12 Signalling System No 7

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250 SIGNALLING SYSTEM NO. 7

In addition, its modular nature ends itself to the development of new user parts whichmay be designed to support almost any new service that can be conceived. The userparts of the system tha t have been developed so far are

0 MTP

0 SCCP

0 TUP

0 DUP

0 ISUP

0 TC

0 TCAP

0 OMAP

0 INAP0 MAP

message transfer part

signalling connection and control part

telephone user part

data user part

ISDN services user part

transaction capabilities (used by intelligent networks)

transaction capabilities application part

operation and maintenance application part

intelligent network application partmobile application part

The MTP and SCCP form the ‘foundations’ of the system, providing for carriage ofmessages. The TUP, DUP and ISUP use the MTP and/or SCCP to convey messagesrelating to call control, for telephone, data , and ISDN networks, respectively. TheOMAP, MAP and INAP are other application parts for operation and maintenanceinteraction, mobile network control and intelligent network services, respectively.

Initially the SS7 system was designed so that the MTP could be used in associationwith any or all of the telephone, data and ISDN user parts. However, following the

emergence of the OS1 model, the SCCP was developed as an adjunct to the MTP; thetwo in combination provide the functions of the OS1 network service (layers 1-3).SS7 signalling can be installed between two exchanges, provided that the necessary

signalling functions are available n both exchanges. Thefunctions reside n aunittermed a signalling po int . This may be a separate iece of hardware to the exchange, butusually it is a software function in the exchange central processor. SS7 signalling poin ts( S P s ) ,basically exchanges, intercommunicate via signalling links and are said to share asignalling relation.

A single SS7 signalling link enables information to be passed directly between twoexchange processors, allowing the set-up, control, and release of not just one, but alarge number of traffic-carrying circuits between the exchanges. Messages over the unit

take the form ‘connect circuit number 37 to the called customer number 01-234 5678’.Theerm comm on hannel signalling aptly escribes hismethod of operation,distinguishing t rom the channel-associated signalling method, wherein callset-upsignals pertinent to a particular circuit are sent down that circuit. SS7 is not the firstcommon channel signalling system to be developed; CCITT 6 (SS6) was also a commonchannel system, but CCITT 6 was less flexible than SS 7 and not so suitable for digitalnetwork use.

Having a common channelfor conveyance of signalling messages saves equipment atboth exchanges, because only one ‘sender’ and one ‘receiver’ is required at each end ofthe link, as against the one per circuit required with channel-associated systems. The



SS7 SIGNALLING NETWORKS 25 1

E x c h a n g e A I I E x c h a n g e B

S T = s igna l l ing e rmina l

Figure 12.1 Linking tw o exchanges using SS7 signalling

combination of a SS7 sender and receiver is normally eferred to as a signallingterminal. In practice signalling terminals are a combination of a software function in theexchange central processor and some hardware to terminate the line and undertake thebasic bit transfer function (OS1 layer 2, datalink).

A label attached to each message as it passes over the signalling link enables thereceiving signalling point to know which of the many circuits it relates to. Figure 12.1illustrates he network configuration of a simple SS7 signalling ink. It shows callsflowing over a arge number of traffic-carrying circuits which are connected to heswitch matrixpart of the xchange.Meanwhile all these circuits are controlledaccording o he nformation passed directly between the exchange processors. Thesignalling terminal ( S T ) function is shown residing within the exchange processor.

12.2 SS7 SIGNALLING NETWORKS

Networks employing SS7 signalling comprise two separate subnetworks. One subnet-work is thenetwork of traffic-carrying circuits nterconnecting the exchanges. Thesecond subnetwork is that of the signalling links. In Figure 12.1 we saw this separationof traffic-carrying circuits from signalling link as it would apply on a single connectionbetween twoexchanges.Figure 12.2 now howsamorecomplicatedexample toillustrate another powerful feature of SS7: the fact that signalling networks and traffic-carrying networks may be designed and implemented almost n solation fromoneanother. Just because there are direct traffic-carrying circuits between two exchanges(they haveadirect trafic-carrying elation) itdoes not follow that he signallinginformation or signalling trafJic) has to travel overdirect signalling links, houghclearly a signalling relation of some sort is needed.




