternate - Assignment of a secondary
communications path to a destination when
the primary path is unavailable.
Routing, Indicator - An address, or group of characters, in the header of a message defining
the final circuit or terminal to which the
message has to be delivered.
Routing, Message - The function of selecting the
route, or alternate route, if required, by
which a message will proceed to its destination.
Sometimes used in place of message switching.
RPT - Repeat.
RS - Request to Send (to Data Set, elicits response
of Clear to Send - CTS).
RT - Reperforator /Transmitter - An integrated unit
consisting of paper tape punch and paper tape
reader for temporary storage of traffic for
RTL - Receive Transmit Leader.
RTTY - Radio telecommunications. A method of
telegraph communication using a radio link.
Running Open - In telegraph applications, a term
used to describe a machine connected to an
open line or a line without battery. A telegraph receiver under such a condition
appears to be running, as the type hammer
continually strikes the type box but does not
move across the page.
SCL - Send Control Leader.
SCR - Serial Clock Receive (data set interface).
SCT - Serial Clock Transmit (data set interface).
SCU - Station Control Unit.
SDC - Send Data Condition.
Sectional Center - A control center connecting primary centers together - a class 2 office.
Seizure - A term used in automatic (dial) telephony
to indicate the establishment of an electrical
connection through the operation of the first
Selective Calling - A form of telegraph communications system. One loop may include several
machines but, with selective calling, only
the machine selected will respond. The
device that controls the individual machines
in response to a selective call (CDC) is
called a stunt box.
Selector - In automatic (dial) telephony, a switch
which in response to dial pulses, hunts for
and chooses an idle circuit to extend a call.
Selector Permanent - A permanent condition in a
selector caused by a short circuit in the
calling line loop or by failure of the calling
telephone user to operate the dial immediately after hearing dial tone.
Semiautomatic Switching - Transfer of traffic from
incoming to outgoing circuits under operator
control (usually push -button).
Serial-By-Bit or Serial Transmis sion - Used to
identify a system wherein the bits of a character occur serially in time. Implies only a
single transmission line.
Serialize - To change from parallel-by-bit to serialby-bit.
Service, Extended Area - An exchange service without toll charges, which extends over an area
where there is a community of interest in
return for a somewhat higher exchange service rate.
Service, Private Line (Wire) - See Private Line,
Sideband - Frequency band above and below the
carrier frequency produced as a result of
Side Circuit - A circuit arranged for deriving a
phantom circuit. In 2-wire side circuits,
the conductors of each side circuit are
placed in parallel to form a side of the
phantom circuit. In 4-wire side circuits,
the lines of the two side circuits, which are
arranged for transmission in the same direction, provide a I-way phantom channel for
transmission in that same direction; the two
conductors of each line are placed in parallel
to provide a side for that phantom channel.
Similarly the conductors of the other two
lines provide a phantom channel for transmission in the opposite direction.
Side-Circuit Loading Coil - A loading coil for introducing a desired amount of inductance in a
side circuit and minimum amount of inductance in the associated phantom circuit.
Side-Circuit Repeating Coil (Side-Circuit Repeat
Coil) - A repeating coil that functions simultaneously as a transformer at a terminal of
a side circuit and as a device for superposing
one side of a phantom circuit on that side
Signal to Noise Ratio (SiN) - Relative power of signal to the noise in a channel.
Simplex Channel - See Channel, Simplex.
Simplex Circuit - A circuit derived from an existing
wire circuit by the use of center-tapped
repeating coil. This additional circuit must
use another wire conductor or ground return
to complete its path.
Simplex Operation - 1. A communication system or
network capable of transmitting or receiving
in one direction only. Not capable of being
2. (Obsolete) A method of
operation in which communication, i. e. ,
transmit-receive operations, take place in
one direction at a time. This meaning now
synonymous with Half Duplex.
Simultaneously - Occurring at substantially the same
instant of time. To be meaningful, the minimum time interval to be considered must be
Simultaneous Transmission - A communication
device capable of transmitting control characters or data on one pair of a Full Duplex
Circuit and receiving information on the
other pair at the same time.
Singing - Unstable oscillations on the line.
Single-Office Exchange - An exchange served by a
single central office.
Single Operation (Simplex Operation) - Operation of
a telegraph system in only one direction at
a time.
Single-Wire Line - A transmission line that utilizes
the ground as one side of the circuit.
