12 Signalling System Noe 7 12.1 The ITU-T signalling system number 7, SS number 7, SS7, CCITT7, C7 or number seven signalling system is the most recently developed of telephone network signalling systems. It is already widely deployed in digital telephone networks and ISDNs across the world, and will also be a ‘cornerstone’ of ‘intelligent networks’ and broadband ISDNs (B-ISDN). It is a complex, common channel signalling system, which enables the controlling processors of two digital exchanges or databases to communicate directly and interact with one another in a manner optimized for digital transmission media. SS7 has also formed the basis of a number of further- developed regional signalling systems. In the United States, for example, ‘ANSI SS7’ is a derivative, while the UK national version is ‘C7/BT’. This chapter describes the overall structure and capabilities of SS7. SS7 SIGNALLING BETWEEN EXCHANGES The SS7 signalling system is described in the 4.700 series of ITU-T recommendations. A common channel signalling system, optimized for digital networks, it allows direct transfer of call information transfer between exchange processors. Comprising a number of layered and modular parts, each with a different function, it is a powerful general-purpose signalling system capable of supporting a range of applications and administrative functions, including e ISDN (integrated services digital network) e intelligent networks (INS) e mobile services (e.g. cellular radio) e network administration, operation and management 249 Networks and Telecommunications: Design and Operation, Second Edition. Martin P. Clark Copyright © 1991, 1997 John Wiley & Sons Ltd ISBNs: 0-471-97346-7 (Hardback); 0-470-84158-3 (Electronic) 250 SIGNALLING SYSTEM NO. 7 In addition, its modular nature lends itself to the development of new user parts which may be designed to support almost any new service that can be conceived. The user parts of the system that have been developed so far are 0 MTP 0 SCCP 0 TUP 0 DUP 0 ISUP 0 TC 0 TCAP 0 OMAP 0 INAP 0 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 part mobile application part The MTP and SCCP form the ‘foundations’ of the system, providing for carriage of messages. The TUP, DUP and ISUP use the MTP and/or SCCP to convey messages relating to call control, for telephone, data, and ISDN networks, respectively. The OMAP, MAP and INAP are other application parts for operation and maintenance interaction, mobile network control and intelligent network services, respectively. Initially the SS7 system was designed so that the MTP could be used in association with 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; the two 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 in both exchanges. The functions reside in a unit termed a signalling point. This may be a separate piece of hardware to the exchange, but usually it is a software function in the exchange central processor. SS7 signalling points (SPs), basically exchanges, intercommunicate via signalling links and are said to share a signalling relation. A single SS7 signalling link enables information to be passed directly between two exchange processors, allowing the set-up, control, and release of not just one, but a large 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’. The term common channel signalling aptly describes this method of operation, distinguishing it from the channel-associated signalling method, wherein call set-up signals pertinent to a particular circuit are sent down that circuit. SS7 is not the first common channel signalling system to be developed; CCITT 6 (SS6) was also a common channel system, but CCITT 6 was less flexible than SS7 and not so suitable for digital network use. Having a common channel for conveyance of signalling messages saves equipment at both exchanges, because only one ‘sender’ and one ‘receiver’ is required at each end of the link, as against the one per circuit required with channel-associated systems. The SS7 SIGNALLING NETWORKS 251 Exchange A I I Exchange B ST = signalling terminal Figure 12.1 Linking two exchanges using SS7 signalling combination of a SS7 sender and receiver is normally referred to as a signalling terminal. In practice signalling terminals are a combination of a software function in the exchange central processor and some hardware to terminate the line and undertake the basic bit transfer function (OS1 layer 2, datalink). A label attached to each message as it passes over the signalling link enables the receiving signalling point to know which of the many circuits it relates to. Figure 12.1 illustrates the network configuration of a simple SS7 signalling link. It shows calls flowing over a large number of traffic-carrying circuits which are connected to the switch matrix part of the exchange. Meanwhile all these circuits are controlled according to the information passed directly between the exchange processors. The signalling terminal (ST) 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 the network of traffic-carrying circuits interconnecting the exchanges. The second subnetwork is that of the signalling links. In Figure 12.1 we saw this separation of traffic-carrying circuits from signalling link as it would apply on a single connection between two exchanges. Figure 12.2 now shows a more complicated example to illustrate another powerful feature of SS7: the fact that signalling networks and traffic- carrying networks may be designed and implemented almost in isolation from one another. Just because there are direct traffic-carrying circuits between two exchanges (they have a direct trafic-carrying relation) it does not follow that the signalling information (or signalling trafJic) has to travel over direct signalling links, though clearly a signalling relation of some sort is needed. 