S38.115 Signaling Protocols Modeling of signaling systems Subscriber signaling
S38.115 Signaling Protocols
ü Modeling of signaling systems
ü Signaling flow charts
ü (Extended) Finite state machines
ü Subscriber signaling
ü Impulse code
ü Multifrequency code (DTMF - dual tone multifrequency)
ü Trunk signaling
ü Register signaling
ü Line signaling
ü R2 signaling system
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Summary of course scope
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Control Part of an Exchange
D ia m et er
Media Gateway or Switching Fabric packets
2 - 2
Signaling Flow Chart illustrates the main events
Calling Subscriber (A)
Called Subscriber (B)
1. Address signal last. nr
The purpose of this slide is to illustrate the method!
Alerting or ringing tone
Last address signal
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Signaling Flow Chart example cont...
Called party (B)
Calling subscriber (A)
Local loop trunk
Information/voice transfer on-hook
• NB: Exchanges must signal both in forward and backward direction on incoming and on outgoing side simultaneously.
• Incoming and Outgoing signaling can be separated, so can
• Incoming Call Control and Outgoing Call control.
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Extended Finite State Machine is very suitable for modeling signaling senders and receivers.
, I, O, U, S, f s
, f o
, f u
s s f f f f
s s o o
O - outgoing signal
S - next state
U - new values of state
I - set of possible incoming signals in state s
s can be unique in the signaling system or context dependent.
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Graphical representation of an FSM
FSM - Finite State Machine
The use of FSMs is well known also in computer languages e.g.
for lexical analysis. In this course it turns out that all most import real time programs in a Switching System are FSMs or sets of FSMs.
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SDL representation of an FSM
SDL - Spesification and Description
State x State z
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A subscriber as an SDL -state machine
Wish to call
Push a digit button
Call State on-hook Wish to disconnect
WF_ ringing tone
B-Answer No-Answer on-hook
What is missing in the figure??
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You can model the world like this
Model of the
Model of the
• Can use verification tools to debug your design.
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How are these methods used in implementing signaling systems?
Signaling Flows may be provided in the protocol specification for all main sequences of events
° if not, they are drawn by the implementor for example by measuring an
existing historic system with no valid documentation
SDLs are drafted for the signaling system independent of the implementation environment
The system independent SDLs are used as a starting point for the implementation specification for the target implementation (computer or system) environment
° details are added gradually. E.g. the execution model is taken into
This approach is one example. More ad hoc approach is also possible but I do not recommend it.
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Execution models of FSM programs
A <- Branch (State, (Secondary state,) Message)
Execute Transition (A)
• Execution model 1: Complete the current Transition always before starting anything else (non-pre-emptive scheduling)
• Execution model 2: A Transition can be interrupted at any time if there is a new task with higher priority (pre-emptive scheduling)
• Depending on implementation a Transition may or may not contain a new
(secondary) Receive Message Statement.
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Table representation of an FSM
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Signaling is used to allocate network resources for the call in a CSN
Signaling carries control information from the end user and another exchange. The info implies that certain circuits and devices in the exchange need to change state.
ü Call state includes records on all resources allocated for the call (time slots, signal receivers and senders, memory, processes, records etc). It is vital that all resources are released when the call is released.
Signals can be decadic impulses, voice band tones or binary signals or messages transported in a packet network.
Signals transferred on a local loop between a terminal and the local exchange form subscriber signaling.
When two exchanges send and receive signals we talk about
trunk signaling (inter-exchange signaling, inter-carrier signaling etc…).
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A Signaling System
A signaling system is a given
, I, O, U, S, f s
, f o
, f u
One of the key structural properties of a signaling system is, how signaling information is associated with the
In the PSTN, depending on penetration of digital exchanges, the following types of signaling are used:
Network Loop signaling Trunk signaling
Analogue Pulse- and multi-frequency Channel Associated
Digital Pulse- and multi-frequency Common Channel
ISDN DSS1 (Q.920…Q.931) Common Channel Signaling
(digital sign systems nr 1) (CCS #7)
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Subscriber or loop signaling
The terminal (a phone) sends information to the network in either rotary impulses or in Dual-Tone-Multifrequency (DTMF-) signals.
A DTMF-signal has two frequencies out of six.
Such Frequencies are used that they have no harmonic components with the other frequencies:
Good immunity to voice signals (incl. whistling) is achieved
No interference between dial tone and the first digit
Impact of local loop is minimized (attenuation is proportional to square root of frequency)
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DTMF-signals are created with a push button phone
1209Hz 1336Hz 1447Hz 1633Hz
1 2 3 A
4 5 6 B
Pushing a button creates a continuous signal with 2 frequences
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Impulse signals are created by the rotary disk
Impulses are created by cutting and reconnecting the local loop (current on and off).
On/off states in an impulse are 40 and 60 ms.
The number of such impulses is a telephony signal, e.g. digit 3.
