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8.8 MTP3 SIGNALING MESSAGE HANDLING 187SMH Message Routing. This function group selects the outgoing signaling linksfor outgoing messages (received from the MTP-users in the signaling point) andtransit messages (received from SMH message discrimination).8.8.5 Selection of Outgoing Signaling LinksSMH message routing first determines the signaling route set for the message, basedon the values of SSF and DPC, which indicate the point code of a national or internationaldestination.The selection of a particular outgoing signaling link in a route set has two objectives.In the first place, a signaling point should distribute its messages to a particulardestination equally among the signaling links in its route set to that destination.In the second place, all messages pertaining to a particular trunk, or a particulartransaction, should be routed on the same signaling route. This is necessary to avoidout-of-sequence delivery of messages pertaining to a trunk, or a transaction. Wehave seen that SS7 messages are delivered in-sequence from the MTP2 at oneend of the link to the MTP2 at the other end (Section 8.3). However, messagesspend some “queueing delay” time in the output and input buffers of the MTP2sat the ends of a signaling link. These delays vary from moment to moment andfrom link to link. If two messages, M 1 and M 2 , which pertain to a trunk or transactionwould traverse different signaling routes, inequalities in the delays on thesignaling links along routes could result in out-of-sequence message delivery.Therefore, the MTP-users assign a particular SLS value to all outgoing messagesrelating to a particular trunk or transaction, and SMH message routing determinesthe outgoing signaling link in a route set from the value of SLS.The assignments of SLS can be made in several ways. For example, in TUP andISUP, each trunk in a trunk group is identified by a circuit indication code (CIC)—see Section 7.2.5—and SLS values can be derived from CIC codes as follows:ITU-T SS7: SLS ¼ CIC mod 16ANSI No: 7: SLS ¼ CIC mod 32Since each SLS value covers approximately the same number of trunks, the loadsof outgoing messages the various SLS values are approximately equal.We shall see in Chapter 15 that SCCP can provide in-sequence delivery of messagesrelating to individual transactions, by assigning the same SLS value to all outgoingmessages of a particular transaction.A number of link-selection examples are outlined below.Example 1. Figure 8.8-3(a) shows the signaling links in the route set between signalingpoints A and F in the original Bell System signaling network (see Section5.1). We examine the signaling link selection for messages from A to F [10].Under normal (nonfailure) conditions in the signaling route set, signaling linksSL 9 and SL 10 are not used.