Slice - To remove those parts of a wave form lying
outside two given amplitude limits on the
same side of the zero axis.
Slicer - A circuit which effectively amplifies a portion of the incoming pulses lying between
two closely spaced amplitude levels.
Smoothline - See Non-Loaded Lines.
SOTUS - In 8 1 D 1 systems, a station control device
located at each station on a multi-station
line. It is the selecting device at each
Source Recording - The recording of data in punched
cards, punched paper tape, magnetic tapes,
etc. Once in this form, the data may be
transmitted, processed or reused without
SND - Send.
Sneak Current - A leakage current that gets into
telephone circuits from other circuits. It is
too weak to cause immediate damage, but
can produce harmful heating effects if allowed to continue.
SNG - Sending.
Solid State - Utilizing transistors, diodes, and
pressure components only; excludes tubes,
relays, and other electromechanical devices.
Sounder - A telegraph receiving instrument in which
an electromagnet attracts an armature each
time a pulse arrives. The armature makes
an audible sound as it hits against its stops
at the beginning and end of each current impulse, and the intervals between these sounds
are translated from code into the received
message by the operator.
Space - The absence of a bit or signal on the line.
In telegraph applications, it is an impulse,
which in a neutral circuit, causes the loop
to open; or in a polar circuit, causes the
loop current to flow in a direction opposite to
that for a mark impulse.
Space-Hold - The normal no-traffic line condition
whereby a steady space is transmitted. May
be a customer selectable option.
Spacing Bias - Bias distortion which lengthens the
spacing impulse by delaying the space-tomark transition. See Distortion, Bias.
Spacing End Distortion - End distortion which
lengthens the spacing impulse by advancing
the mark-to-space transition. See Distortion, End.
Space-to-Mark Transition - The transition, or
switching, from a spacing impulse to a marking impulse. Also may denote a type of bias
SPX - Simplex.
Standard Test-Tone Power - 1 milliwatt (0 dbm) at
1,000 cps.
Standby - Condition of equipment which will permit
complete stable operation within a short
period of time.
Start Element - The first element of a character in
certain serial transmission, used to permit
Start-Stop Transmission - Asynchronous signaling
utilizing a start bit preceding the character
and a stop bit following the character for
synchronization purposes.
Station Battery - The electrical power source for
signaling in telegraphy.
Step-by-Step Automatic Telephone System - A switching system characterized by the use of successive step-by-step selector switches
actuated by current impulses produced by
rotation of a telephone call. The selectors
are electromagnetically operated and contain
a number of tiers of fixed contacts with each
tier arranged in a semicircle. A moving
contact arm first rises to the height of the
desired tier, then swings around horizontally
and stops over the required contact.
STL - Send Transmit Leader.
Stop Element - Stop Bit - The last element of a
character in asynchronous serial transmissions, used to insure recognition of next
start element. In telegraph applications, it
is 1. 42 times as long as the unit data bit.
Store and Forward - The interruption of data flow
from the originating terminal to the designated
receiver by storing the information enroute,
in physical media (punched card or tape, etc.)
and forwarding it at a later time. See Switching Message.
STR - Synchronous Transmitter Receiver - This is
the transmitted receiver section of the 1009,
1013, 7702, Autodin and other medium speed
IBM Teleprocessing equipment.
Stunt Box - A device used in telegraph terminals to
perform nonreadout functions such as carriage return, line feed, ring singal bell,
answer CDC's and TSC's, etc.
SUB RCD - Substitute Record (Replaces Characters
of Bad Parity).
Subscriber's Loop - See Local Loop
Subset - 1. In communications, a subscriber set,
such as a telephone.
2. A modulation and demodulation device.
See Data Set.
Sub-Voice Grade - Denotes a communications channel
wider than telegraph but narrower than voice
grade channels. Considered capable of transmitting data at from 180 to 600 bits/second.
Superposed Circuit - An additional channel obtained
from one or more circuits, normally provided for other channels, in such a manner
that all the chann~ls can be used simultaneously without mutual interference.
Superposed (Superimposed) Ringing - Party-line
ringing in which a combination of alternating
and direct currents is utilized, the direct
currents, of both polarities, being provided
for selective ringing.
Supervisory Control - A system for the selective
control and automatic indication of the operation of remotely located units by electrical
means, over a relatively small number of
common transmission lines. Carrier current channels on power line can be used for
this purpose.