Between two signals an interval of 400-800 ms is used to separate signals.
Signals are created on the backward rotation of the disk
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Responses to the terminal
Terminal receives the following indications as responses to the signals it has sent:
Semantics Frequency Timing
Dial tone 425 Hz continuous
Ringing tone 425 Hz 1s on, 4s silence
Engaged/Busy 425 Hz 300 ms on, 300 ms off
Queueing 950 Hz 650 ms
950 Hz 325 ms
1400 Hz 1300 ms on, 2600 ms off
In terms of modelling the signaling flow, tones are like signals.
However, tones are transported in the voice band and intermediate exchanges usually do not see them!
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Call establishment procedure or signaling sets
up the call between two parties across the network
Trunk signaling can be divided into two phases: call set-up control or inter-register signaling and line signaling.
In setting up a call, devices called incoming and outgoing
register were used in earlier exchange types, thus register signaling.
Call set up (register phase) ends in the ringing state, and devices seized for the call (such as registers) are released for use by other calls.
Incoming and outgoing registers were used in crossbar and relay exchanges. In digital exchanges the same functions are performed by programs. Allocating Register phase call processing and signaling to separate programs may save memory, but will make call control more difficult during the call. When computer memory became plentiful and ISDN emerged, the separating of register and line signaling phases lost its importance.
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Line signaling takes care of call supervision and tear-down (release)
Line signaling is used to control the state of line or channel specific equipment.
ü Line signaling starts when the call has been set up and call routeing has been performed.
Line signaling supervises call tear-down and may also send charging information to a charging point (Finland).
Call signaling ends with the release commands to exchange devices and circuits that the call was using.
ü Another name: supervisory signaling.
Often physically line signals look quite different from register signals.
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Number Analysis links the information received from signaling to call routeing
Analysis result is determined by
° Dialed digits ( from call set up signaling)
° Incoming circuit group,
° Origin or subscriber category (e.g. operator - see R2 group II)
Analysis may return
° a set of routeing alternatives
° an instruction to perform number translation (e.g. 0800-numbers):
In this case, the analysis may need to be repeated
Analysis trees are built by MML-commands issued by the operator based on a route plan
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An example of route descriptions
Primary routeing alternative
The tree is traversed according to some algorithm until and idle outgoing circuit is found or the tree ends, in which case the call is blocked.
Second routeing alternative
Nodes of this tree may contain information that is needed in signaling, for example:
When to start end-to-end signaling etc...
seizure = search and reservation of a free circuit or trunk
Outgoing circuits or trunks
Different algorithms exist for seizure.
Circuit groups can be either unidirectional or bi-directional (as cmp. to call set-up)
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Some Signals used in trunk signaling
Line/Set-up Signal Direction
L seizing signal --> (forward)
L seizing-acknowledgement <-- (backward)
S request for an address signal <--
S address signal -->
S congestion signals
S address complete signals
S subscriber free (charge)
S subscriber free (no charge)
S subscriber line busy
L answer signal
L charging pulse <--
L clear-back signal <--
L release-guard signal <--
L clear-forward -->
L blocking <--
L remove blocking <--
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Channel Associated Signaling (CAS)
ü A category of trunk signaling between exchanges
ü Is originally based on properties of electrical circuits typical in crossbar and relay exchanges.
In Channel Associated signaling the association of the voice path with the signal path may be based on space or frequency or time division multiplexing.
Space division: each voice copper pair is associated with a signaling copper pair. Wastes a lot of copper, therefore, different multiplexing schemes have been developed.
In frequency and time division multiplexing (TDM), the location of the signaling channel determines the associated voice channel. PCM
(pulse code multiplexing) is an example of a TDM system, that uses
time-slot 16 to carry signaling of the voice channels. A multi-frame structure is used to establish the association between the voice and the signaling channels.
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R2 and N2
Among CAS systems, in Finland, the most widely spread is probably R2. A CAS system called N2, developed by Siemens was also widely used especially by the Helsinki Telephone
R2 is the most powerful among anologue CAS systems and was originally specified by ITU-T and elaborated by national standardization.
R2 is a forward and backward compelled signaling system for
call establishment. Sender continues sending a signal until it sees an acknowledgement signal from the the other end. This ensures reliable and fast operation.
ü Each R2 signal is a continuous signal of two voice band frequencies on the voice path. R2 frequencies are not the same as
DTMF that are used in the local loop.
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R2 is a call establisment signaling system
Call establishment signals are sent in-band. In-band means using the voice path for signaling
(subscribers can not talk at the same time!)
ü Originally, R2 was specified for trunk signaling =
I.e. between public network exchanges in analogue
Later digital R2 appeared (analogue signals are represented in a digital form but the signals are basically the same)
Later R2 was adopted for PABXs. Direct Dialling
In (DDI) can be implemented for PABX subscribers using R2. This use has survived the longest.
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Compelled signaling method
Beginning of a signal
Signal is detected.