U

II H ,

IExchDange IU

Signal l inginks

'm r a f f i c - c a r r y i n gi r c u i t s

Figure 12.2 Traffic-carrying and signalling networks in SS7

Figure 12.2 shows heraffic-carryingnetworks and signallingnetworks nter-connecting four exchanges, A , B, C and D. The traffic circuits directly connect A-C,A-B, B-C and B-D. All traffic to or from exchange D passes via exchange B and alltraffic to or from exchange A passes either via B or C , and so on.The signallingnetwork, however, is different. Signalling links only exist between A-B, B-C and B-D,so that signalling trafic has to be routed differently from the actual traffic. In the case oftheactual traffic from A to B, there exist both direct traffic circuits and a directsignalling link. In effect, this is the same as Figure 12.1, so that both signalling messagesand traffic can be passeddirectly between the wo. Similarly exchange B maypasssignalling messages and traffic directly either to exchange C or exchange D, and mayalso act as a normal ransit exchange for two-link routing of traffic from exchange A to

either of exchanges C or D. These are all examples of associated mode signalling, inwhich signalling links and traffic circuits have a similar configuration, and signallingmessages and traffic both route in the same manner. In short, there is a signalling linkassociated with each link of direct traffic-carrying circuits.

By contrast, although exchange A is directly connected to exchange C by traffic-carryingcircuits, here is no direct signalling ink. Signalling nformation for thesecircuits must be passed on another route via exchange B. This is known as the quasi-associated mode of signalling, and the signalling point (SP) in exchange B is said inthis instance to perform the function of a signal transfer point ( S T P ) , as illustrated inFigure 12.3.



THE STRUCTURE OF SS7 SIGNALLING 253

Exchange

n

Exchangexchange

S P sp / / / / / / / / / l s pP/ / / / U

Ass ociate d mode Ouosi - associatedode

slgnallingin k SP = signalling point.m raff ic-ar ry ingircuits STP = signalransfer point

Figure 12.3 Modes of SS7 signalling

Signalling information is passed over SS7 signalling links in short bursts; indeed aSS7 signalling network is like a powerful packet-switched data network. To identifyeach of the signalling points for the purpose of signalling message delivery around thenetwork, each is assigned a numerical identifier, called a signalling point code (SPC) .This code enables an SP to determine whether received messages are intended for it, orwhether they are to be transferred (in S T P m o d e )to another SP. The codes are allocatedon a network by network basis. Thus the code is only unique within, say, nationalnetwork A, national network B or the international network.

12.3 THE STRUCTURE OF SS7 SIGNALLING

Thanks o hemodularmanner n which the SS 7 system has been designed, itencourages the development of new modules in support of future telecommunicationsservices and functions. Figure 12.4 illustrates the functional structure of the SS7 system,relative to the layers of the Open Systems Interconnection (OSI) model (see Chapter 9).

In hesame way as he OS1 modelhasanumber offunctional ayers, eachanimportant foundation for the layers above i t, so SS 7 signalling is designed in a numberof functional levels. Note in Figure 12.4, that the component levels and par t s of SS7 donot align with the OS1 layered model. The lack of alignment of signalling levels withOSZ layers is unfortunate and it rises from the fact that the two models ere developedconcurrently but for different purposes. The lack of alignment of levels with layersmeans hatnot allhigher ayer OS1 protocolsarecurrently uitable or use inconjunction with the lower evels of SS7 signalling. The various standards developmentbodies are trying to rationalize the component parts of SS7 to conform with the OS1model. The signalling connection and con trol par t (SCCP), or example, delivers the S1network service (OS1 layer 3 service), so that a communication system can use heSCCP (and MTP below it) to support ayers 4-7 OSI-based protocols. The evels in SS 7signalling provide a convenient separation of signalling functions, and in the remainderof the chapter the signalling level model is used in explanation.




