Concepts of
Communication and
Networking
UNIT 4 COMMUNICATION MEDIUMS
Structure
4.0
4.1
4.2
4.3
4.4
4.5
4.6
4.7
4.8
4.9
4.10
4.11
4.12
4.13
4.0
Digital Data Transmission
Objectives
Serial and Parallel Transmission
Guided and Unguided Mediums
Twisted Pair
UTP Cable
STP Cable
Coaxial Cable
Fiber Optic Cables
Unguided Mediums
Connectors
Summary
References/Further Reading
Solutions/Answers
Page No.
60
60
61
61
62
63
63
64
65
67
69
71
71
72
DIGITAL DATA TRANSMISSION
The term digital refers to the way it is conveyed: usually by a binary code consisting
of a long string of 1s and 0s. Digital transmission or digital communications is a
literal transfer of data over a point to point (or point to multipoint) link using
transmission medium –such as copper wires, optical fibers, wireless communications
media, or storage media. The data that is to be transferred is often represented as an
electro-magnetic signal (such as a microwave). Digital transmission transfers
messages discretely. These messages are represented by a sequence of pulses via a
line code. Digital data transmission can occur in two basic modes: serial or parallel.
The serial and parallel transmission is shown in Figure 1 below. Data within a
computer system is transmitted via parallel mode on buses with the width of the
parallel bus matched to the word size of the computer system. Data between computer
systems is usually transmitted in bit serial mode. Consequently, it is necessary to
make a parallel-to-serial conversion at a computer interface when sending data from a
computer system into a network and a serial-to-parallel conversion at a computer
interface when receiving information from a network. The type of transmission mode
used may also depend upon distance and required data rate.
4.1
OBJECTIVES
After going through this unit, you should be able to:
60
•
Know the concept of communication mediums
•
Differentiate between Serial and Parallel Transmission
•
Differentiate between Guided and Unguided Mediums
•
Know the features and limitations of different wired mediums
•
Understands the use of Twisted Pair, Coaxial and Fiber Optic Cables
•
Know the functions of Unguided Mediums
•
Understand the use of different connectors
4.2
SERIAL AND PARALLEL TRANSMISSION
Communication
Mediums
Serial Transmission: In serial transmission, bits are sent sequentially on the same
channel (wire) as shown in Figure 1, which reduces costs for wire but also slows the
speed of transmission. Also, for serial transmission, some overhead time is needed
since bits must be assembled and sent as a unit and then disassembled at the receiver.
Serial transmission can be either synchronous or asynchronous. In synchronous
transmission, groups of bits are combined into frames and frames are sent
continuously with or without data to be transmitted. In asynchronous transmission,
groups of bits are sent as independent units with start/stop flags and no data link
synchronization, to allow for arbitrary size gaps between frames. However, start/stop
bits maintain physical bit level synchronization once detected.
In parallel transmission, multiple bits (usually 8 bits or a byte/character) are sent
simultaneously on different channels (wires, frequency channels) within the same cable
as shown in Figure 1, or radio path, and synchronized to a clock. Parallel devices have a
wider data bus than serial devices and can therefore, transfer data in words of one or
more bytes at a time.
Figure 1: Serial and parallel communication
As a result, there is a speedup in parallel transmission bit rate over serial transmission
bit rate. However, this speedup is a tradeoff versus cost since multiple wires cost more
than a single wire, and as a parallel cable gets longer, the synchronization timing
between multiple channels becomes more sensitive to distance. The timing for parallel
transmission is provided by a constant clocking signal sent over a separate wire within
the parallel cable; thus parallel transmission is considered synchronous.
4.3
GUIDED AND UNGUIDED MEDIUMS
Figure 2: Classification of Transmission Mediums
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Concepts of
Communication and
Networking
Transmission Media: The transmission medium is the physical path between
transmitter and receiver in a data transmission system. Transmission media can be
classified as guided or unguided as depicted in Figure 2. With guided media, the
waves are guided along a solid medium, such as twisted pair, coaxial cable, and
optical fiber. The atmosphere and outer space are examples of unguided media that
provide a means of transmitting electromagnetic signals but do not guide them; this
form of transmission is usually referred to as wireless transmission.
