FAULT DETECTION AND ISOLATION IN A SYNCHRONOUS OPTICAL NETWORK (SONET) AND IN A SYNCHRONOUS DIGITAL HIERACHY (SDH) NETWORK

An in-band fault detection and isolation system and method for networks such as Synchronous Optical Network/Synchronous Digital Hierarchy (SONET/SDH) networks. Fault detection may be quickly and efficiently performed by a network without requiring modification to any add/drop multiplexers (ADMs) and/or without the involvement of an external network management system (NMS). To accomplish this without violating SONET/SDH standards, a new signal, called a fault isolation signal in the path (FIS-P), is proposed. The FIS-P may be used in lieu of an AIS-P to support special applications on certain digital cross connect (DCS)-to-DCS path segments. Moreover, the present invention may be implemented such that the behavior of path terminating equipment (PTE) is unaffected.