Motivated by the excessive link status changes observed in some field operations of the common channel signaling (CCS) network, the authors provide a detailed analysis of the signaling link error monitoring algorithms in the Signaling System No. 7 (SS7) protocol. These algorithms determine when to fail a link due to excessive error rates and when to put a failed link back into service. The analysis shows that, under current SS7 specifications of the error monitoring algorithms, the probability of a signaling link oscillating in and out of service could be high, depending on the traffic load, signal unit size, and the statistical nature of errors (bursty or random). The link oscillation phenomenon could become worse as longer Transaction Capability Application Part (TCAP) messages for transaction-based services (e.g., 800 Service) are carried in the CCS networks. While the risk to the existing network may not be high due to the light loads carried at present, there is still a need to study the error monitoring issues thoroughly.
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