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182 SS7 MESSAGE TRANSFER PARTMSUs, it starts sending SIB (link busy) status units to LC-B, at intervals of80–120 ms. It continues sending outgoing MSUs and FISUs but discards incomingMSUs and “freezes” the value of BSN in the SUs it sends out. The delay in acknowledgmentswould normally cause timer (T 1 -B) of LC-B to time out. However, timers(T 1 ) are also restarted each time a SIB is received from the distant MTP2. Therefore,T 1 -B does not expire as long as SIBs are being received.When LC-B receives the first SIB, it also starts a timer T 2 -B, which expires in3–6 s. If the congestion at MTP2-A abates, it again acknowledges received signalunits, and MTP2-B starts to receive SUs with “new” values of BSN. If thishappens before T 2 -B has expired, LC-B stops the timer and resumes normal operation.However, if T 2 -B expires and LC-A is still sending SUs with the “frozen”BSN value, LC-B passes a link failure indication to its MTP3-B.8.6.5 Processor OutageWith this procedure, the MTP3 at either end of the signaling link can temporarilysuspend the operation of the link. Assume that LC-A in Fig. 8.6-1 has received acommand (C) from MTP3-A to suspend incoming and outgoing message trafficon the link. LC-A then starts sending a continuous stream of SIPOs (processoroutage) and discards received MSUs. On receipt of the SIPOs, link control(LC-B) starts sending fill-in signal units (FISUs)—see Fig. 8.3-3(b)—and indicatesto its MTP3-B that the signaling link is out of operation.When LC-A receives a command from its MTP3-A to resume operation, it stopssending SIPOs and resumes sending MSUs and FISUs. LC-B, which no longerreceives SIPOs, indicates to its MTP3-B that the signaling link is back in operationand resumes sending MSUs.8.6.6 Outgoing CongestionLink controls (LCs) constantly monitor N, the total number of outgoing MSUs intheir output and retransmit buffers, OB and RB (Fig. 8.3-1). When N exceeds athreshold value N on , LC passes an onset of congestion indication to its MTP3,which then takes measures to reduce the outgoing MSU traffic temporarily. WhenN drops below a threshold value N off , the link control passes an end of congestionindication, and MTP3 resumes normal outgoing message flow. To avoid rapid fluctuationsin link status, N off is set below N on . This gives a certain amount of hysteresisin the status transitions.8.7 OVERVIEW OF MTP LEVEL 3The message transfer part level 3 (MTP3) is divided into the fol1owing two groupsof functions (Fig. 8.7-1).Signaling Message Handling (SMH). This handles the transfer of messagesbetween “peer” MTP-users–Telephone User Part (TUP), Integrated Services