Supervisory Relay - A relay which, during a call, is
controlled by the transmitter current supplied
to a subscriber line to receive from the associated station signals that control the actions
of operators or switching mechanisms.
Supervisory Signal - A signal for attracting the
attention of an attendant to a duty.
Suppressed Carrier Transmission - That method of
communication in which the carrier frequency
is suppressed either partially or to the maxi~um degree possible. One or both of the
sidebands may be transmitted.
Switch Hook - A switch on a telephone set, associated with the structure supporting the receiver or handset. It is operated by the
removal or replacement of the receiver or
handset on the support.
Switching Center - Location where incoming data
from one circuit is transferred to the proper
outgoing circuit.
Switching Center, Automatic Message - A location
where an incoming message is automatically
directed to one or more outgoing circuits
according to intelligence contained in the
Switching Center, Semi-Automatic Message - A
location where an incoming message is
displayed to an operator who directs the
message to one or more outgoing circuits
according to information read from the
Switching Center, Torn Tape - A location where
operators tear off the incoming printed and
punched paper tape and transfer it manually
to the proper outgoing circuit.
Switching Central - An installation in a wire system
where telephone or teletypewriter switchboards are installed to interconnect telephone or teletypewriter circuits.
Switching, Circuit or Line - A switching technique
where the connection is made between the
calling party and the called party, prior to
the start of a communication.
Switching, Message - The technique of receiving a
message, storing it until the proper outgoing
circuit is available, and then retransmitting
Switching Through - Extension of a calling line loop,
clear of attachments, through one switch to
a succeeding switch of a switch train. Sometimes referred to as Cutting Through, or
Switching-Through Relay - A control relay of a linefinder, selector, connector, or other stepping
switch, which extends the loop 9f a calling
telephone through to the succeeding switch in
a switch train. Usually designated relay D.
Switching Trunk (LD) - Trunk from a long distance
office to a local exchange office used for
completing a long distance call.
Switch Room - That part of a central office building
that houses switching mechanisms and associated apparatus.
Switch Train - In telephone, a sequence of switches
through which connection must be made to
establish a circuit between a calling telephone
and a called telephone.
Synchronize - To lock one element of a system into
step with another. Usually refers to locking
a receiver to a transmitter, or can refer to
locking data terminal equipment bit rate to
data set frequency.
Synchronous - Having a constant time interval between successive bits, characters or events.
Synchronous Transmission - Continuous bit-stream
transmission, with no start-of-character
System - An over-all term used to describe communication facilities from an engineering
aspect, including all the associated equipment.
Tandem - Method of interconnecting central offices
by trunks when the central offices do not
have trunks directly to each other.
Tandem Central Office - A central office primarily
used as a switching point for traffic between
other central offices.
Tandem Completing Trunk - A trunk, extending from
a tandem office, used as part of a telephone
connection between stations.
Tandem Office - A central office used exclusively
for interconnection of other end offices within the same exchange and near-by exchanges.
Handles no direct connection to subscribers.
A class 5 exchange.
Tandem Trunk - A trunk, extending from a central
office to a tandem office, used as part of a
telephone connection between stations.
Tape Chadless - A tape used in printing telegraphy /
teletypewriter operation. The perforations
are not completely severed from the tape,
thereby permitting the characters representing the perforations in the tape to be printed
on the same tape.
Tape Relay - A method of receiving and retransmitting messages in tape form.
Tariff - 1. The published rate for a particular approved commercial service of a common
2. A list of the specifications for a service.
TD - Transmitter - Distributor - The device in a
telegraph terminal which makes and breaks
the line in timed sequence. Modern usage of
the term refers to a paper tape transmitter.
TEL - Request for coordinating communication between terminals.
Telautograph - A writing telegraph instrument, in
which movement of a pen in the transmitting
apparatus varies the current in two circuits
in such a way as to cause corresponding
movement of a pen at the remote receiving
instrument. Also called Telewriter.
Telecommunication - Transmission or reception of
signals, writing, sounds, or intelligence of
any nature, by wire, radio, visual or other
electromagnetic systems. Often used interchangeably with Communication.