OS1 l aye rppl icat ion

[7

6

5L

3 I SCCP IljU P

-

MTP

Message ransferover signallingn e t w o r k

over ingle ink

d a t ain k

Signal l inglevel

L Userlevel

Networkl e v e l

Link* l eve l

, O a t a l i n kl e v e l

Figure 12.4 The structure of SS7 signalling. ASE, Application service element; TCAP, Trans-action capability; ISP, Intermediate service part; ISUP, ISDN services user part; TUP, Telephoneuser part; DUP, Data user part; SCCP, Signalling connection and control part; MTP, Messagetransfer part

12.4 THE MESSAGE TRANSFER PART (MTP)

The foundation evel of the SS7 signalling system s the message transfer part defined byITU-T Recommendations Q.701-4.707. The message transfer part takes care of theconveyance of messages, fulfilling signalling level functions 1 to 3 (sometimes labelledMT Pl , MTP2, MTP3) as follows.

Level 1 (datalink functions)The first level defines the physical, electrical and functional requirements of the signal-

ling data link. The level one function is attuned to the particular transmission mediumas laid down by ITU-T G series recommendations. The level 1 function allows for anunstructured bit stream to be passed between SPs over an isolated signalling data link.

Level 2 (signalling link jun cti on s)This level defines the functions and procedures relating to the structure and transfer f asignal. Message flow control, and error detection and correction are included. (Flowcontrol prevents the over-spill and consequent loss of messages that result if a messageis sent when the receiving end was not ready to eceive it; error detection and correctionprocedures eliminate message errors introduced on the link.)



THE MESSAGE TRANSFERART 255

L e v e l 3 ( s i g n a l l i n g n e t wo r k f u n c t i o n s )This level defines the unctions andprocedures or conveyingsignalling messagesaround an entire ignalling network. It provides for the routing of messages around thesignalling network. In this role it has a number of ‘signalling network management’capabilities ncluding ‘loadsharing’ of signalling traffic between different signallinglinks and routes illustrated in Figure 12.5) and e-routing around signalling inkfailures. Link sharing on the same route between signalling points (SPs) guards againstlineplant failure (Figure 12.5(a)). Route sharing may additionally provide protectionagainst failure of STPs. Thus in Figure 12.5 the signalling traffic from SP A to SPs Band C is shared over the two STPs, D and E. In the event of a failure of any of theroutes shown, signalling messages could be re-routed.

MTP is useless on its own for setting up telephone or other connections. To performthese functions MTP needs to be used in association with one of the SS 7 user partswhich are level 4 or user fun ctio ns. Examples are the telephone user part (TUP) and theintegratedservicesdigitalnetwork user part ( I S D N - U P or I S U P ) . These define the

content and interpretation of the message, and they provide for connection control.The structure of an MTP message is shown in Figure 12.6. It comprises four parts,transmitted in the following order.

FlagTheJag is the first pattern of bits sent. This is an unmistakeable pattern to distinguishthe beginningof ach message, and delimit t romheprevious message. It iscomparable to the synchronization (SYN) byte in data communications (Chapter 9) .

M T P i n f o r m a t i o nThe flag is followed by a number of j e lds of information, which together ensure the

correct message transfer. These fields include: the message sequence numbers that keep

S P S P

S P S T P S P

A - B and A - C s i g n a l l i n gmessages venly dividedt o ro u t e via both D and E .

STP SP

Figure 12.5 Load sharing over signalling. A-B and A-C signalling messages evenly divided toroute via both D and E




Nextmessage

rFirst bittransmitted

U i t s lessage sequence

Checkand us e r pa r t ’mes s age s ubs tance 1

F l a gu m b e r s , eng thignalling nformation ield

type n fo rmat ion

(Inser ted by appropr ia te‘ l e v e l I ’ ‘user part’)

Figure 12.6 CCITT 7 MTP message structure

the messages in the correct order on receipt, and allow lost messages to be resent; andinformation about the type and length of the information held in the main ‘signallinginformation field’; it might say which user part is in operation and record the length ofthe message.

