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8.6 SIGNALING LINK MANAGEMENT 181In the link management examples that follow, a LSSU with a particular value ofLSI is denoted by the corresponding acronym. For example, a “SIN” denotes aLSSU in which LSI ¼ 001.8.6.1 Initial AlignmentWhen a signaling link is turned on, both LCs start sending SIOs. When LC-Aacquires alignment (i.e., recognizes the flags in the incoming bit stream), it startssending SINs. When LC-A begins to receive SINs, indicating that LC-B also hasachieved alignment, a proving period of a few seconds is started. If both LCs maintaintheir alignment during this period, they start sending FISUs and indicate to theirrespective MTP3s that the link is aligned and ready for service.If LC-A has acquired alignment but keeps receiving SIOs or SIOSs, it knows thatLC-B is not in working condition. LC-A then passes an alignment failure indicationto MTP3-A.8.6.2 Error MonitoringWhen the signaling link is in service, each LC monitors the error rate of receivedsignal units. When one of the following conditions occurs, the MTP3 in the signalingpoint is alerted with a link failure indication:1. Sixty-four consecutive signal units have been received with errors.2. The error rate of received signal units exceeds one error per 256 signal units.3. An “impossible” bit pattern, consisting of at least seven consecutive 1’s, hasbeen received, and a flag has not been detected within 16 octets following thatpattern.8.6.3 Delayed AcknowledgmentsEach LC expects that, under normal conditions, its outgoing MSUs are acknowledgedwithin a short time. Excessive delay of acknowledgments is an indicationof problems on the signaling link. Acknowledgment delays are checked by bothLCs, using timers (T 1 ), which expire in 0.5–2 s (Fig. 8.6-1).The procedure at MTP2-A is as follows. Timer T 1 -A is restarted each time anacknowledgment with a new value of BSN (Section 8.3) is received fromMTP2-B, and there is at least one MSU (waiting for acknowledgment) in the RB.If T 1 -A expires, LC-A interprets the absence of received acknowledgments duringthe time-out interval as a signaling link problem and passes a link failure indicationto its MTP3-A.8.6.4 Level 2 Flow ControlWhen a link control, say, LC-A, detects that the number of received MSUs in its IBexceeds a particular value, because MTP3-A has fallen behind in taking out these