Upload
akhil-murali
View
214
Download
0
Embed Size (px)
Citation preview
8/6/2019 5[1].Signalling
1/23
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 .
43
8/6/2019 5[1].Signalling
2/23
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.
44
8/6/2019 5[1].Signalling
3/23
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
45
8/6/2019 5[1].Signalling
4/23
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.,
46
8/6/2019 5[1].Signalling
5/23
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
47
8/6/2019 5[1].Signalling
6/23
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
48
8/6/2019 5[1].Signalling
7/23
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
49
8/6/2019 5[1].Signalling
8/23
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.
50
8/6/2019 5[1].Signalling
9/23
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.
51
8/6/2019 5[1].Signalling
10/23
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.
52
8/6/2019 5[1].Signalling
11/23
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 .
53
8/6/2019 5[1].Signalling
12/23
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.
54
8/6/2019 5[1].Signalling
13/23
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.
55
8/6/2019 5[1].Signalling
14/23
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
56
8/6/2019 5[1].Signalling
15/23
8/6/2019 5[1].Signalling
16/23
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).
58
8/6/2019 5[1].Signalling
17/23
- 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
59
8/6/2019 5[1].Signalling
18/23
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
60
8/6/2019 5[1].Signalling
19/23
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)
61
8/6/2019 5[1].Signalling
20/23
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
62
8/6/2019 5[1].Signalling
21/23
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 :
63
8/6/2019 5[1].Signalling
22/23
(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.
64
8/6/2019 5[1].Signalling
23/23
- 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.
.