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Chapter-5

SIGNALLING IN TELECOMMUNICATION

A telecommunication network establishes and realizes temporary connections,in accordance with the instructions and information received from subscriber

lines and inter exchange trunks, in form of various signals. Therefore, it is

necessary to interchange information between an exchange and it externalenvironment i.e. between subscriber lines and exchange, and between different

exchanges. Though these signals may differ widely in their implementation

they are collectively known as telephone signals.

A signalling system uses a language which enables two switching

equipments to converse for the purpose of setting up calls. Like any other

language. it possesses a vocabulary of varying size and varying precision, ie. alist of signals which may also vary in size and a syntax in the form of a complex

set of rules governing the assembly of these signals.This handout discusses the

growth of signalling and various type of signalling codes used in IndianTelecommunication.

Telephony started with the invention of magneto telephone which used amagneto to generate the ringing current, the only signal, sent over a dedicated

line between two subscribers. The need for more signals was felt with the advent

of manual switching. Two additional signals were, therefore, introduced toindicate call request and call release. The range of signals increased further with

the invention of electro-mechanical automatic exchanges and is still growingfurther at a very fast pace, after the advent of SPC electronic exchanges.

The interchange of signaling information can be illustrated with the help

of a typical call connection sequence.

1. A request for originating a call is initiated when the calling subscriber

lifts the handset.

2. The exchange sends dial-tone to the calling subscriber to indicate to him to startdialing.

3. The called number is transmitted to the exchange, when the calling subscriber

dials the number.4. If the number is free, the exchange sends ringing current to him.

Feed-back is provided to the calling subscriber by the exchange by sending,

a) Ring-back tone, if the called subscriber is free.b) Busy tone if the called subscriber is busy , or

c)Recorded message, if provision exists, for non completion of call due to

some other constraint .

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5 The called subscriber indicates acceptance of the incoming call by lifting the

handset6. The exchange recognizing the acceptance terminates the ringing current and the

ring-back tone, and establishes a connection between the calling and called

subscribers.7 The connection is released when either subscriber replaces the handset.When the

called subscriber is in a different exchange, the following inter-exchange

trunk. signal functions are also involved, before the call can be set up.

8 The originating exchange seizes an idle inter exchange trunk, connected to a digitregister at the terminating exchange.

9. The originating exchange sends the digit. The steps iv to viii are then performed

to set up the call.

Types of Signalling

1.Subscriber Line signalling

Calling Subscriber Line Signaling

In automatic exchanges the power is fed over the subscribers loop by the

centralized battery at the exchange. Normally, it is 48 V. The power is fedirrespective of the state of the subscriber, viz., idle, busy or talking.

Call request

When the subscriber is idle, the line impedance is high. The line impedance falls,as soon as, the subscriber lifts the hand-set, resulting in increase of line current.

This is detected as a new call signal and the exchange after connecting an

appropriate equipment to receive the address information sends back dial-tonesignal to the subscriber.

Address signalAfter the receipt of the dial tone signal, the subscriber proceeds to send the

address digits. The digits may be transmitted either by decade dialing or by

multifrequency pushbutton dialling.

1. Decadic Dialling

The address digits may be transmitted as a sequence of interruption of the

DC loop by a rotary dial or a decadic push-button key pad. The number ofinterruption (breaks) indicate the digit, exept0, for which there are 10

interruptions. The rate of such interruptions is 10 per second and the make/break

ration is 1:2. There has to be a inter-digital pause of a few hundred millisecondsto enable the exchange to distinguish between consecutive digits. This method is,

therefore, relatively slow and signals cannot be transmitted during the speech

phase.

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2. Multifrequency Push-button Dialling

This method overcomes the constraints of the decadic dialling. It usestwo sets of four voice frequencies. Pressing a button (key), generates a signal

comprising of two frequencies. one from each group. Hence, it is also called

Dual-Tone Multi-frequency (DTMF) dialling.

By this method, the dialling time is reduced and almost 10 digits can be

transmitted per second. As frequencies used lie in the speech band, information

may be transmitted during the speech phase also, and hence, DTMF telephonescan be used as access teminals to a variety of systems, such as computers with

voice output. The tones have been so selected as to minimize harmonic

interference and probability of simulation by human voice.

End of selection signal

The address receiver is disconnected after the receipt of complete address. After

the connection is established or if the attempt has failed the exchange sends anyone of the following signals.