Signalling information jieldThis is the main information field or the ‘substance’ of the message. The informationis inserted by one of he user parts, asappropriate or the particular application(e.g. telephone user part (TUP), or integrated services user part (ISUP)). The structureof this$eld depends on which user par t is in use.

Check bitsFinally, each MTP message is concluded with a check bit field. This is the data (cyclicredundancy check code or C R C )needed to perform the error detection and correctionmechanism of the MTP level 2. The check bitsare followed by the flag at the start of thenext message.

12.5 THE USER PARTS OF SS7

The various user parts of SS 7 are alternative functions meeting the requirements f level4 of the signalling level model. The user parts may be used in isolation, or sometimesmay be used together. Thus the telephone user part ( T U P )and the MTP together aresufficient to provide telephone ignalling between exchanges. The data user part (DUP),ISDN user part (ISUP) and other user par ts need not be built in to a pure telephoneexchange. An example where more than one user part is employed is the combination ofSCCP (signalling connection and control part) , ISP (intermediate service part) andTCAP (transaction capability application part). These are all necessary for the supportof the intelligentnetworks described in Chapter 11). The remainder of the chapterdescribes the capabilities of each of the level 4 user parts of SS7.



THE TELEPHONE USER PARTTUP) 257

12.6 THE TELEPHONE USER PART (TUP)

The telephone user part comprises all the signalling messages needed in a telephonenetwork to set up telephone calls (we described the sequence of call set-up in Chapter 7).Thus an exchange using the SS7 signalling system carries out the normal rocess of digitanalysis and route selection, seizes the outgoing circuit and sends the dialled digit trainon to the next exchange in the connection by using the SS7 signalling link, conveyingTUP encoded messages using the MTP. Crudely put, an example of a TUP messagemight be ‘connect the call on circuit number 56 to the destination directory number071-234 5678’. Backward messages such as ‘destination busy’ are also included in thetelephone user part.

The structure of TU P messages is shown in Figure 12.7. TUP messages occupy thesignalling information Jield of the underlying MTP message. The messages comprise aTUP signalling information field which is used to convey ‘dialled digits’, ‘line busy’,‘answer’ signals, and other circuit-related nformation, ogether with four adminis-

trative fields as follows.Destination point code (DPC)

Thiscode identifies the signallingpoint to which the signalling message is to bedelivered by the MTP. (The destination of a signalling message is not necessarily thesame as the final destination of the call.) The signalling point is in the exchange thatforms the next link of the connection (for orwardmessages) or in the previous exchange(for backward messages).

Originating point code ( O P C )Thiscode identifies the signalling point which originated he message (again not

necessarily the origination point of the call).

Circuit identijication code (CIC)This is a number that ndicates to the exchange at the receiving end of the signalling linkwhich traffic circuit each message relates to.

The telephony user part is defined in ITU-T Recs. Q.721-Q.725.

TU Pmessoge>

TU Psignallinginformation

CIC= Circui t ident i f icat lon

(ot her s as SCCP fields)

CIC OPC codeP C

\/

\

] /[[ bit se n t

0

/

information field

MTPmessage

Figure 12.7 TUP message structure and relation to MTP



258 SIGNALLING SYSTEM NO . 7

12.7 THE DATA USER PART (DUP)

The Data user part is similar to the telephone user part, but it is optimized for use oncircuit-switched data networks. The message structure of the DUP is very similar tothat of the TUP, illustrated n Figure 12.7. The DUP is defined by ITU-T recommenda-tions 4.741 but was hardly ever used. It has been largely superseded by the ISUP.