Telectrograph - A system of phototelegraphy in which
the original subject copy is composed of lines
on a metal base, prepared by photograph
through a single screen of parallel lines. A
metal stylUS moving over the plate makes
contact with the metal base only in regions
corresponding to dark areas of the picture,
so that current flows for dark portions. At
the receiver, this current makes a colored
mark by electrolytic action on paper moving
Telegraph - System of communication by coded signals. As used today, hand sending and ear
receiving are usually implied.
Telegraph Channel - A channel suitable for the
transmission of telegraph signals.
NOTE: Three basically different kinds of
telegraph channels used in multichannel telegraph transmission are:
1. One of a number of paths for simultaneous
transmission in the same frequency range
as in bridge duplex, differential duplex,
and quadruplex telegraphy.
2. One of a number of paths for simultaneous
transmission in different frequency ranges,
as in carrier telegraphy.
3. One of a number of paths for successive
transmission, as in multiplex printing
telegraphy. Combinations of these three
types may be used on the same circuit.
Telegraph Circuit - The complete circuit over which
signal currents flow between transmitting
and receiving apparatus in a telegraph system:
It sometimes consists of an overhead wire or
cable and a return path through the ground.
Telegraph Equipment Abbreviations·ASR
Automatic Send and Receive
Keyboard Send and Receive
Receive Only
Receive Only Typing ReperROTR
/Tran smitter
Telegraph Switching Systems - Some of the existing
switching systems are:
Full Duplex
Half Duplex
Full Duplex
Semiautomatic System
Western Union
Full Duplex
Plan 57
Plan 111
Plan 115
Half Duplex
Plan 117
Full Duplex
Telegraph Transmission Speed - The rate at which
signals are transmitted, and may be measured by the equivalent number of dot-cycles
per second or by the average number of
letters or words transmitted and received
per minute.
NOTE: A given speed in dot-cycles per
second (often abbreviated to dots per second)
may be converted to bauds by multiplying by
2. The baud is the unit of signaling transmission speed recommended by the International Consultative Committee on Telegraph Communication. Where words per
minute are used as a measure of transmission speed, five letters and·a space per word
are assumed.
Telemeter - To transmit digital and/or analog data
by radio waves, e. g. , data can be telemetered from a missile and recorded at a ground
Telephony - The transmission of speech current
over wires, by means of which two persons
can converse effectively over a distance.
Teleprinter - Trade name used by Western Union to
refer to its telegraph terminal equipment.
Teletype - Trademark of the Teletype Corporation.
A system for transmitting messages over
some distance employing keyboard or paper
tape sending and printing receiving.
Teletypewriter - Trade name used by AT&T to refer
to its telegraph terminal equipment.
Teletypewritcr Exchange Service (TWX) - A forn1 of
teletypewriter service in which suitably
arranged teletypewriter stations are provided
with lines to a central office where connections
can be established between any such stations
and any similar station (in the same city or
in other cities) under control of the subscriber provided by AT&T.
Telewriter - See Telautograph.
Telex (TEX) - An automatic Teletype Exchange
Service provided by Western Union.
Telpak - Telpak service makes available wide-band
communication channels of various sizes
suitable for large-volume point-to-point
transmission of data, voice, teletypewriter,
facsimile, or other services for a flat rate
regardless of usage. Telpak can be used as
a single large channel or a group of smaller
channels. For example, a Telpak "A" channel may be divided into 12 voice-grade channels. See Carrier System.
Terminal, Job Oriented - A terminal specially
designed to receive source data in an environment associated with the job to be performed, and capable of transmission to and
from the system of which it is a part.
Terminal Repeater - A repeater for use at the end
of a trunk or line.
Terminal Room - A room, associated with a central
office, private branch exchange or private
exchange, which contains distributing frames,
relays, and similar apparatus except that
mounted in the switchboard sections.
Terminal Unit - Communication-channel equipment
that can be used for either input or output.
Terminated Line - A transmission line terminated
in a resistance equal to the characteristic
impedance of the line, so there is no reflection and no standing waves.
Test Board - Switchboard equipped with apparatus
for making tests and for temporary interconnection and rearrangement of circuits.
Test, Busy - Test to determine if circuits are
available and may be taken for use.
Test Desk - In exchange testing, a board with trunks
to distributing frames and testing apparatus
for testing users' lines, switching trunks,
and cable pairs.
Test Handset (Hand Test Set) - A handset used for
test purposes in a central office or in the
outside plant. It may contain in the handle
other components in addition to the transducer; for example, a dial, keys, capacitors,
and resistors.