1. Ring-back tone to the calling subscriber and ringing current to the

called subscriber, if the called line is free.2. Busy-tone to the calling subscriber, if the called line is busy or

otherwise inaccessible.

3. Recorded announcement to the calling subscriber, if the provisionexists, to indicate reasons for call failure, other than called line

busy.

Ring back, tone and ringing current are always transmitted from the calledsubscriber local exchange and busy tone and recorded announcements, if any, by

the equipment as close to the calling subscriber as possible to avoid unnecessarybusying of equipment and trunks.

Answer Back Signal

As soon as the called subscriber lifts the handset, after ringing, a battery reversal

signal is transmitted on the line of the calling subscriber. This may be used tooperate special equipment attached to the calling subscriber, e.g., short-circuiting

the transmitter of a CCB, till a proper coin is inserted in the coin-slot.

Release signal

When the calling subscriber releases i.e., goes on hook, the line impedance goes

high. The exchange recognizing this signal, releases all equipment involved inthe call. This signal is normally of more than 500 milliseconds duration.

Permanent Line (PG) Signal

Permanent line or permanent glow (PG) signal is sent to the calling subscriber ifhe fails to release the call even after the called subscriber has gone on-hook and

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the call is released after a time delay. The PG signal may also be sent, in case the

subscriber takes too long to dial. It is normally busy tone.

Called subscriber line signals.

Ring Signal

On receipt of a call to the subscriber whose line is free, the terminating exchange

sends the ringing current to the called telephone. This is typically 25 or 50Hz

with suitable interruptions. Ring-back tone is also fed back to the callingsubscriber by the terminating exchange.

Answer SignalWhen the called subscriber, lifts the hand-set on receipt of ring, the line

impedance goes low. This is detected by the exchange which cuts off the ringing

current and ring-back tone.

Release Signal

If after the speech phase, the called subscriber goes on hook before the calling

subscriber, the state of line impedance going high from a low value, is detected.The exchange sends a permanent line signal to the calling subscriber and releases

the call after a time delay, if the calling subscriber fails to clear in the meantime.

Register Recall Signal

With the use of DTMF telephones, it is possible to enhance the services, e.g., by

dialing another number while holding on to the call in progress, to set up a callto a third subscriber. The signal to recall the dialling phase during the talking

phase, is called Register Recall Signal. It consists of interruption of the callingsubscribers loop for duration less than the release signal. it may be of 200 to 320milliseconds duration.

2.Inter-exchange Signaling

Inter-exchange signaling can be transmitted over each individual inter exchange

trunk. The signals may be transmitted using the same frequency band as for

speech signals (inband signaling), or using the frequencies outside this band (out-of-band signaling).

Line signals

DC Signaling

The simplest cheapest, and most reliable system of signaling on trunks,was DC signaling, also known as metallic loop signaling, exactly the same as

used between the subscriber and exchange, i.e.,

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i. Circuit seizure/release corresponding to off/on-hook signal of the

subscriber.

ii. Address information in the from of decade pulses.

In-Band and Out-of-Band Signals

Exchanges separated by long distance cannot use any form of DC linesignaling. Suitable interfaces have to be interposed between them, for conversion

of the signals into certain frequencies, to enable them to be carried over long

distance. A signal frequency (SF) may be used to carry the on/off hook

information. The dialing pulses can also be transmitted by pulsing of the states.The number of signals is small and they can be transmitted in-band or out-of

band. The states involved are shown in Table 1.

TABLE 1. SINGLE FREQUENCY SIGNALING STATES TONE SIGNAL

CONDITION

State Forward Backward

Idle (On hook)

FORWARDSeizure(off hook)

Release (on hook)

BACKWARDAnswer(off hook)

Clear Back (on hook)

Blocking (off hook)

On

off

on

off

off

on

On

on

off/on

off

on

off

For in band signaling the tone frequency is chosen to be 2600Hz. or 2400 Hz. As

the frequency lies within the speech band, simulation of tone-on condition

indicating end-of call signal by the speech, has to be guarded against, for pre-mature disconnection.

Out-of- Band signaling overcomes the problem of tone on condition imitation bythe speech by selecting a tone frequency of 3825 Hz which is beyond the speech

band. However, this adds up to the hard-ware costs.

