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8.9 MTP3 SIGNALING NETWORK MANAGEMENT 193TABLE 8.9-1 Messages for Signaling Network ManagementMessage Name Acronym H0 H1 ParameterChangeover Order COO 0001 0001 FSNR (1)Changeover Acknowledgment COA 0001 0010 FSNR (1)Changeback Declaration CBD 0001 0101 CBC (2)Changeback Acknowledgment CBA 0001 0110 CBC (2)Emergency Changeover ECM 0010 0001 —Emergency Changeover Acknowledgment ECA 0010 0010 —Transfer Prohibited TFP 0100 0001 PAD (3)Transfer Allowed TFA 0100 0101 PAD (3)Transfer Controlled TFC 0011 0010 PAD (3)Notes: (1) Forward sequence number of last accepted MSU. (2) Changeback code. (3) Point code of affecteddestination.Source: Rec. Q.701. Courtesy of ITU-T.8.9.3 Procedures for Congestion ControlThe MTP2s of each signaling link constantly monitor the number of waiting messagesin their output and retransmission buffers [11]. When this number exceeds apreset threshold value, SNM is alerted with an onset of congestion indication(Section 8.6.6). Outgoing signaling link congestion is determined independentlyat each end of the link. A signaling link between signaling points A and B canthus be congested at A and not congested at B.When a signaling link is congested, SNM considers all destinations whose signalingroute sets include that link as “congested.” In Fig. 8.9-3, signaling end point Ahas two signaling links (SL 1 and SL 2 ), to the signal transfer points B and C in itsregion. The route set at A for all destinations thus consists of SL 1 and SL 2 . WhenSNM-A receives an “onset of congestion” indication from the MTP2 of signalinglink SL 1 , it therefore concludes that its route sets to all destinations are congested.Figure 8.9-3. Congestion of SL 1 at signaling end point A.