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192 SS7 MESSAGE TRANSFER PARTFigure 8.9-2. MTP3 message with a signaling network management message in the usermessage field. (From Rec. Q.704. Courtesy of ITU-T.)8.9.2 SNM MessagesTo send and receive messages to/from other signaling points, SNM uses the servicesof SMH in the same manner as the MTP-users.The format of a MTP3 message with a SNM message in its user message (UM)field is shown in Fig. 8.9-2. The contents of routing label (RL) are shown in theITU-T SS7 format.SNM messages have service indicator code (SI) ¼ 0000. RL contains OPC andDPC and, in lieu of the signaling link selector (SLS) parameter, the parameter signalinglink code (SLC).Signaling Link Code (SLC). Some SNM messages have status informationabout a particular signaling link. In these messages, the signaling link is identifiedby the combination of the parameters: DPC, OPC, and SLC. In SNM messagesthat do not pertain to a particular signaling link, the link code is set to SLC ¼ 0.Since the MTP3 messages that carry SNM messages do not have a SLS parameter,SMH message routing has to use special routing rules. When SLC ¼ 0, the messagecan be routed on any link in the route set to the message destination. When SLC = 0,it identifies a particular signaling link. In this case, the message has to be sent eitheron the identified link or on an alternate of the identified link, depending on the SNMmessage type. We will see the reason for this in the sections that follow.Headings and Parameters. SNM messages always contain a heading and mayinclude a parameter. The heading consists of the H0 and H1 fields. The code in H0represents a group of functionally related messages, and the code in H1 indicates aparticular message in the group—see Table 8.9-1. The table also lists the messageacronyms and—if the message includes a parameter—the parameter name andacronym. In the sections that follow, the messages and parameters are denoted bythese acronyms.When a procedure involves signaling network management at several signalingpoints, we shall denote the SNM at a particular signaling point—say, point A—by SNM-A, and so on.