E & M Signals

E & M lead signaling may be used for signaling on per-trunk basis. An

additional pair of circuit, reserved for signaling is employed. One wire isdedicated to the forward signals ((M-Wire for transmit or mouth) which

corresponds to receive or R-lead of the destination exchange, and the other wire

dedicated to the backward signals (E-wire for receive or ear) which corresponds

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transmit or send wire or S-Lead of the destination exchange. The signaling states

are shown in table2.

TABLE 2. E & M SIGNALING STATES

State Outgoing Exchange

M- lead E-lead

Incoming Exchange

M- lead Elead

Idle

(On hook)

Earth Open Earth Open

FORWARDseizure

(off hook)

Battery Open Earth Earth

Release

(On hook)BACKWARD

Earth Earth/open Battery/Earth Open

Answer

(off hook)

battery Earth Battery Earth

Clear Back

(On hook)

battery Open Earth earth

Blocking Earth Earth Battery Open

This type of signaling is normally used in conjunction with an interface to change

the E & M signals into frequency signal to be carried along with the speech.

Register Signals

It was, however felt that the trunk service could not be managed properly

without the trunk register which basically is an address digit receiver, with suchdevelopment, the inter-exchange signaling was sub- divided into two categories.

1. Line signaling in which the signals operate throughout the duration of call, and

2. Register signaling during the relatively short phase of setting up the call,

essentially for transmitting the address information.

In other words, register signals are interchanged between registers during a phase

between receipt of trunk seizure signal and the exchange switching to the speech

phase. These signals are proceed-to-send (PTS) signals, address, signals, andsignals indicating the result of the call attempt.

The register signals may be transmitted in band or out of band. however, in thelatter case, the signaling is relatively slow and only limited range of signals may

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be used. For example, a single out-of-band frequency may be selected and

information sent as pulses.

In-band transmission can be used easily as there can be no possible interference

with the speech signals. To reduce transmission time and to increase reliability, a

number of frequencies are used in groups. Normally 2 out of 6 frequencies areused. To make the system more reliable compelled sequence is used. Hence, this

system is normally called compelled sequence Multi-frequency (CSMF)

signaling as shown in Fig.3. In CCITT terminology it is termed as R2 system. As

the frequencies need be transmitted only for a short duration to convey the entireinformation, the post dialling delay is reduced.

R2 Signalling

CCITT standardized the R2 signaling system to be used on national and

international routes. However, the Indian environment requires lesser number of

signals and hence, a slightly modified version is being used.

There is a provision for having 15 combinations using two out of six frequencies

viz., 1380, 1500, 1620, 1740, 1860 and 1980 Hz, for forward signals and another15 combination using two out of six frequencies viz., 1140,1020, 900, 780, 660

and 540 Hz, for backward signals. In India, the higher frequency in the forward

group i.e., 1980 Hz, and the lower frequency in the backward group, i.e., 540 hz,are not used. Thus, there are 10 possible combinations in both the directions. The

weight codes for the combinations used are indicated in Table 3 and the

significance of each signal is indicated in Table 4 and 5.

TABLE 3- SIGNAL FREQUENCY INDEX AND WEIGHT CODE

Signal Frequency (Hz)

Forward 1380 1500 1620 1740 1860

Backward 1140 1020 900 780 660

Index f 0 f 1 f 2 f 3 f 4

Weight Code 0 1 2 4 7

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TABLE 4-FORWARD SIGNALS

Signal Weight Group I Group II

1 0+1 Digit 1 Ordinary subscriber

2 0+2 Digit2 Subscriber with priority Test / Mtce,equipment

3 1+2 Digit3 Spare

4 0+4 Digit4 STD Barred

5 1+4 Digit5 Spare

6 2+4 Digit6 CCB

7 0+7 Digit7 Changed Number to

Operator

8 1+7 Digit8 Closed Number

9 2+7 Digit9 Closed Number

10 4+7 Digit0 Spare

TABLE 5 -BACKWARD SIGNALS

Signal No. Weight Code Group A Group B

1 0+1 Send next digit Called line free with

out metering

2 0+2 Restart Changed number

3 1+2 Address complete,

Changeover toreception of group B

signals

Called line busy

4 0+4 Calling line

identification formalicious calls

Local congestion

5 1+4 send calling

subscribers category

Number unobtainable

6 2+4 Set up speech

connection

called line fee, with

metering

7 0+7 Send last but 1 digit Route congestion

8 1+7 Send last but 2 digit Spare9 2+7 Send last but 3 digit Route Breakdown

10 4+7 Spare Malicious call

blocking

Note : Signals A2, and A7 to A9 are used in Tandem working only.