12.8 THE INTEGRATED SERVICES USER PART (ISUP)

Used in conjunction with the MTP, the S7 integrated services digital networkuser par t,I S D N - U P or I S U P, is the signalling system designed for use in ISDNs. In effect it is acombination of capabilities similar o TUP and DUP, which allow voice and dataswitched services to be integrated within a single network. The message structure is

similar to that of TUP and DUP, but the messages used are incompatible with both ofthe other systems. ISUP is defined by CCITT Rec Q.761-Q.764.The ISDN user part ISUP) nteracts as necessarywith he ISDN D-channel,

signalling ( D S S I , digital subscriber signalling 1, as defined by recommendation Q.931)to convey end-to-end nformation between ISDN user erminals. Such nformationincludes the terminal compatibilitychecking procedure which ensures that a compatiblereceiving terminal is available at the location dialled by the caller. As we learned inChapter 10, the procedure prevents, for example, the connection of a group 4 facsimilemachine to a videoconference at the receiving end.

12.9 THE ENHANCED TELEPHONE USER PART (TUP+)

The TUP+ is an enhanced version of the TUP, though incompatible with it. It wasdeveloped by CEPT as recommendation TjSPS 43-02 for use as an interim ISDN-likesignalling system supporting an early pan-European ISDN. It is used in Europe byFrance Telecom for international ISDN signalling, but is likely to be superseded byISUP.

12.10 THE SIGNALLING CONNECTION CONTROL PART (SCCP)

TheSCCP is used to convey non-circuit-related information between exchanges ordatabases, between an exchange and a database or between two exchanges (for certaintypes of ISDN supplementary services). By non-circuit-related we mean that although asignalling relation is established between an exchange and a database, no traffic circuitis intended to be set up. In essence the SCCP (in conjunction with the TC and relevantapplicat ion pa rt ) provides a means for querying a reference store of information, as isnecessary during call et-up on intelligent networks. It is an ideal data ransfermechanism for



THE SIGNAQLLINGONNECTIONONTROLARTSCCP) 25 9

0 interrogation of a central database (Chapter 11)

0 updating cellular radio ‘location registers’ (Chapters 11 and 15)

0 remote activation and control of services or exchanges

0 data transfer between network management or network administration and controlcentres

The SCCP is a user part which in conjunction with the MTP allows a SS7 signallingnetwork o conform o he OS1 network service (OS1 layers l-3), and o supportprotocols designed according to layers 4-7 of the OS1 model. Most importantly, thisallows new user parts to conform with the OS1 model.

SCCP controls the type of connection made available between the two signallingpoints in the exchange and the database. In effect it establishes the signalling relation inpreparation for one of the higher level application parts.Four classes of transjer servicecan be made available as defined by the OS1 model. Thesecan begrouped ntoconnectionless and connection-oriented types, as we learned in Chapters 1 and 7 . Theseare shown in Table 12.1.

In the context of the SCCP classes shown in Table 12.1, connection-oriented datatransfer (classes 2 and 3) is that in which an association is established between senderand receiver before the data are sent. In eality this means the establishment of a virtualor packet-switched connection between the ends (as we discussed in Chapter 9) , and nota circuit-switched connection. Thus before the application part signalling commencesoperat ion over the SS7 signalling link, a virtual connection is created by the SCCP. Inthe alternative connectionless mode of data transfer (SCCP classes 0 and l), messagesare despatched onto he S S 7 signalling network without first ensuring that the recipientis ready to receive them. Connection-oriented procedures are useful when a argeamount of data needs to be transferred. The connectionless mode is better suited tosmall and short messages, because it avoids the burden of extra messages to establishthe connection. By their nature, connectionless messages always include ddressinformation.