The characteristics and quality of a data transmission are determined both by the
characteristics of the medium and the characteristics of the signal. In the case of
guided media, the medium itself is more important in determining the limitations of
transmission.
For unguided media, the bandwidth of the signal produced by the transmitting antenna
is more important than the medium in determining transmission characteristics. One
key property of signals transmitted by antenna is directionality. In general, signals at
lower frequencies are Omni-directional; that is, the signal propagates in all directions
from the antenna. At higher frequencies, it is possible to focus the signal into a
directional beam.
4.4
TWIATED PAIR
Twisted pair is most widely used media for local data distribution. Twisted-pair cable
is a type of cabling that is used for telephone communications and most modern
Ethernet networks. A pair of wires forms a circuit that can transmit data. The pairs are
twisted to provide protection against crosstalk, and noise generated by adjacent pairs.
When electrical current flows through a wire, it creates a small, circular magnetic
field around the wire. When two wires in an electrical circuit are placed close
together, their magnetic fields are the exact opposite of each other. Thus, the two
magnetic fields cancel each other out. They also cancel out any outside magnetic
fields. Twisting the wires can enhance this cancellation effect. Using cancellation
together with twisting the wires, cable designers can effectively provide self shielding
for wire pairs within the network media. The twisted pair cable is shown in Figure 3.
Figure 3: Twisted pair Cable
While twisted-pair cable is used by older telephone networks and is the least
expensive type of local-area network (LAN) cable, most networks contain some
twisted-pair cabling at some point along the network.
Since some telephone sets or desktop locations require multiple connections, twisted
pair is sometimes installed in two or more pairs, all within a single cable. For some
business locations, twisted pair is enclosed in a shield that functions as a ground. This
is known as shielded twisted pair (STP). Ordinary wire to the home is unshielded
twisted pair (UTP).
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4.5
Communication
Mediums
UTP CABLE
Unshielded twisted pair is the most common kind of copper telephone wiring. UTP
cable is a medium that is composed of pairs of wires. UTP cable is used in a variety
of networks. Each of the eight individual copper wires in UTP cable is covered by an
insulating material. In addition, the wires in each pair are twisted around each other
as shown in Figure 4 (a).
Figure 4: UTP and STP Cables
UTP cable relies solely on the cancellation effect produced by the twisted wire pairs
to limit signal degradation caused by electromagnetic interference (EM!) and radio
frequency interference (RFI). To further reduce crosstalk between the pairs in UTP
cable, the number of twists in the wire pairs varies. UTP cable must follow precise
specifications governing how many twists or braids are permitted per meter (3.28
feet) of cable.
4.6
STP CABLE
STP is similar to UTP in that the wire pairs are twisted around each other. STP also
has shielding around the cable to further protect it from external interference. The
shielding further reduces the chance of crosstalk but the shielding increases the
overall diameter and weight of the cable. The maximum segment length of STP cable
is 100 meters.
Shielded twisted pair is a special kind of copper telephone wiring used in some
business installations. An outer covering or shield is added to the ordinary twisted
pair telephone wires; the shield functions as a ground. The STP cable is shown in
figure above in Figure 4(b).
Shielded twisted-pair (STP) cable combines the techniques of shielding, cancellation,
and wire twisting. Each pair of wires is wrapped in a metallic foil. The four pairs of
wires then are wrapped in an overall metallic braid or foil. It is usually a 150-ohm
cable, as specified for use in Ethernet network installations. STP reduces electrical
noise both within the cable (pair-to-pair coupling, or crosstalk) and from outside the
cable (EMI and RFI).
Check Your Progress 1
1.
Define parallel transmission.
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Concepts of
Communication and
Networking
2.
List guided transmission mediums?
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3.