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It can be seen from the tables that, forward signals are used for sending the address

information of the called subscriber, and category and address, information of the calling

subscriber and backward signals are used for demanding address information andcallers category and for sending condition and category of called line. R2 signaling is

fully compelled and the backward signal is transmitted as an acknowledgement to the

forward signal. This speeds up the interchange of information, reducing the call set uptime. It is a self checking system. Each signal is acknowledgement appropriately at the

other end after the receiver checks the presence of only 2 and only 2 out of 5 proper

frequencies.

An example of CSMF signaling between two exchanges may be illustrated by

considering a typical case.The various signals interchanged after seizure of the circuit

are ,

1. Originating exchange sends first digit

2. Receipt of the digit is acknowledged by the terminating exchangesby sending A5 (demanding the callers category).

3. A5 is acknowledgement by sending a group II signal by the

originating exchange

4. Terminating exchange acknowledges this by A1, demanding for

next digit.

5. Originating exchange, acknowledges A1 by sending the next digit.

6. The digits are sent in succession by interchange of steps 5 and 6.

7. On receipt of last digit, the terminating exchange carries out group

and line selection and then sends A3, indicating switching over to

group B signals.

8. This is acknowledgement by the originating exchange by sending

the callers category again.

9. The terminating exchange acknowledgements by sending the

called line condition by sending any of B2 to B6.

10. In response to B6, the originating exchanges switches through the

speech path and the registers are released. Alternatively, in

response to B2 to B5, the registers are released and appropriate

tone is fed to the calling subscriber by the originating exchange.

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Digital Signalling

All, the systems discussed so far, basically, are on per line or per trunk basis, as

the signals are carried on the same line or trunk. With the emergence of PCM

systems, it was possible to segregate the signaling from the speech channel.

Inter exchange signalling can be transmitted over a channel directly associated

with the speech channel, channel-associated signalling (CAS) , or over adedicated link common to a number of channels, common channel signalling

(CCS). The information transmitted for setting up and release of calls is same in

both the cases. Channel associated signalling requires the exchanges, to have

access to each trunk via the equipment which may be decentralised, whereas, incommon channel signalling, the exchange is connected to only a limited number

of signalling links through a special terminal.

Channel- Associated signalling

In the PCM systems the signalling information is conveyed on a separate channelwhich is rigidly associated with the speech channel. Hence, this method is known

as channel associated signalling (CAS). Though the speech sampling rate is 8

Khz, the signals do not change as rapidly as speech and hence, a lower samplingrate of 500 Hz, for digitisation of signals can suffice. Based on this concept, TS

16 of each frame of 125 microseconds is used to carry signals of 2 speech

channels, each using 4 bits.

Hence, for a 30 channel PCM system, 15 frames are required to carry all thesignals. This constitute a 2 millisecond multiframe of 16 frames numbered from

F0 to F 15. TS 16 of the frame F0 is used for multiframe synchronisation. TS 16

of F1 contains signal for speech channels 1 and 16 being carried in TS 1and TS 17, respectively, TS16 of F2 contains signals of speech channels 2 and 17

being carried in TS2 and TS 18, respectively and so on.

Although four bits per channel are available for signalling only two bits are used.

As the transmission is separate in the forward and backward direction, the bits in

the forward link are called af and bf, and those in the backward link are called ab

and bb. Values for these bits are assigned as shown in Table 6.

However, the utilisation of such a dedicated channel for signalling for each

speech channel is highly inefficient as it remains idle during the speech phase.Hence, another form of signalling known as common-channel signalling evolved.

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StateBit Value

Forward Backward.

af bf ab bb

Idle 1 0 1 0

Seizure 0 0 1 0

Seizure

acknowledge

0 0 1 1

Answer 0 0 0 1

Clear Forward 1 0 0/1 1

Clear Back 0 0 1 1

Table 6

COMMON CHANNEL SIGNALLING SYSTEM No. 7

(CCS#7)Communication networks generally connect two subscriber terminating equipment units

together via several line sections and switches for message exchange (e.g. speech, data,

text or images). Control information has to be transferred between the exchanges for call

control and for the use of facilities. In analog communication networks, channel-

associated signalling systems have so far been used to carry the control information.

