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8.7 OVERVIEW OF MTP LEVEL 3 183Figure 8.7-1. MTP3 primitives. SIO, Service information octet; SSF, sub service field; SI, serviceindicator; SIF, signaling information field; RL, routing label; OPC, origanting point code; DPC,destination point code; SLS, signaling link section; UM, user message; PAD, point code ofaffected destination. (From Rec. Q.701 and Q.704. Courtesy of ITU-T.)Digital Network User Part (ISUP), and Signaling Connection Control Part (SCCP).SMH is described in Section 8.8.Signaling Network Management (SNM). This keeps the message trafficflowing under abnormal conditions (congestion, failures) in the signaling network.SNM is described in Section 8.9.8.7.1 MTP-Transfer PrimitivesMTP-users pass their outgoing messages to SMH in MTP-transfer requests, andSMH delivers incoming messages to the MTP-users in MTP-transfer indications(Section 7.3.2).The primitives include the parameters in the service information octet (SIO) andthe signaling information field (SIF) (Fig. 7.3-3). The parameters are discussed inSection 8.8.8.7.2 MTP Status, Pause, and Resume IndicationsSNM passes these indications to the MTP-users at its signaling point, to inform themabout abnormal conditions (failures, congestion) in the signaling routes to particulardestinations. The affected destinations are identified by their point codes.