252 SIGNALLING SYSTEM NO. 7 U I r I H, I ExchDange I U Signalling links 'm Traffic-carrying circuits Figure 12.2 Traffic-carrying and signalling networks in SS7 Figure 12.2 shows the traffic-carrying networks and signalling networks inter- 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 all traffic to or from exchange A passes either via B or C, and so on. The signalling network, 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 of the actual traffic from A to B, there exist both direct traffic circuits and a direct signalling link. In effect, this is the same as Figure 12.1, so that both signalling messages and traffic can be passed directly between the two. Similarly exchange B may pass signalling messages and traffic directly either to exchange C or exchange D, and may also act as a normal transit 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, in which signalling links and traffic circuits have a similar configuration, and signalling messages and traffic both route in the same manner. In short, there is a signalling link associated with each link of direct traffic-carrying circuits. By contrast, although exchange A is directly connected to exchange C by traffic- carrying circuits, there is no direct signalling link. Signalling information for these circuits 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 in this instance to perform the function of a signal transfer point (STP), as illustrated in Figure 1 2.3. THE STRUCTURE OF SS7 SIGNALLING 253 Exchange n Exchange Exchange SP sp /////////l sp SP ////U Associated mode Ouosi - associated mode slgnalling link SP = signalling point .m traffic- carrying circuits STP = signal transfer point Figure 12.3 Modes of SS7 signalling Signalling information is passed over SS7 signalling links in short bursts; indeed a SS7 signalling network is like a powerful packet-switched data network. To identify each of the signalling points for the purpose of signalling message delivery around the network, 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, or whether they are to be transferred (in STP mode) to another SP. The codes are allocated on a network by network basis. Thus the code is only unique within, say, national network A, national network B or the international network. 12.3 THE STRUCTURE OF SS7 SIGNALLING Thanks to the modular manner in which the SS7 system has been designed, it encourages the development of new modules in support of future telecommunications services 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 the same way as the OS1 model has a number of functional layers, each an important foundation for the layers above it, so SS7 signalling is designed in a number of functional levels. Note in Figure 12.4, that the component levels and parts of SS7 do not align with the OS1 layered model. The lack of alignment of signalling levels with OSZ layers is unfortunate and it arises from the fact that the two models were developed concurrently but for different purposes. The lack of alignment of levels with layers means that not all higher layer OS1 protocols are currently suitable for use in conjunction with the lower levels of SS7 signalling. The various standards development bodies are trying to rationalize the component parts of SS7 to conform with the OS1 model. The signalling connection and controlpart (SCCP), for example, delivers the OS1 network service (OS1 layer 3 service), so that a communication system can use the SCCP (and MTP below it) to support layers 4-7 OSI-based protocols. The levels in SS7 signalling provide a convenient separation of signalling functions, and in the remainder of the chapter the signalling level model is used in explanation. 254 SIGNALLING SYSTEM NO. 7 OS1 layer Application [l 7 6 5 L 3 I SCCP Ilj DUP - MTP Message transfer over signalling network over single link data link Signalling level L User level Network level Link * level , Oatalink level 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, Telephone user part; DUP, Data user part; SCCP, Signalling connection and control part; MTP, Message transfer part 12.4 THE MESSAGE TRANSFER PART (MTP) The foundation level of the SS7 signalling system is the message transfer part defined by ITU-T Recommendations Q.701-4.707. The message transfer part takes care of the conveyance of messages, fulfilling signalling level functions 1 to 3 (sometimes labelled MTPl, 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 medium as laid down by ITU-T G series recommendations. The level 1 function allows for an unstructured bit stream to be passed between SPs over an isolated signalling data link. Level 2 (signalling link junctions) This level defines the functions and procedures relating to the structure and transfer of a signal. Message flow control, and error detection and correction are included. (Flow control prevents the over-spill and consequent loss of messages that result if a message is sent when the receiving end was not ready to receive it; error detection and correction procedures eliminate message errors introduced on the link.) THE MESSAGE TRANSFER PART 255 Level 3 (signalling network functions) This level defines the functions and procedures for conveying signalling messages around an entire signalling network. It provides for the routing of messages around the signalling network. In this role it has a number of ‘signalling network management’ capabilities including ‘load sharing’ of signalling traffic between different signalling links and routes (illustrated in Figure 12.5) and re-routing around signalling link failures. Link sharing on the same route between signalling points (SPs) guards against lineplant failure (Figure 12.5(a)). Route sharing may additionally provide protection against failure of STPs. Thus in Figure 12.5 the signalling traffic from SP A to SPs B and C is shared over the two STPs, D and E. In the event of a failure of any of the routes shown, signalling messages could be re-routed. MTP is useless on its own for setting up telephone or other connections. To perform these functions MTP needs to be used in association with one of the SS7 user parts which are level 4 or user functions. Examples are the telephone user part (TUP) and the integrated services digital network user part (ISDN-UP or ISUP). 