Fault free operation is guaranteed with the channel-associated signalling systems in

analog communication networks, but the systems do not meet requirements in digital,

processor-controlled communication network. Such networks offer a considerably larger

scope of performance as compared with the analog communication networks due, for

instance, to a number of new services and facilities. The amount and variety of the

information to be transferred is accordingly larger. The information can no longer be

economically transported by the conventional channel-associated signalling systems. For

this reason, a new, efficient signalling system is required in digital, processor-controlled

communication networks.

The CCITT has, therefore, specified the common channel signalling system no.7 .

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CCS-7 is optimised for application in digital networks. It is characterised by the

following main features :

internationally standardized (national variations possible).

suitable for the national, international and intercontinental network level.

suitable for various communication services such as telephony, text services, data

services digital network (ISDN).

high performance and flexibility along with a future-oriented concept which well

meet new requirements.

high reliability for message transfer.

processor-friendly structure of messages (signal units of multiples of 8 bits).

signalling on separate signalling links; the bit rate of the circuits is, therefore,exclusively for communication.

signalling links always available, even during existing calls.

use of the signalling links for transferring user data also.

used on various transmission media

- cable (copper, optical fiber)

- radio relay

- satellite (up to 2 satellite links)

use of the transfer rate of 64 kbit/s typical in digital networks.

used also for lower bit rates and for analog signalling links if necessary.

automatic supervision and control of the signalling network.

Signalling Points (SP) and signalling transfer points (STP).

The SPs are the sources (originating points) and the sinks (destination points) of

signalling traffic. In a communication network these are primarily the exchanges.

The STPs switch signalling messages received to another STP or to a SP on the basis ofthe destination address. No call processing of the signalling messages occurs in a STP. A

STP can be integrated in a SP (e.g. in an exchange) or can form a node of its own in the

signalling network. One or more levels of STPs are possible in a signalling network,

according to the size of the network.

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All SPs in the signalling network are identified by means of a code within the framework

of a corresponding numbering plan and, therefore, can be directly addressed in a

signalling message.

Signalling links

A signalling link consists of a signalling data link (two data channels operating together

in opposite directions at the same date rate) and its transfer control functions. A channel

of an existing transmission link (e.g. a PCM30 link) is used as the signalling data link.

Generally, more than one signalling link exists between two SPs in order to provide

redundancy. In the case of failure of a signalling link, functions of the CCS7 ensure that

the signalling traffic is rerouted to fault-free alternative routes. The routing of the

signalling links between two SPs can differ. All the signalling links between two SPs are

combined in a signalling link set.

Signalling Network

In contrast to channel-associated signalling, which has been standardpractice until now, in CCS7 the signalling messages are sent via separate signalling links

(See Fig. 1). One signalling link can convey the signalling messages for many circuits.

The CCS7 signalling links connect signalling points (SPs) in a

communication network. The signalling points and the signalling links form an

independent signalling network which is overlaid over the circuit network.

Fig. 1

Signalling Modes

Two different signalling modes can be used in the signalling networks for CCS7, viz.

associated mode and quasi-associated mode.

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In the associated mode of signalling, the signalling link is routed together with the

circuit group belonging to the link. In other words, the signalling link is directly

connected to SPs which are also the terminal points of the circuit group (See Fig.2). This

mode of signalling is recommended when the capacity of the traffic relation between the

SPs A and B is heavily utilized.

Fig. 2

Associated Mode of Signalling

In the quasi-associated mode of signalling, the signalling link and the speech circuitgroup run along different routes, the circuit group connecting the SP A directly with the

SP B. For this mode, the signalling for the circuit group is carried out via one or more

defined STPs (See Fig. 3.3). This signalling mode is favourable for traffic relations with

low capacity utilization, as the same signalling link can be used for several destinations.

Fig. 3Quasi-associated Mode of Signalling

Signalling Routes

The route defined for the signalling between an originating point and a destination point

is called the signalling route. The signalling traffic between two SPs can be distributed

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correct order without information loss, duplication or sequence alteration and without

any bit errors.