Table 12.1 The classes of SCCP

Class Type

Class 0 Connectionless

Message sequence not guaranteedClass 1 Connectionless

Message sequence guaranteed

Class 2 Basic connection-orientedMessage segmentation and reassembly

Class 3 ConnectionrientedMessage segmentation and reassemblyFlow controlDetection of message loss and mis-sequence




1

SCCPmessage

SCCP user data DPCPCL S

\ //

\0

information ield

M TP me ssag e

Figure 12.8 SCCP message structure and relation to MTP. SLS, signalling link selection; OPC,originating point code; DPC, destination point code

The structure of SCCP messages is similar to that of the TUP, as we can see fromFigure 12.8, except that the SCCP includes no circuit identzjcation code ( C I C ) . TheCIC is superfluous because no circuit will be established.

Figure 12.9 shows an example of the use of SCCP and TC for a database queryduring the call set-up phase of a Freephone (800) call. The caller (who happens to be anISDN subscriber) dials the 0800 number, which is conveyed to the ISDN exchange bythe D-channel signalling protocol, DSSl Q.931). Following analysis of the number, theISDN exchange realizes that it must refer to the intelligent network databases for anumber translation. It does so using the SCCP and TC. Meanwhile the circuit set-up is

@ Exchange efers toIntelligent da ta ba se for numbernetworkranslation

d a t a b a s e

@ I SCCP and T CI ( + M T P )I

- Y-*

_ _ - . I S D N m- - - - - -O ISUP ( + M T P ) @ l S D N/ %

B exchange Traff ic clrcuit exchange

Dials 0800 12345 @) Circuitxtendedslng ISUP

signalling to nextexchange

circuit establlshed- - - - ignalling elation

Figure 12.9 A database query using SCCP and TC



TRANSACTION 261

suspended. When he database nteraction is over and he ISDN exchange has heappropriate nformation, he ircuit an be connectedusinghe standard SUPsignalling.

The SCCP is defined by ITU-T Recs Q.711-Q.714.

12.11 TRANSACTION CAPABILITIES TC)

Building on the oundat ion of MTP and SCCP the ransaction capabilities ( T C )are thatpart of the SS7 signalling systemwhichconveys non-circuit-related nformation. Itsdevelopment as been intertwined withhe evelopment of intelligent networks(Chapter 11 ) . The transaction capabiZity is ideally suited to supervising short ‘ping-pong’ style dialogue between signalling points, typically between an exchange and anintelligent network database. Transaction apabilities ( T C ) breakdown into threecomponent parts,andundertake he unctions of OS1 layers 4-7. The underlyingfoundation (OS1 layers 1-3) is the SCCP and MTP. The ntermediate service part ( Z S P )

c 3p p l i c a t i o n

S i g n a l l in g l e v e l

T C User( A p p l i c a t i o n e n t i t y , A E )

T C C o m p o n e n tA p p l i c a t i o nu b l a y e r

p a r t( T C A P )

Tr a n s a c t i o ns u b l a y e rH

r a n s a c t i o nc a p a b i l i t i e s ( T C )

I

I I n t e r m e d i a t es e r v i c e p a r t ( I S P I

S w i t c h i n g c o n n e c t i o n

c o n t r o l p a r t ( S C C P )I I - - -

M e s s a g e t r a n s f e r

p a r t ( M T P )

Figure 12.10 The ransaction capabilities




carriesout he unctions of OS1 layers 4-6, while the componentublayer andtransaction ublayer exist within OS1 layer 7 and ogether orm he transactioncapabil it ies application part ( T C A P ) .

Transaction capabilities exist to serve a TC-user, normally called an application entity( A E ) . An AE contains the necessary functions to serve a particular application. Inaddit ion, every application entity also contains the transaction capabilities applicationpart (TCAP). TCAP s, in essence, a copy of the ‘rules’ which enable the messages to beinterpreted. (For example an A E may support VPN service. Another might supportf reephone. ) Figure 12.10 illustrates the architecture of TC.

The ISP is required only when large amounts of data are to be transferred, using oneof the SCCP connection-oriented classes.

The TCAP contro ls the ialogue between the exchange and the database, overseeingrequests (questions or instructions) and making sure that corresponding responses aregenerated. The content of the request (the question itself) is prepared by the TC user(i.e. the applicat ion enti ty ( A E ) ) .