Acknowledgement sending is started
Acknowledgement is detected. Signal is stopped
Signal end is detected.
Acknowledgement is stopped.
End of Acknowledgement is detected. New signal begins.
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R2 and carriage of signals
R2 - system is based on end-to-end signaling. Intermediate exchanges just pick the information they need for routeing the call, then they through connect the voice path and the rest of the signals can travel transparently onwards.
ü R2 uses MF -coding, in which a signal is a combination of two voice band frequencies. Both forward and backward directions have their own set of six frequencies producing 15 possible signals in both directions.
° R2 is not the same as DTMF: different frequencies and different semantics of signals. Similar physical representation of signals.
ü These signals are grouped into two subgroups (I.e. each physical signal is used twice!) the use of which is controlled by the receiving end.
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Signal Group I Group II
1 1 Ordinary subscriber
Subscriber with priority
Data transmission call
Data transmission call
10 0 Operator
11 Special serv operator Forwarded call
12 Negative ack National signal
13 Test equipment National signal
14 Network Operator specific National signal
15 End of pulsing National signal
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Signal Group A
1 Send next digit
Group B subscriber line free
2 Repeat last but one address signal Send special info tone
3 Hop to receiving Group B signals subscriber line busy
4 Congestion in national network Congestion
5 Send A-subscriber category unallocated number
6 Connect to voice path subscriber line free, charge
7 Repeat number n - 2 subscriber line free, no charge
8 Repeat number n - 3 subscriber line out of order
9 Send country code of A-subs reroute to operator
10 Network Operator Specific subscriber number changed
NB: Because of many variants, the exact signals may be different in different implementations. Naturally, both ends need to follow exactly the same implementation!
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K0 K1 K2 K3 K4 K5 K6 K7 K8 K9 K10 K11 K12 K13 K14 K15
T1 T2 T3 T4 T5 T6 T7 T8 T9 T10 T11 T12 T13 T14 T15 T16 T17 T18 T19 T20 T21 T22 T23 T24 T25 T26 T27 T28 T29 T30 T31
Frame alignement kehyslukitus-
B8 d merkinantosignaling bits kanavan 16
Channel 16 signaling bits
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Even numbered PCM 30 -frame
K0 K1 K2 K3 K4 K5 K6 K7 K8 K9 K10 K11 K12 K13 K14 K15
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T2 T3 T4 T5 T6 T7 T8 T9 T10 T11 T12 T13 T14 T15 T16 T17 T18 T19 T20 T21 T22 T23 T24 T25 T26 T27 T28 T29 T30 T31
Frame alignement kehyslukitustime slot T0 aikav li T0
0 1 1
7 bitin lukitusmerkki
7 bits for alignement in even frames merkinantotime slot T16
0 0 1
1 lukitusmerkki alignement in frame 0 puhekanava 26 aikaväli T27
B1 B2 B3
Applies only to K0, other even numbered, look at the previous slide
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R2 - line signals
There are a number of variants of Line signaling for R2. A typical variant in Finland was (is) PCM -line signals. PCM -line signals are sent in timeslot 16 of the PCM -frame, so that the four bits (a, b, c, d) in the multi-frame dedicated to the corresponding voice channel are used as follows:
NB first abcd are forward bits second abcd are backward bits
Signal a b c d a b c d
Idle 1 0 0 1 1 0 0 1
Seizure 0 0 0 1 1 0 0 1
Seizing ack 0 0 0 1 1 1 0 1
B-answer 0 0 0 1 0 1 0 1
Charging 0 0 0 1 1 0 0 1
B off-hook 0 0 0 1 1 1 0 1
Clear-back 0 0 0 1 0 0 0 1
Clear-forward 1 0 0 1 1 1 0 1
Clear forward 1 0 0 1 0 1 0 1
Clear forward 1 0 0 1 0 0 0 1
Blocking 1 0 0 1 1 1 0 1 forward-transfer 0 1 0 1 1 1 0 1
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Signaling after set-up of the call
ü It is typical in CAS systems that after setting up the call, terminals can not control the network in any way except initiate release.
ü This is due to closing the signaling “connection” between the phone and the local exchange.
ü Workaround methods have been developed. An LE can supervise the voice channel traffic and possible DTMF signals on the voice path or the line card can detect
° It must be possible to detect DTMF -signals among voice.
° Polarity reversal can cause seizure of a register during a call. The register can reserve other signaling resources as needed.
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Limitations of analogue signaling systems
Only a small set of signals -> difficult to add new services.
Context dependent semantics of signals --> modularization of programs is difficult.
Signaling FSM controls the state of Exchange resources on a micro -level --> complex call control..
Separate, e.g. DSPs are needed for signal detection and translation of R2 and DTMF signals.
Voice channel and signaling channel have a fixed mapping. No signaling unless voice channel has been seized.
Difficult to control the call after the setup.
A lot of national and vendor specific variants.
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