Functional Levels

Level I (Signalling Data Link) defines the physical, electrical and functionalcharacteristics of a signalling data link and the access units. Level 1 represents the bearer

for a signalling link. In a digital network, 64-kbit/s channels are generally used as

signalling data links. In addition, analog channels (preferably with a bit rate of 4.8 kbit/s)

can also be used via modems as a signalling data link.

Level 2 (Signalling Link) defines the functions and procedures for a correct exchange

of user messages via a signalling link. The following functions must be carried out at

level 2 :

- delimitation of the signal units by flags.

- elimination of superfluous flags.

- error detection using check bits.

- error correction by retransmitting signal units.

- error rate monitoring on the signalling data link.

- restoration of fault-free operation, for example, after disruption of the signallingdata link.

Level 3 (Signalling Network) defines the interworking of the individual signalling

links. A distinction is made between the two following functional areas :

- message handling, i.e. directing the messages to the desired signalling line, or to

the correct UP.

- signalling network management, i.e. control of the message traffic, for example,

by means of changeover of signalling links if a fault is detected and changeback to

normal operation after the fault is corrected.

The various functions of level 3 operate with one another, with functions of other levels

and with corresponding functions of other signalling of other SPs.

Signal Units (SU)

The MTP transport messages in the form of SUs of varying length. A SU is formed by

the functions of level 2. In addition to the message it also contains control information

for the message exchange. There are three different types of SUs :

- Message Signal Units (MSU).

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- Link Status Signal Units (LSSU).

- Fill-in Signal Units (FISU).

Using MSUs the MTP transfers user messages, that is, messages from UPs (level 4) and

messages from the signalling network management (level 3).

The LSSUs contain information for the operation of the signalling link (e.g. of the

alignment).

The FISUs are used to maintain the acknowledgement cycle when no user messages are

to be sent in one of the two directions of the signalling link.

The structure of the three types of message units is shown in Fig.5.

Fig. 5

Format of Various Signal Units

Protocol Information Bits

Flag (F) : (8 bits) The SUs are of varying length. In order to clearly separate them from

one another, each SU begins and ends with a flag. The closing flat of one SUs is usually

also the opening flag of the next SU. However, in the event of overloading of the

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signalling link, several consecutive flags can be sent. The flag is also used for the

purpose of alignment. The bit pattern of a flg is 01111110.

Backward Sequence Number (BSN) : (7 bits) The BSN is used as an

acknowledgement carrier within the context of error control. It contains the forward

sequence number (FSN) of a SU in the opposite direction whose reception is being

acknowledged. A series of SUs can also be acknowledged with one BSN.

Backward Indicator Bit (BIB) : (1 bit) The BIB is needed during general error

correction. With this bit, faulty SUs are requested to be retransmitted for error

correction.

Forward Sequence Number (FSN) : (7 bits) A FSN is assigned consecutively to each

SU to be transmitted. On the receive side, it is used for supervision of the correct order

for the SUs and for safeguarding against transmission errors. The numbers 0 to 127 are

available for the FSN.

Forward Indicator Bit (FIB) : (1 bit) The FIB is needed during general error

correction. It indicates whether a SU is being sent for the first time or whether it is being

retransmitted.

Length Indicator (LI) : (6 bits) The LI is used to differentiate between the three SUs. It

gives the number of octets between the check-bit (CK) field and the LI field. The LI

field contains different values according to the type of SU; it is 0 for FISU, 1 or 2 for

LISU and is greater than 2 for MSU.

The maximum value in the length indicator fields is 63 even if the signalling informationfield (SIF) contains more than 63 octets.

Check bits (CK) : (16 bits) The CKs are formed on the transmission side from the

contents of the SU and are added to the SUs as redundancy. On the receive side, the

MTP can determine with the CKs whether the SU was transferred without any errors.

The SUs acknowledged as either positive or faulty on the basis of the check.

Fields specific to MSUs :

Service Information Octet (SIO) : (8 bits) It contains the Service Indicator (SI, 4 bits)

and Subservice field (SSF, 4 bits) whose last 2 bits are Network Indicator (NI).

An SI is assigned to each user of the MTP. It informs the MTP which UP has sent the

message and which UP is to receive it. Four SI bits can define 16 UPs (3-SCCP, 4-TUP,

5-ISUP, 6-DATAUP, 8-MTP test, etc.). The NI indicates whether the traffic is

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international (00,01) or national (10,11). In CCS7 a SP can belong to both national and

international network at the same time. So SSF field indicate where the SP belongs.

