Signaling System No. 7 (SS7/C7) - Protocol, Architecture and Services (Full Book)
Signaling System No. 7 (SS7/C7) - Protocol, Architecture and Services (Full Book) |
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Signaling Link Error MonitoringError rate monitoring is performed both for an in-service link and when the initial alignment procedure is performed. Signal Unit Error Rate Monitor (SUERM) and the Alignment Error Rate Monitor (AERM) are the two link error rate monitors that are used [51]. The SUERM performs monitoring when the link is in service, and the AERM performs monitoring when the link is undergoing initial alignment to bring it into service. The following sections describe these two link error rate monitors. SUERMThe SUERM is active when a link is in service, and it ensures the removal of a link that has excessive errors. It employs a leaky bucket counter, which is initially set to 0. The counter is increased by 1 for each SU that is received in error. The counter is decreased by 1 for each block of D consecutive SUs received without error, if it is not at 0. If the link reaches a threshold of T, MTP2 informs MTP3, which removes it from service. For a 64-kbps link, the values of D and T are 256 and 64, respectively. NOTE In ANSI networks, high-speed links (1.536 Mbps) use an errored interval monitor, which differs in its threshold and counting values from those used by the SUERM on low-speed links (see Figure 6-13). Refer to ANSI T1.111 for more information. Figure 6-13. SUERM Counter
The SUERM enters octet counting mode if an SU fails the acceptance procedure (seven or more consecutive 1s, length is not a multiple of 8 bits, or SU length is not between 6 and 279 octets). For every block of N octets counted during octet counting mode, the SUERM is increased by 1. If the octet counting mode continues for a significant period of time (meaning that SUs cannot be identified from the received data), the link is removed from service. The SUERM reverts to normal mode if a correctly checked SU is received. AERMThe AERM is active when the link is in the proving period of the initial alignment procedure. The counter is initialized to 0 at the start of the proving period and is increased for every LSSU that is received in error. If octet counting mode is entered during the proving period, the counter is increased for every block of N octets that is counted. The proving period is aborted if the counter reaches a threshold value of Ti; it is reentered upon receiving a correct LSSU, or upon the expiration of the aborted proving period. Different threshold values Tin and Tie are used for the normal and emergency alignment procedures, respectively. If the proving is aborted M times, the link is removed from service and enters the idle state. The values of the four parameters for 64-kbps and lower bit rates (both for ITU and ANSI) are as follows:
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