Test Tone - Tone used in identifying circuits or for
trouble location.
Test-Tone Power, Standard - See Standard TestTone Power.
TEX - See Telex.
Text - The information portion of a message.
Three-Row Keyboard - Utilized for transmission of
5-level codes and requiring figures/letters
shifts. As opposed to four row.
Tie Line - A leased channel, usually voice-gr~de.
Tie Trunk - A telephone line or channel directly
connecting two branch exchanges.
Tip - The tip of a plug is the contacting part at the
end of the plug.
Tip Side (Tip Wire) - That conductor of a circuit
which is associated with the tip of a plug, or
the tip spring of a jack.
NOTE: By extension, it is common practice
to designate by these terms the conductors
having similar functions or arrangements in
circuits where plugs or jacks may not be
TL - Transmit Leader.
Toll - 1. A charge made for a connection beyond an
exchange boundary.
2. By extension, any part of telephone plant,
circuits, or service for which toll charges
are made.
Toll Center - Toll Office - Toll Point - Basic toll
switching entity. A central office where
channels and toll message circuits terminate.
Usually this is one particular central office
in a city; however, larger cities may have
several central offices where toll message
circuits terminate. A class 4 office.
Torn Tape Switching - Manual transfer of incoming
messages in the form of punched paper tape
to outgoing circuits for retransmission to
their destination. See Switching Center,
Torn Tape.
Touch-Tone Dialing - The use of keys or pushbuttons
instead of a rotary dial to generate a sequence
of digits to establish a circuit connection.
TR - Transmit.
Traffic (Communication) - All transmitted and
received messages.
Train - Sequence of pieces of apparatus joined to- \
gether to forward or complete a call. See
Switch Train.
TRAN - Transmit.
Transceiver - A terminal which is capable of both
transmitting and receiving traffic.
Translator - Devices used in cross-bar systems
which change information coded by one system
to equivalent information coded by .another
system. It may convert a multifrequency
dialing code to a cross-bar marker code, a
3-digit area code to a special local code, or
an equipment number to a telephone number.
Transmission - The electrical transfer of a signal,
message, or other form of intelligence from
one location to another.
Transmission Level - The transmission level of the
signal power at any point in a transmission
system is the ratio of the power of that point
to the power at some point in the system
chosen as a reference point. This ratio is
usually expressed in decibels. The transmission level at the transmitting switchboard
is frequently taken as the zero level reference
point. See Decibel.
Transmission Section (Communications) - One of two
or more portions of a long message, each of
which is transmitted separately. All transmission sections of the same complete message use the same date-time group'.
Transition - The instance of switching from one
state (e. g. , positive voltage) to a second
state (negative) in a serial transmission.
Translators - Code conversion devices which may
generate a character sequence in response
to a single character, e. g., $ = dlrs.
Transposition - Interchanging the position of open
wire conductors to reduce noise and interference.
Transposition Section - A length of open-wire line to
which a fundamental transposition design or
pattern is applied as a unit.
Transverse Cross Talk Coupling - Between a disturbing and a disturbed circuit in any given
section, the vector summation of the direct
couplings between adjacent short lengths of
the two circuits, without dependence on intermediate flow in other nearby circuits.
Triplex System - A telegraph system in which two
messages in one direction and one message
in the other direction can be sent simultaneously over a single circuit.
Trouble, Ticket - In testboard work, a small form
for reporting any circuit condition that requires a testboardman' s attention.
Trouble, Unit (LD) - Weighting figure applied to
circuits or circuit to indicate their expected
performance in a given period of time.
Trunk - A channel connecting switching centers or
exchanges, as opposed to local loops.
Trunk Group - Consists of those trunks between two
pOints both of which are switching centers
and/or individual message distribution points,
and which employ the same multiplex terminal
Trunk Hunting - A method of switching incoming
calls to the next consecutive number if the
first called number is busy.
TSC - Transmitter Start Code - Usually a two-letter
call that is sent to an outlying telegraph terminal which automatically turns its. tape
transmitter on.
TT - Transmit Trailer.
TTY - Teletypewriter equipment.
Turn Around Time - The actual time required to
reverse the direction of transmission of a
haU-duplex channel, data set and terminal.
(Nominally 200 milliseconds.)