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. Flag TheJag is the first pattern of bits sent. This is an unmistakeable pattern to distinguish the beginning of each message, and delimit it from the previous message. It is comparable to the synchronization (SYN) byte in data communications (Chapter 9). MTP information The flag is followed by a number ofjelds of information, which together ensure the correct message transfer. These fields include: the message sequence numbers that keep SP SP SP STP SP A-B and A-C signalling messages evenly divided to route via both D and E. STP SP Figure 12.5 Load sharing over signalling. A-B and A-C signalling messages evenly divided to route via both D and E 256 SIGNALLING SYSTEM NO. 7 Next message r First bit transmitted U bits l Message sequence Check and ‘user part’ (message substance 1 Flag numbers, length Signalling information field type information (Inserted by appropriate ‘level 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; and information about the type and length of the information held in the main ‘signalling information field’; it might say which user part is in operation and record the length of the message. Signalling information jield This is the main information field or the ‘substance’ of the message. The information is inserted by one of the user parts, as appropriate for the particular application (e.g. telephone user part (TUP), or integrated services user part (ISUP)). The structure of this$eld depends on which user part is in use. Check bits Finally, each MTP message is concluded with a check bit field. This is the data (cyclic redundancy check code or CRC) needed to perform the error detection and correction mechanism of the MTP level 2. The check bits are followed by the flag at the start of the next message. 12.5 THE USER PARTS OF SS7 The various user parts of SS7 are alternative functions meeting the requirements of level 4 of the signalling level model. The user parts may be used in isolation, or sometimes may be used together. Thus the telephone user part (TUP) and the MTP together are sufficient to provide telephone signalling between exchanges. The data user part (DUP), ISDN user part (ISUP) and other user parts need not be built into a pure telephone exchange. An example where more than one user part is employed is the combination of SCCP (signalling connection and control part), ISP (intermediate service part) and TCAP (transaction capability application part). These are all necessary for the support of the intelligent networks described in Chapter 11). The remainder of the chapter describes the capabilities of each of the level 4 user parts of SS7. THE TELEPHONE USER PART (TUP) 257 12.6 THE TELEPHONE USER PART (TUP) The telephone user part comprises all the signalling messages needed in a telephone network 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 process of digit analysis and route selection, seizes the outgoing circuit and sends the dialled digit train onto the next exchange in the connection by using the SS7 signalling link, conveying TUP encoded messages using the MTP. Crudely put, an example of a TUP message might be ‘connect the call on circuit number 56 to the destination directory number 071-234 5678’. Backward messages such as ‘destination busy’ are also included in the telephone user part. The structure of TUP messages is shown in Figure 12.7. TUP messages occupy the signalling information Jield of the underlying MTP message. The messages comprise a TUP signalling information field which is used to convey ‘dialled digits’, ‘line busy’, ‘answer’ signals, and other circuit-related information, together with four adminis- trative fields as follows. Destination point code (DPC) This code identifies the signalling point to which the signalling message is to be delivered by the MTP. (The destination of a signalling message is not necessarily the same as the final destination of the call.) The signalling point is in the exchange that forms the next link of the connection (for forwardmessages) or in the previous exchange (for backward messages). Originating point code (OPC) This code identifies the signalling point which originated the message (again not necessarily the origination point of the call). Circuit identijication code (CIC) This is a number that indicates to the exchange at the receiving end of the signalling link which traffic circuit each message relates to. The telephony user part is defined in ITU-T Recs. Q.721-Q.725. TUP messoge > TUP signalling information CIC= Circuit identificatlon (others as SCCP fields) CIC OPC code DPC \ / \ ] /[[ bit sent 0 / information field MTP message 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 on circuit-switched data networks. The message structure of the DUP is very similar to that of the TUP, illustrated in 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 SS7 integrated services digital network user part, ISDN-UP or ISUP, is the signalling system designed for use in ISDNs. In effect it is a combination of capabilities similar to TUP and DUP, which allow voice and data switched 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 of the other systems. ISUP is defined by CCITT Rec Q.761-Q.764. The ISDN user part (ISUP) interacts as necessary with the ISDN D-channel, signalling (DSSI, digital subscriber signalling 1, as defined by recommendation Q.931) to convey end-to-end information between ISDN user terminals. Such information includes the terminal compatibility checking procedure which ensures that a compatible receiving terminal is available at the location dialled by the caller. As we learned in Chapter 10, the procedure prevents, for example, the connection of a group 4 facsimile machine 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 was developed by CEPT as recommendation TjSPS 43-02 for use as an interim ISDN-like signalling system supporting an early pan-European ISDN. It is used in Europe by France Telecom for international ISDN signalling, but is likely to be superseded by ISUP. 12.10 THE SIGNALLING CONNECTION CONTROL PART (SCCP) The SCCP is used to convey non-circuit-related information between exchanges or databases, between an exchange and a database or between two exchanges (for certain types of ISDN supplementary services). By non-circuit-related we mean that although a signalling relation is established between an exchange and a database, no traffic circuit is intended to be set up. In essence the SCCP (in conjunction with the TC and relevant application part) provides a means for querying a reference store of information, as is necessary during call set-up on intelligent networks. It is an ideal data transfer mechanism for