Evaluation of the APS protocol for SDH rings reconfiguration

The synchronous digital hierarchy (SDH) architecture is one of the underlying technologies used by asynchronous transfer mode (ATM) networks. The SDH includes various protection mechanisms. One main design issue is probably the reconfiguration process in case of failure. In case of SDH works on optical fibers with a ring network topology, the automatic protection switching (APS) protocol can be used. This paper addresses the problem of the maximum allowed recovery time in the four fibers ring architecture. We analyze the APS protocol and derive upper bounds for the processing time in each node of the network in order to cope with the maximum reconfiguration time of 50 ms, as specified in the standard. We finally analyze the behavior of the system in case of two interleaved failures. A worst case analysis is carried out, showing that a 100-ms reconfiguration time can be guaranteed.