Two-Tone Keying - See Frequency Shift.
Two-Wire - See Channel, Two Wire.
Two-Way Trunk - Traffic originating either end.
TWX Service - See Teletypewriter Exchange Service.
Unattended Operation - A data set option allowing a
data call to be automatically answered by the
receiving station. As opposed to attended
Unbalanced to Ground - When applied to a cable pair
or two wire line it means that the impedance
to ground as measured from one side of the
line is not equal to the impedance to ground
as measured from the other side of the line.
Unbalanced to ground in cable pairs can make
the unbalanced pairs susceptible to noise and
cross talk and can cause the unbalanced pairs
to cross talk to other pairs.
Unipolar - See Neutral Circuit.
User's Set - Apparatus located on the premises of a
user of a communication or signaling service
and designed to function with other parts of a
Validity Check - A check for accuracy of character
Variolosser I - A device whose loss can be controlled
by a voltage or current.
VOGAD - Voice-Operated Gain-Adjusting Device A device somewhat similar to a compandor
and used on some long radio systems. A
voice-operated device which removes fluctuation from input speech, and sends it out at a
constant level. No restoring device is needed
at the receiving end.
Voice- Frequency Carrier Telegraphy - That form of
carrier telegraphy in which the carrier currents have frequencies such that the modulated
currents may be transmitted over a voicefrequency telephone channel.
Voice- Frequency Telegraph System - A telegraph
system permitting use of up to 18 channels on
a single circuit. A different audio frequency
generated by a tuning-fork-controlled vacuumtube oscillator is used for each channel, being
keyed in the conventional manner. The various audio frequencies at the receiving end are
separated by suitable filter circuits and fed
to their respective receiving circuits.
Voice-Grade Channel - A channel which permits
transmission of speech.
Voice-Operated Device - A device used on a telephone
circuit, the object of which is to permit the
presence or some quality of the telephone
currents to effect a desired control. This device is used in most echo suppressors.
VRC - Vertical Redundancy Check.
VU - Voice Units - The gross amplitude of an electrical speech or program wave is called its
volume and is expressed in VU' s. The reference volume is usually 0 VU which is one
millivolt of steady sine wave into 600-ohm
resistive load. A good volume is usually
between -10 to -30 VUe Anything over 0 VU
is definitely too loud; anything under -55 VU
is definitely too soft. Readings will depend
on the meter's frequency response and calibration criteria.
WATS - See Wide-Area Telephone Service.
Way Station - Western Union terminology for a
station on a multipoint circuit.
Wide-Area Telephone Service (WATS) - This service
is arranged for subscribers who make many
outgoing long distance calls to many points.
Monthly charges are based on the size of the
area in which the calls are placed, not on the
number or length of calls. Under the WATS
arrangement, the United States is divided into six zones. The subscriber is billed a flat
rate according to the zones to be called on a
full time or measured time basis. This can
be an advantageous arrangement for data
Willful Intercept - The act of intercepting messages
intended for stations having equipment or
line trouble. See Miscellaneous Intercept.
WM - Word Mark.
Word - 1. In telegraph, six operations or characters
(five characters plus one space). (Group is
also used in place of word.)
2. In computing, an ordered set of characters which is the normal unit in which information may be stored, transmitted, or operated upon within a computer.
WPM - Words Per Minute - Generally six characters
(including space) per word.
WRS - Working Reference System - An actual system
working under specified conditions and used
as a reference for other telephone systems.
Zero Transmission Level Reference Point - An
arbitrarily chosen point in a circuit to which
all relative transmission levels are referred.
The transmission level at the transmitting
switchboard is frequently taken as the zero
transmission level reference point. See
Relative Transmission Level.
Miscellaneous Grouping
o - Phase.
4W/HD - Four Wire Half Duplex - IBM term only.
Does not refer to communication line serviees.
L21A - Transmission Measuring Set.
WE2B - Noise Measuring Set - A device for measuring noise in telephone channels. It is speech
related and employs weighted networks to
simulate the action of the ear.
901 Data Test Set - This is an interface adapter
designed to make out-of-service installation
and routine tests on all Bell System data sets
to date. The 901 Data Test Set includes an
"adapter" which also permits in-service
monitoring and measuring of signals in the
interface between the data set and business
902 Data Test Set - This is a portable test set designed primarily for the measurement of
peak distortion, and for the detection of transmission errors in serial data sets. The 902
data set is intended to be connected to, and
operated in conjunction with, the 903 Data
Test Set (63-bit word generator).