An application entity is the logical set of questions, responses and instructions whichconstitute the dialogue necessary to support an application. An A E comprises one or anumber of simple functions, called application service elements (ASEs).The idea is thata small set of multi-purpose ASEs can be combined together in different permutationsto serve different applications. The intelligent network (IN) architecture, for example,defines a set of primitive network actions which i t calls unctional componentsor serviceindependent building blo cks .These are examples of ASEs. Figure 12.11 illustrates theconcept of application service elements.

Three well known application entities defined by ITU-T are the mobile applicationp a r t ( M A P ) , used to suppor t roaming mobile telephone networks, theoperationan dmaintenance appl ica tion part ( O M A P ) ,used for control and maintenance of remote

equipment and exchanges and the intel l igent network applicat ion part (INAP),used inintelligent networks. MAP comprises one complex ASE, OMAP comprises two: MRVTand SRVT (the MTP and SCCP Routing Verification Tests).

Return ing to the TCAP itself, let us briefly describe the functions of the transactionand component sublayers.The transactionublayer is responsible ornitiating,

Application 1 = A S E 1 + A SE 2

Application 2 = A S E 2 + A S € 3-pplication 3 = A S E L

1ApplicationService

( A S E s 1Elemen ts

(Within OS1layer 7 I

A E = Application enti ty

Figure 12.11 Permutation of ASEs to serve different applications



THE MOBILEPPLICATION PART (MAP) 263

maintaining and closing the dialogue between the signalling points. Classification ofmessages into one of the four types listed below helps the two end signalling pointdevices to relate each message back to the previous dialogue and to check that thecommunication is occurring in an orderly ashion. Thuseach message also hasatransaction identity code.

e Begin (dialogueor ransaction)

e Continue

e End

e Abort

The component sublayer provides machine discipline to the dialogue, controlling theinvocation of requests and making sure that they receive proper responses. The ASEinformation within the requests and responses is thus classified into one of five types

e invoke an action

e return the final response (to a sequence)

0 return an intermediate (but not final) response

e return a message to signal an error

e reject a message (if a request is not understood, or is out of sequence)

Although the information content of the requests and responses is not known by thecomponent layer (it s understood only by the ASE or SEs), the component sublayer sable to make sure that commands are undertaken and responses are given.

The transaction capabilities are defined in ITU-T Recs Q.771-4.775.

12.12 THE MOBILE APPLICATION PART (MAP)

The mobile application pa rt is an example of an application entity of SS7 signalling,developed to serve aparticular application. It is used between a mobile elephonenetwork exchange and an intelligent network database, called a home ( H L R ) r visitorlocation register (VLR). he database is kept informed of the current location of themobileelephonehandset. Thus he mobileelephone ustomer’s ncoming and

outgoing calls can be handled at any time. Chapter 15 on cellular telephone networksdescribes this application more fully.

12.13 OPERATION AND MAINTENANCEAPPLICATION PART (OMAP)

OMAP is another application entity of SS7 signalling. It provides for networkmaintenance as well as other network operations and management functions f remoteexchanges and equipment. OMAP contains 2 ASEs: the M T P routing verijication test




( M V RT ) and he S C C P routing erijication test ( S RV T ) . These areproceduresdesigned to enable the network operator to test the integrity of the signalling networksand identify faults.

12.14 INTELLIGENT NETWORK APPLICATION PART (INAP)

The intelligent etwork pplication art ( Z N A P ) comprises et f pplicationfunctions or service building blocksfrom which complex intelligent network services anbebuilt.The initial unctionality defined by the beststandardized INAP that ofETSI)comprises unctions defined n ts capabilityse t 1 ( C S I ) . These are a rangeof relatively simple intelligent network services,but will be very important because theywill be standardized across many different manufacturers’ switch and service controlpoint equipment.