What are the advantages of STP over UTP?
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4.7
COAXIAL CABLE
Coaxial cable like twisted pair, consists of two conductors, but is constructed
differently to permit it to operate over a wider range of frequencies. It consists of a
hollow outer cylindrical conductor that surrounds a single inner wire conductor. The
inner conductor is held in place by either regularly spaced insulating rings or a solid
dielectric material. The outer conductor is covered with a jacket or shield. A single
coaxial cable has a diameter of from 0.4 to about 1 inch. Because of its shielding,
concentric construction, coaxial cable is much less susceptible to interference and
cross-talk than is twisted pair. Coaxial cable can be used over longer distances and
supports more stations on a shared line than twisted pair.
Coaxial cable is perhaps the most versatile transmission medium and has widespread use
in a wide variety of applications; the most important of these are
i)
Television distribution
ii)
Long-distance telephone transmission
iii)
Short-run computer system links
iv)
Local Area Networks
Coaxial cable is spreading rapidly as a means of distributing TV signals to individual
homes - cable TV. A cable TV system can carry dozens or even hundreds of TV
channels ranging up to a few tens of miles.
Coaxial cable has traditionally been an important part of the long-distance telephone
network. Today, it is getting replaced by optical fiber, terrestrial microwave, and
satellite. Using frequency-division multiplexing, a coaxial cable can carry over
10,000 voice channels simultaneously. Coaxial cable is also commonly used for
short-range connections between devices. Using digital signaling, coaxial cable can
be used to provide high-speed I/O channels on computer systems. A co-axial cable is
shown in Figure 5 below.
Figure 5: Coaxial cable
64
Another application area for coaxial cable is local area networks. Coaxial cable can
support a large number of devices with a variety of data and traffic types, over distances
that encompass a single building or a complex of buildings.
Communication
Mediums
Coaxial cable is used to transmit both analog and digital signals. Coaxial cable has
frequency characteristics that are superior to those of twisted pair, and can hence be
used effectively at higher frequencies and data rates. The principal constraints on
performance are attenuation, thermal noise, and inter modulation noise.
For long-distance transmission of analog signals, amplifiers are needed every few
kilometers, with closer spacing required if higher frequencies are used. The usable
spectrum for analog signaling extends to about 400 MHz. For digital signaling,
repeaters are needed every kilometer or so, with closer spacing needed for higher data
rates.
4.8
FIBRE OPTIC CABLES
Now day’s optical fiber is widely used as a back bone for network due to its higher
data transmission rate, lighter in weight, low interferences, less number of repeaters
required, long distance coverage etc. An optical transmission system has three
components; the light source, the transmission medium, and the detector.
Conventionally, a pulse of light indicates a bit 1 and absence of light indicates bit 0.
Transmission medium is an ultra-thin fiber of glass. The transmitter generates the
light pulses based on the input electrical signal. The detector regenerates the electrical
signal based on the light signal it detects on the transmission medium. By attaching a
light source to one end of an optical fiber and a detector to the other, we have an
unidirectional data transmission system that accepts an electrical signal, converts and
transmits it by light pulse, and then reconverts the output to an electrical signal at the
receiving end. Figure 6 given blow shows optical fiber cable.
Figure 6: Optical Fiber Cable
An optical fiber is a thin (2 to 125 nm – nano meter – 10-9 meter), flexible medium
capable of conducting an optical ray. Various glasses and plastics can be used to make
optical fibers. The lowest losses have been obtained using fibers of ultrapure fused
silica. Ultrapure fiber is difficult to manufacture; higher-loss multi-component glass
fibers are more economical and still provide good performance. Plastic fiber is even
less costly and can be used for short-haul links, for which moderately high losses are
acceptable.