903 Data Test Set - This is a self-contained, portable
signal generator used to simulate business
machine outputs. It provides a 63-bit binary
word suitable for testing binary serial-type
data sets. The 903 Data Test Set also generates alternating-mark space signals
called a dot pattern.
Acknow ledgements
The following sources were utilized in preparing this
Electrical Communications Systems Engineering
Definitions and Abbreviations. Department
of the Army Technical Manual, TMll-486-11.
Department of the Army, Washington 25,
D. C., 18 January 1957.
"Glossary", Data Systems Design Vol, 1, No. 11,
November, 1964. Pages 4, 8.
Holmes, James E. Communications Dictionary.
New York. John F. Rider Publishers, Inc.,
a Division of Hayden Publishing Company,
Inc., 1962.
McKenzie, Alexander A. "New Era in Telephony:
Electronic Switching," Electronics, October
19, 1964, Pages 72-85.
Rexroad, William D. Teletypewriter Fundamental
Handbook. West Concord, Massachusetts.
Computer Design Publishing Company, 1965.
Miscellaneous IBM Publications.
1. 1941 with the IBM 040 and IBM 057
2. a. To provide rapid transfer of source data
to processing location.
b. To provide accurate, timely reports to
management for decision making.
c. To accomplish a and b with r.educed time,
effort, and expense.
3. Telephone, Telegraph
4. 60, 75, 100
5. 48, 60, 80
6. 21,600
7. IBM Start-Stop (Asynchronous), Synchronous,
Parallel Data, Telegraph
8. 2260
9. 7770 or 7772
f - - - 5 Miles ------l
(a) Purpose
(b) Number and size of messages
(c) Priority of messages
(d) Accuracy required and cost of system
3. Message Switching (through DPC-B), Circuit
Switching (direct)
4. (a) Single drop
5. True
6. 900 (30 per TI, 30 TI's per line) 30 x 30 = 900
7. (a) Reliable reservation control
(b) Advance hotel reservations for passengers
(c) Advance car rental arrangements
(d) Special meal' considerations, medical
conveniences, etc.
8. (a) Maintenance scheduling
(b) Accurate crew time logging
(c) Weather Information Control
(d) Accurate control of seats, arrivals,
9. No, only through the DPC.
10. Federal Communications Commission, State
Public Utilities Commissions.
11. Use of IBM equipment for transmitting and
receiving data at locations physically removed from
each other utilizing Te~ephone, Telegraph and Radio
communication facilities.
1. parallel
2. serial
3. A signal which causes a terminal to change
from a transmitter to a receiver or from a receiver
to a transmitter.
4. Modulation is the method of. causing one signal to vary, depending upon the characteristics of
another signal, for the purpose of conveying intelligence. (In one case, in a form acceptable by the
channel. )
5. parallel
6. serial
7. modulated
8. demodulates, serial
9. parallel, 4 of 8, BCD
10. (a) Punched cards
(b) Magnetic tape
(c) Punched paper tape
(d) Visual display
(e) Printed copy
Disk or drum file
11. to provide a dividing line of responsibility
12. Interface specifications publication from
data set manufacturer
13. serialization
14. de serialization
15. For the purpose of checking validity by
agreement of odd/even count of bits in message
16. start of record
17. inquiry
18. acknowledgement
19. (a) Inquiry
(b) End of transmission or message
(c) Aclmowledgement
Channel, circuit, line
b, c, a
Telegraphic, Subvoice, Voice, Broadband
Baudot, 5, start, stop
Yes, one
No, . Zero
7. Asynchronous signaling requires th..:.t each
character be prefixed by a Start bit and followed by
a Stop bit while special clocking circuits make this
wasteful use of time unnecessary by controlling beginning and ending of characters based on a count.
8. Serial bit transmission disassembles a character and sends it one bit at a time over one circuit,
while parallel-bit transmission requires one circuit
for each bit value and sends the entire character at
9. Herzians - abbreviated Hz
10. 3,000
11. True
12. False, see Figure 14 for example.
13. Modulation is the process of causing one
signal to vary its form, dependent upon the characteristics of another signal.