Signalling Information Fields (SIF) : (2 to 272 octets) It contains the actual user

message. The user message also includes the address (routing label, 40 bits) of the

destination to which the message is to be transferred. The maximum length of the user

message is 62 octets for national and 272 octets for international networks (one octet = 8

bits). The format and coding of the user message are separately defined for each UP.

Fields Specific to LSSUs

Status Field (SF) : (1 to 2 octets) It contains status indications for the alignment of the

transmit and receive directions. It has 1 or 2 octets, out of which only 3 bits of first octet

are defined by CCITT, indicating out (000), normal (001), Emergency (010) alignments,

out-of-service (011), Local processor outage (100) status, etc.

Addressing of the SUs (in SIF)

A code is assigned to each SP in the signalling network according to a numbering plan.

The MTP uses the code for message routing. The destination of a SU is specified in a

routing label. The routing label is a component of every user message and is transported

in the SIF. The routing label in a MSU consists of the following (See Fig. 6).

Fig. 6

Routing Label of a Message Signal Unit

Destination Point Code (DPC) : (14 bits) identifies the SP to which this message is to

be transferred.

Originating Point Code (OPC) : (14 bits) specifies the SP from which the message

originates.

The coding of OPC and DPC is pure binary and using 14 bits linear encoding, it is possible to identify 16,384 exchanges. The number of exchanges in DOT network

having CCS7 capability are expected to be within this limit.

Signalling Link Selection (SLS) field : (4 bits) The contents of the SLS field determine

the signaling route (identifying a particular signalling link within s link set or link sets)

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along which the message is to be transmitted. In this way, the SLS field is used for load

sharing on the signalling links between two SPs.

The SIO contains additional address information. Using the SI, the destination MTP

identifies the UP for which the message is intended. The NI, for example, enables a

message to be identified as being for national or international traffic.

LSSUs and FISUs require no routing label as they are only exchanged between level 2 of

adjacent MTPs.

The message sent from a user to the MTP for transmission contains : the user

information, the routing label, the SI, the NI and a LI. The processing of a user message

to be transmitted in the MTP begins in level 3 .

The MTP is responsible for (a) transmitting, (b) receiving SUs, (c) for correcting

transmission errors, (d) for the signalling network management, and (e) for the

alignment. Its functions are spread over the functional levels 1, 2 and 3.

The message routing (level 3) determines the signalling link on which the user message

is to be transmitted. To do this, it analyzes the DPC and the SLS field in the routing label

of the user message, and then transfers the message to the appropriate signalling link

(level 2).

The transmission control (level 2) assigns the next FSN and the FIB to the user

message. In addition, it includes the BSN and the BIB as an acknowledgement for the

last received MSU. The transmission control simultaneously enters the part of the MSU

formed so far in the transmission and retransmission buffers. All MSUs to be transmittedare stored in the retransmission buffer until their fault-free reception is acknowledged by

the receive side. Only then are they deleted.

The check bit and flag generator (level 2) generates CKs for safeguarding against

transmission errors for the MUS and sets the flag for separating the SUs. In order that

any section of code identical to the flag (01111110) occurring by chance is not mistaken

for the flag, the user messages are monitored before the flag is added to see if five

consecutive ones (1) appear in the message. A zero (0) is automatically inserted after

five consecutive 1s. On the receive side, the zero following the five 1s is then

automatically removed and the user message thereby regains its original coding.

The check-bit and flag generator transfers a complete MSU to level 1. In level 1, the

MUS is sent on the signalling data link.

The bit stream along a signalling data link is received in level 1 and transferred to level

2. Flag detection (level 2) examines the received bit stream for flags. The bit sequence

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between two flags corresponds to one SU. The alignment detection (level 2) monitors

the synchronism of the transmit and receive sides with the bit pattern of the flags.

Using the CKs also transmitted, error detection (level 2) checks whether the SU was

correctly received. A fault-free SU is transferred to the receive control, while a faulty SU

is discarded. The reception of a faulty SU is reported to error rate monitoring, in order to

keep a continuous check on the error rate on the receive side of the signalling link. If a

specified error rate is exceeded, this is reported to the signalling link status control by

error rate monitoring. The signalling link status control then takes the signalling link out

of service and sends a report to level 3.