An optical fiber cable has a cylindrical shape and consists of three concentric
sections: the core, the cladding, and the jacket. The core is the innermost section and
consists of one or more very thin strands, or fibers, made of glass or plastic. Each
fiber is surrounded by its own cladding, a glass or plastic coating that has optical
properties different from those of the core. The outermost layer, surrounding one or a
bundle of cladded fibers, is the jacket. The jacket is composed of plastic and other
65
Concepts of
Communication and
Networking
material layered to protect against moisture, abrasion, crushing and other
environmental dangers.
One of the most significant technological breakthroughs in data transmission has
been the development of practical fiber optic communications systems. Optical fiber
already enjoys considerable use in long-distance telecommunications. The continuing
improvements in performance and decline in prices, together with the inherent
advantages of optical fiber, have made it increasingly attractive for local area
networking and metropolitan networks. Optical fiber is of two types.
i)
Single mode optical fiber.
ii)
Multimode Optical Fiber.
Single mode optical fiber: Single mode uses step-index fiber and a highly focused
source of light that limits beams to a small range of angles, all close to the horizontal.
The fiber itself is manufactured with a much smaller diameter than that of multimode
fibers, and with substantially lowers density (index of refraction). The decrease in
density results in a critical angle that is close enough to 90 degrees to make the
propagation of beams delays are negligible. All of the beams arrive at the destination
“together” and can be recombined without distortion to the signal as depicted in
Figure 7 (c).
Multi-Mode: Multimode is so named because multiple beams from a light source
move through the core in different paths. How these beams move within a cable
depends on the structure of the core. Multi-mode is categorized into step-index
multimode and graded index mode.
1.
Step-index Mult-mode: In step-index multimode, the density of the core
remains constant from the center to the edges. A beam of light moves through
this constant density in a straight line until it reaches the interface of the core
and the cladding. At the interface there is an abrupt change to a lower density
that alters the angle of the beam’s motion. The term step-index refers to the
suddenness of this change. Figure 7 below shows various beams (or rays)
traveling through a step-index fiber. Some beams in the middle travel in straight
lines through the core and reach the destination without reflecting or refracting.
Some beams strike the interface of the core and cladding at an angle smaller
than the critical angle; these beams penetrate the cladding and are lost. Still
others hit the edge of the core at angles greater than the critical angle and reflect
back into the core and off the other side, bouncing back and forth down the
channel until they reach the destination.
Every beam reflects off the interface at an angle equal to its angle of incidence
as shown in Figure 7(a). The greater the angle of incidence, the wider the angle
of refraction. A beam with a smaller angle of incidence will require more
bounces to travel the same distance than a beam with a larger angle of
incidence. Consequently, the beam with the smaller incident angle must travel
farther to reach the destination. This difference in path length means that
different beams arrive at the destination at different times. As these different
beams are recombined at the receiver, they result in a signal that is no longer an
exact replica of the signal that was transmitted. Such a signal has been distorted
by propagation delays. This distortion limits the available data rate and makes
multimode step-index cable inadequate for certain precise applications.
2.
66
Graded-index Mode: A second type of fiber, called graded-index, decreases
this distortion of the signal through the cable. The word index here refers to the
index of refraction. As we saw above, index of refraction is related to density. A
graded-index fiber, therefore, is one with varying densities. Density is highest at
the center of the core and decreases gradually to its lowest at the edge.
Figure 7(b) shows the impact of this variable density on the propagation of light
beams.
Communication
Mediums
Figure 7: Types of Optical Fiber Cables
Check Your Progress 2
1.
List the applications of Coaxial cable.
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2.
What is Single mode optical fiber?
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4.9
UNGUIDED MEDIUMS
Unguided Media: Unguided media transport electromagnetic waves without using a
physical conductor. Signals are broadcast though air or water, and thus are available
to anyone who has a device capable of receiving them. The EM spectrum covers
frequencies from 3 Hz (ELF) to gamma rays (30 ZHz, Zetta Hertz - 1021 Hz) and
beyond (cosmic rays). But only frequencies ranging from 3 KHz to 900 THz are used
for wireless communication.