14. To provide a buffer zone between channels to
minimize the possibility of interference between
channels (crosstalk)
15. a. Amplitude, b. Frequency (FSK or FSM),
c. Phase (PSK or PSM), specifically t,his is 2 phase
16. Federal Communications Commission
17. Bit rate (speed)
18. a. Telegraphic, b. Subvoice, c. Voice
grade, d. Subvoice
19. 2.2V P-P, 1.1V P-P, 0.87V P-P
20. a. To regenerate distorted signals into good
quality signals
b. To improve signal to noise ratio and compensate for line loss
21. Echo suppressors. The time required from
end of sending in one direction until transmission
may begin in the opposite direction. Reduces
throughput due to idle line time.
22. 1. Crosstalk by maintaining more uniform
signal levels on line
2. Noise effects reduced
23. great, little, compression
24. Distributed
25. Compensators
26. .001, 600
27. Low noise, low or no echo problem, better
frequency response, handle slightly higher data
rates with a lower error rate than ordinary lines.
28. Channel
29. Modulation, De-modulation
1. Queueing is the procedure of holding messages in a programmed sequence ready to be forwarded to another destination.
2. Message switching is store and forward
while circuit switching is initial routing control to
addressee direct without intermediate receive and
3. Contention
4. Polling is direction for the terminal to send
to the control station. Addressing is direction from
control station to terminal to receive messages.
5. The message itself
6. b. skip that terminal and proceed to process
the remainder of the Polling List.
7. False - Customer may establish sequence
8. False
9. False - May vary to meet priority or peakload requirements
10. False - Use intercept procedure to route to
nearest active terminal
11. False - Sequence and Journal Control
12. False - Priority coding can alter Queue
13. a. Flagging
b. Retention of priority addresses in main
c. Transmit prior to logging,
d. Priority Queues
14. a. Source of error check information
b. Retrieval file if message gets lost
15. False - Must have proper access key. Example: Normally, production department should not
be able to have access to Payroll data
16. While terminals are being addressed or polled
they are in control mode and such data is usually not
stored. Usually begin accumulating LRC in text
mode and in storing or outputting data.
1. Serialize - Deserialize
2. Parallel, Serial
3. Serial, Parallel
4. Off
5. Slightly after bit time 8 (delay Ion)
6. So last bit will be still on line while new
character is being loaded (after T's are all reset at
about 8.4 time)
7. BCD, 4 of 8
8. To allow time to place last bit of character
in Line Control before resetting register.
9. To make sure old character is cleared before
a new one is loaded
10. Mark
11. 8, parallel, tran slator
12. On
13. Counter Circuit reaches 8
15. Space, 4
16. Zero, space, 4
17. Counter circuit at 8
18. SERDES holds character and permits each
bit to be sampled to the line while shift register
moves character up through the registers while
·presenting the character one bit at a time to the line.
19. SERDES through bit timer permits each
storage element a definite period during which it
may be turned On or left Off under control of the
condition existing on the Receive Data Line. The
Shift Register sequentially samples each bit interval of the Receive Data Line to the TN while stepping the previously sampled bits and no bits up
through the register until the counter circuit informs
the machine that adequate time has been allowed and
a complete character is in the register.
1. Data Set
2. Frequencies
3. Mark
4. Phase
5. Bit errors
6. + (Plus), - (negative)
1. Bi-Polar
1. Provide communication circuits for public
use in accordance with prescribed schedules and
2. Federal Communications Commission
3. State Public Utilities Commission
4. Tariffs
5. Standardization, Coordination
6. a. Wide Area Telephone Service, b. Subscriber Teletypewriter Exchange
7. A subscriber teletypewriter service
9. facsimile
Please fill in the following blanks:
Rate of Understanding
List any technical errors you found in this manual
Were you given your own copy of this manual to write in and keep '?
How many hours were required to complete this course?
Were you interrupted during this time?
Did you require assistance during this course?
If so, state in which area you required help.
List any additional comments you may have pertaining to this course.
Man No.
Branch Office
Date of Employment
Fold and staple or tape and return via IBM mail
For Form Z25-2522-0
IBM Corporation
FE Education
Department 911, Bldg 005-3
Poughkeepsie, N. Y. 12602
L _ _ _ _....I
Was this manual useful for you? yes no
Thank you for your participation!

* Your assessment is very important for improving the work of artificial intelligence, which forms the content of this project

Download PDF