The receive control (level 2) checks whether the transferred SU contains the expected

FSN and the expected FIB. If this is the case and if it is a MSU, the receive control

transfers the user message to level 3 and causes the reception of the MSU to be

positively acknowledged. If the FSN of the transferred MSU does not agree with that

expected, the receive control detects a transmission error and causes this and all

subsequent MSU to be retransmitted (see subheading "Correction of Transmission

Errors").

The message discrimination (level 3) accepts the correctly received user message. It

first determines whether the user message is to be delivered to one of the immediately

connected UPs or to be transferred to the another signalling link (quasi-associated

message). This preselection is achieved in the message discrimination by evaluation of

the DPC. A user message which only passes through a SP (STP) is transferred by the

message discrimination to the message routing, where it is treated as a user message to

be transmitted.

If a received user message is intended for one of the connected UPs (SP), it is transferred

to message distribution (level 3). The message distribution evaluates the SIO, thereby

determining the UP concerned, and delivers the user message there.

Signalling Network Management

The signalling network management is a function of level 3. It controls the operation and

the interworking of the individual signalling links in the signalling network. To this end,

the signalling network management exchanges messages and control instructions with

the signalling links of level 2, sends message to the UPs and works together with thesignalling network management in adjacent SPs. For the interworking with other SPs the

signalling network management uses the transport function of the MTP. Management

messages are transferred in MSUs like user messages. For discrimination, the

management messages have their own SI. The signalling network management contains

3 function blocks :

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(a) The signalling link management controls and monitors the individual signalling

links. It receives the messages concerning the alignment and status of the individual

signalling links, or concerning operating irregularities and effects any changes in status

which may be necessary. In addition, the signalling link management controls the

putting into service of signalling links, including initial alignment and automaticrealignment of signalling links after failures or alignment losses due to persistent faults.

If necessary, the signalling link management transfers messages to the signalling traffic

management or receives instructions from there.

(b) The signalling route management controls and monitors the operability of

signalling routes. It exchanges messages with the signalling route management in the

adjacent STPs for this purpose. The signalling route management receives, for example,

messages concerning the failure or non availability of signalling routes or the

overloading of STPs. In cooperation with the signalling traffic management, it initiates

the appropriate actions in order to maintain the signalling operation to the signallingdestinations involved.

(c) The signalling traffic management controls the diversion of the signalling

traffic from faulty signalling links or routes to fault-free signalling links or routes. It also

controls the load distribution on the signalling links and routes. To achieve this, it can

initiate the following actions :

- changeover; on failure of a signalling link the signalling traffic management

switches the signalling traffic from the failed signalling link to a fault-free signalling

link.

- change back; when signalling link becomes available again after a fault has been

corrected, the signalling traffic management reverse the effect of the changeover.

- rerouting; when SP can no longer be reached on a normal route, the signalling

traffic management diverts the signalling traffic to a predefined alternative route.

When overloading occurs, the signalling traffic management sends messages to the users

in its own SP in order that they reduce the load. The management also informs the

adjacent SPs of the overloading in its own SP and requests them to also reduce the load.

The signalling traffic management accomplishes its functions by

- receiving messages from the signalling link and signalling route management.

- sending control instructions to signalling link and signalling route management.

- directly accessing the signalling links, e.g. during emergency alignment.

- modifying the message routing on failure of signalling routes.

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- exchanging management messages with the signalling traffic management in

adjacent SPs.

As discussed earlier, level 4 functions, which include formatting of messages based on

the applications, are allotted to UPs. Each UP provides the functions for using the MTP

for a particular user type. Some of the UPs as currently specified by the CCITT are :

- telephone user part (TUP)

- integrated services digital network user part (ISDN-UP)

- the signalling connection control part (SCCP)

- the transaction capabilities application part (TCAP)

For Intelligent Network (IN) application, Intelligent Application Part (INAP) and TCAP

are used. SCCP forms the interface between these UPs and MTP.

Fig.9 shows the users of the MTP as well as their relationship to one another and to theMTP. CCS7 can be adapted to all requirements due to the modular structure. Expansion

for future applications is also possible. Each CCS7 user can specify its own UP, for

example, the mobile user part (MUP) is Siemen's own specification for the mobile

telephone network C450.

.

Documents

5[1].Signalling