Propagation of Radio Waves: Radio technology considers the earth as surrounded
by two layers of atmosphere: the troposphere and the ionosphere. The troposphere is
the portion of the atmosphere extending outward approximately 30 miles from the
earth's surface. The troposphere contains what we generally think of as air. Clouds,
wind, temperature variations, and weather in general occur in the troposphere. The
ionosphere is the layer of the atmosphere above the troposphere but below space.
Unguided signals can travel from the source to destination in several ways. There is
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Concepts of
Communication and
Networking
ground propagation, sky propagation, and line-of-sight propagation. In ground
propagation, radio waves travel through the lowest portion of the atmosphere, hugging
the earth. These low-frequency signals emanate in all directions from the transmitting
antenna and follow the curvature of the earth. The distance depends on the power of
the signal. In Sky propagation, higher-frequency radio waves radiate upward into the
ionosphere where they are reflected back to earth. This type of transmission allows for
greater distances with lower power output. In Line-of-Sight Propagation, very high
frequency signals are transmitted in straight lines directly from antenna to antenna.
Antennas must be directional, facing each other and either tall enough or close enough
together not to be affected by the curvature of the earth.
Radio Waves: Radio wave frequencies are between 3 KHz to 1 GHz, and uses
omnidirectional antenna. Omniderectional antenna propagates signal in all direction.
This means that the sending and receiving antennas do not have to be aligned. But it
has disadvantage too, it is susceptible to interference wherein a radio wave
transmitted by one antenna may be interfered by another antenna that may send
signals using the same frequency or band.
Radio waves are used for multicast communications, such as radio (AM and FM
radio), maritime radio, television, cordless phones and paging systems.
Microwaves: Frequencies between 1 and 300 GHz are called microwaves.
Microwaves are unidirectional. When an antenna transmits microwave waves, they
can be narrowly focused. This means that the sending and receiving antennas need to
be aligned. Its advantage is that a pair of antennas can be aligned without interfering
with another pair of aligned antennas.
The propagation of microwave is line-of-sight. The problem with this propagation is
that towers that are far apart from each other need to be very tall. The curvature of the
earth as well as other blocking obstacles does not allow two short towers to
communicate. For long distance communication, repeaters are often needed. Another
disadvantage is that very high frequency microwaves cannot penetrate walls.
In a unidirectional antenna, there are two types: the parabolic dish and the horn. A
parabolic dish antenna is based on the geometry of the parabola. Every line parallel to
the line of symmetry reflects off the curve at angles such that all the lines intersect in
a common point called focus. The parabolic dish works as a funnel, catching a wide
range of waves and directing them to a common point.
A horn antenna on the other hand looks like a gigantic scoop. Outgoing transmissions
are broadcast up a stem and deflected outward in a series of narrow parallel beams by
the curved head. Received transmissions are collected by the scooped shape of the
horn, in a manner similar to the parabolic dish, and are deflected down into the stem.
There is another type of microwave transmission with the use of satellite relay. It
requires geo-stationary orbit with the height of 35,784km to match the earth’s
rotation. It has uplink that receives transmission on one frequency and a downlink that
transmits on a second frequency. It Operates on a number of frequency bands known
as transponders.
It can operate in two ways:
68
a)
Point to point- Ground station to satellite to ground station
b)
Multipoint (Broadcast link)- Ground station to satellite to multiple receiving
stations.
Microwaves are used in unicast communication such as cellular telephones, satellite
networks, and wireless LANs.
Communication
Mediums
Infrared Waves: Infrared signals with frequencies from 300 GHz to 400 THz
(wavelengths from 1 mm to 700 nm), can be used for short-range communication.
high frequencies cannot penetrate walls. This characteristic prevents interference
between one system and another; a short-range communication cannot be affected by
another system in the next room. The same characteristic makes infrared signals
useless for long range communication. Infrared waves cannot be used outside a
building because the sun’s rays contained infrared waves can interfere with the
communication. The infrared band, almost 400 THz, has an excellent potential for
data transmission. Such a wide bandwidth can be used to transmit digital data with a
very high data rate. The infrared Data Association (IrDA), an association for
sponsoring the use of infrared waves, has established a standard for using these
signals for communication between devices such as the keyboard, mice, PCs, and
printers. Infrared signals defined by the IrDA transmit through line of sight; the IrDA
port on the keyboard needs to point to the PC for transmission occurs.