12.15THE USE ANDEVOLUTION OF CCITT7 SIGNALLING

SS7 is an adaptable and continuously volving signalling system hat has been designedto meet the challenging and ever-changing service needs of public and major privatenetwork exchanges making up he ISDN, he B-ISDN and he intelligentnetwork.Network operators may choose o implement the subset of user parts which mostmatches their needs, adopting new user parts as they become available.

Depending on their particular ircumstances, some network operators may choose toimplement an adapted version of some of the SS7 standards, taking up some of thepermitted signalling options. The options allow the operator to ‘tailor’ the system toparticular national requirements (e.g. C7/BT is the UK national version of TUP/ISUPand T1-ISUP s the version of ISUP used between public networks n the United States.Because of the considerable capital investment already committed to SS7, there is apressure for use of a common system, and there is pressure also from established SS7users to ensure that new developments are backward com patible with previous versionsof the system.

One of the ways in which backward compatibility is ensured is by building into allSS7 implementations mechanism orhandlingunrecognized nformation.Suchinformation is bound to be sent occasionally from a more advanced exchange to anexchange with an older version of SS7. The common methods for dealing with thisinformation are

0 to discard t

0 to ignore t

0 to assume that some other expected response (or defau l t ) was actually received

0 to reply with a message of ‘confusion’

0 to terminate he call and reset

0 to raise an alarm to a human



SIGNALLINGETWORKNNING AND TESTING 265

Such a backward compatibilitymechanism obviates any need to stop the evelopment ofSS7, or the extension of its services. If backward compatibility is not built into any newsignalling standard henoint discussions etween he operators interconnectednetworks usingdifferent versions willbe necessary to agree amendments allowingcompatible operation.

12.16 SIGNALLING NETWORK PLANNING AND TESTING

Theignallinginks of a SS7 signallingetwork need careful lanningndimplementation just like any other data network. The links need to be sufficient innumber to handle the overall signalling raffic demand, and to be oriented in a topologythat gives good resilience to network failures.

Twopossible opologies are illustrated in Figure 12.12. Figure 12.12(a) shows ameshed network in which exchanges are capable f both SP and STP functions and elyon one another for theesilience of their signalling relations. n contrast, Figure 2.12(b)shows a topology commonly used in Nor th America, where dedicated and duplicatedcomputers perform the STP function alone; the exchanges are not capable of the STPfunction.

Because of the very complex nature of SS7 signalling, and because of the heavynetwork reliance on it, it is normal to undertake a comprehensive validation testingprogramme prior to the introduction of each new link and exchange. Exchanges built

0 xchange - erforms SPand STP functions

- ignallingin k

( a ) Me she d signalling ne twork

SP

STP only - not anormalexchange

Normal exchange -SP only

- ignalling link(tr af fic circuits not shown)

0

( b ) Use of STPs

Figure 12.12 Typical S S 7 signalling networks




by different manufacturers can sometimesbe ncompatible at first, and a testingprogramme is invaluable in ensuring that their problems are ironed-out before beingbrought into service.

12.17 INTERCONNECTION OF SS7 NETWORKS

Within a network owned and operated by a single network operator, the SS7 signallingsystem (or a variant of it) will make for close control and monitoring of the networkand highly efficient call routing. However,whennetworksbelonging to differentnetwork operators are connected together, neither operator is likely to want the otherto have full control of his network.

In an international network, an operator n one country is unlikely to let the operatorin another control his network management and routing rearrangements.

Competing network operators in a single country may have their networks interc-

onnected but each guards the monitoring and control of his own network fiercely.Public network operators may allow direct SS7 signalling fromcompany privateexchanges but they are unlikely to give up control of the network.

For these reasons, a number of options are permitted within the signalling systemspecifications. These allow operators by mutual agreement to restrict the capability ofthe signalling system when it is used for network interconnection. Thus a period ofnegotiation is necessary prior to the interconnection of networks. A mutual testingperiod confirms that the options have been selected correctly.

12 Signalling System No 7

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