Signaling Data Link Signaling Link Control

134 Communication Networks Copyright © 2005 PragSoft • The message routing function handles the further routing of an MSU by choosing a signaling link to forward it on. Routing is facilitated by a routing label which consists of the source and destination addresses and the SLS field. The SLS guides link selection so that the load is shared between alternate links. • The message distribution function accepts an MSU destined for this signaling point and uses the SIO field to determine the user part the message should be forwarded to. Figure 10.13 Signaling network functions. Level 3 Level 2 Level 4 Message Distribution Message Discrimination Message Routing Signaling Management Network Signaling Handling Message The signaling network management function is responsible for monitoring and maintaining the signaling service by overcoming signaling failures and by congestion control. It provides re-routing information to signaling points and controls the activation and deactivation of links in response to changed link status. SS7 levels 1-3 i.e., signaling data link, signaling link control, and signaling network functions are collectively referred to as the Message Transfer Part MTP. The MTP provides a highly reliable connectionless datagram service. It is described by CCITT recommendations Q.701-Q.710. Methods for monitoring and measuring MTP performance are described by CCITT recommendation Q.791.

10.3.4. Signaling Connection Control Part

The network level functions provided by the MTP are relatively minimal. The Signaling Connection Control Part SCCP augments the services of MTP to provide user parts with enriched connectionless or connection-oriented services. It is specified by CCITT recommendations Q.711-Q.714. The combination of MTP and SCCP is called the network service part. www.pragsoft.com Chapter 10: Telephone Networks 135 SCCP provides services similar to those of the OSI network layer to the SS7 user parts. It serves as a means of using higher layer OSI protocols to communicate with SS7. Five classes of SCCP service are identified: 0. Basic connectionless class. 1. Sequenced connectionless class. 2. Basic connection-oriented class. 3. Flow control connection-oriented class. 4. Error recovery and flow control connection-oriented class. Class 1 denotes the MTP service described earlier. The connection-oriented service is defined in terms of the same set of service primitives as presented for the OSI network layer see Chapter 4 and will not be further discussed here. SCCP messages appear as the SIF field in the signal unit. Figure 10.Error Bookmark not defined. illustrates the general structure of an SCCP message. Figure 10.14 SCCP message structure. Field Description Routing Label The routing label in SIF as explained earlier. Message Type Denotes the type of SCCP message. Mandatory Fixed Part Denotes the mandatory message parameters. Mandatory Variable Part Denotes additional parameters for variable messages. Optional Part Denotes optional parameters.

10.3.5. User Parts

User part messages appear as the SIF field in signal units. Also important, is the SIO field which, as mentioned earlier, guides message distribution. A subfield in SIO determines the message category as one of: • SCCP message • Telephone user part message • ISDN user part message • Data user part message • Signaling network management message • Signaling network testing and maintenance message The Telephone User Part TUP defines the SS7 telephone signaling messages and functions for national and international calls. It directly uses the services of the