4.10 CONNECTORS
The connectors are the interface for communication between computers/ computers to
hub, switch, router etc. In LAN basically used connector are discussed as follows:
1.
RJ-45 Connector: RJ stands for registered jack. RJ45 is a standard type of
connector for network cables. RJ45 connectors are most commonly seen with
Ethernet cables and networks. RJ45 connectors feature eight pins to which the
wire strands of a cable interface electrically. Standard RJ-45 pin-outs define the
arrangement of the individual wires needed when attaching connectors to a
cable. RJ-45 connectors are of two types: male RJ-45 and female RJ-45. The
Figure 8 shows RJ -45 connector.
Figure 8: RJ-45 connectors
2.
BNC connector: The BNC connector (Bayonet Neill–Concelman) is miniatures
quick connect/disconnect RF connector used for coaxial cable. It features two
bayonet lugs on the female connector; mating is achieved with only a quarter
turn of the coupling nut. BNCs are ideally suited for cable termination for
miniature-to-subminiature coaxial cable (e.g., RG-58, 59, to RG-179, RG-316).
It is used with radio, television, and other radio-frequency electronic equipment,
test instruments, video signals, and was once a popular computer network
connector. BNC connectors are made to match the characteristic impedance of
cable at either 50 ohms or 75 ohms. It is usually applied for frequencies below
3 GHz and voltages below 500 Volts.
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Concepts of
Communication and
Networking
Figure 9: BNC connectors
Fiber optic cable connector: Fiber optic cable connectors are hardware installed on
fiber cable ends to provide cable attachment to a transmitter, receiver or other cable.
In order for information to be transmitted efficiently, the fiber cores must be properly
aligned. They are usually devices that can be connected and disconnected repeatedly.
There are many types of fiber optic cable connectors also shown in Figure 10:
1.
ST Connectors: ST stands for Straight Tip. Slotted bayonet type connector
with long ferrule, a common connector for multi-mode fibers. The ST connector
has been the main stay of optical fiber connectors for many years. It can be
found in almost every communications room worldwide, but used mainly in
data communications systems. The simple to use bayonet locking mechanism
reduces the risks of accidental disconnection of fiber connections.
2.
SC (Standard Connector) Connectors: Push/pull connector that can also be
used with duplex fiber connection. The SC connector comprises a polymer
body with ceramic ferrule barrel assembly plus a crimp over sleeve and rubber
boot. These connectors are suitable for, 900µm and 2-3mm cables. The
connector is precision made to demanding specifications. The combination of a
ceramic ferrule with precision polymer housing provides consistent long-term
mechanical and optical performance.
3.
MT Connector: The MT-RJ connector is a development of the now legendary
MT ferrule. MT stands Multi-fiber Connector. The MT ferrule in its various
designs has the ability to connect anything from 2 fibers in the MTRJ to 72
fibers in the latest versions of the MPO connector.
Figure 10: Fiber optic cable connector
70
Check Your Progress 3
1.
Communication
Mediums
What are microwaves? Explain their properties.
……………………………………………………………………………………
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2.
What is BNC connector?
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3.
Explain the use of SC Connectors.
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4.11 SUMMARY
After completing this unit, you must have knowledge of different transmission
mediums, cables and connectors. In the beginning serial and parallel communication
is explained. In serial transmission, bits are sent sequentially on the same channel
(wire). In parallel transmission, multiple bits (usually 8 bits or a byte/character) are
sent simultaneously on different channels (wires, frequency channels) within the
same cable. In this unit, we have seen that transmission media can be classified as
guided or unguided. Twisted-pair cable is a type of cabling that is used for telephone
communications and most modern Ethernet networks. Coaxial cable like twisted pair,
consists of two conductors, but is constructed differently to permit it to operate over a
wider range of frequencies. Today’s optical fiber is widely used as a back bone for
network due to its higher data transmission rate, lighter in weight, low interferences,
less number of repeaters required, long distance coverage etc.. Optical fiber is of two
types i.e. Single mode optical fiber and Multimode Optical Fiber. Further medium of
communication is unguided. Unguided media transport electromagnetic waves
without using a physical conductor. Signals are broadcast though air or water, and
thus are available to anyone who has a device capable of receiving them. The
connectors are the interface for communication between computers/ computers to
hub, switch, router etc. In LAN basically used connector.
4.12 REFERENCES/FURTHER READING
1.
Computer Networks, A. S. Tanenbaum 4th Edition, Practice Hall of India, New
Delhi. 2003.
2.
Introduction to Data Communication & Networking, 3rd Edition, Behrouz
Forouzan, Tata McGraw Hill.
3.
Computer Networking, J.F. Kurose & K.W. Ross, A Top Down Approach
Featuring the Internet, Pearson Edition, 2003.
4.
Communications Networks, Leon Garcia, and Widjaja, Tata McGraw Hill,
2000.
5.
www.wikipedia.org
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Concepts of
Communication and
Networking
6.
Data and Computer Communications, Willian Stallings, 6th Edition, Pearson
Education, New Delhi.
7.
Larry L. Peterson, Computer Networks: A Systems Approach, 3rd Edition (The
Morgan Kaufmann Series in Networking).
4.13 SOLUTIONS/ANSWERS
Check Your Progress 1
1.
In parallel transmission, multiple bits (usually 8 bits or a byte/character) are
sent simultaneously on different channels (wires, frequency channels) within
the same cable
2.
Following are the guided transmission mediums
3.
i)
twisted pair,
ii)
coaxial cable,
iii)
optical fiber
STP is similar to UTP in that the wire pairs are twisted around each other. STP
also has shielding around the cable to further protect it from external
interference. The maximum segment length of STP cable is 100 meters.
Shielded twisted-pair (STP) cable combines the techniques of shielding,
cancellation, and wire twisting. Each pair of wires is wrapped in a metallic foil.
Check Your Progress 2
1.
2.
Following are the main applications of Coaxial cable.
i)
Television distribution
ii)
Long-distance telephone transmission
iii)
Short-run computer system links
iv)
Local Area Networks
Single mode uses step-index fiber and a highly focused source of light that
limits beams to a small range of angles, all close to the horizontal. The fiber
itself is manufactured with a much smaller diameter than that of multimode
fibers, and with substantially lowers density (index of refraction). The decrease
in density results in a critical angle that is close enough to 90 degrees to make
the propagation of beams delays are negligible.
Check Your Progress 3
1.
Frequencies between 1 and 300 GHz are called microwaves. Microwaves are
unidirectional. When an antenna transmits microwave waves, they can be
narrowly focused. This means that the sending and receiving antennas need to
be aligned. Its advantage is that a pair of antennas can be aligned without
interfering with another pair of aligned antennas.
The propagation of microwave is line-of-sight. The problem with this
propagation is that towers that are far apart from each other need to be very tall.
The curvature of the earth as well as other blocking obstacles does not allow
two short towers to communicate. For long distance communication, repeaters
are often needed. Another disadvantage is that very high frequency microwaves
cannot penetrate walls.
72
2.
3.
The BNC connector (Bayonet Neill–Concelman) is miniatures quick
connect/disconnect RF connector used for coaxial cable.
This is a fiber optics cable connector. Push/pull connector that can also be used
with duplex fiber connection. The SC connector comprises a polymer body
with ceramic ferrule barrel assembly plus a crimp over sleeve and rubber boot.
These connectors are suitable for, 900µm and 2-3mm cables. The connector is
precision made to demanding specifications.
Communication
Mediums
73
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