Automatic Protection Switching (APS) for Synchronous Optical NETwork (SONET)/Synchronous Digital Hierarchy (SDH) technology according to the GR-253-CORE specification entitled “Synchronous Optical Network (SONET) Transport Systems: Common Generic Criteria”, for example, requires fault detection in SONET/SDH connections to subtending equipment, or in the SONET/SDH equipment itself. In a 1+1 linear APS implementation, traffic is electrically bridged from a working connection to a protection connection at transmitting equipment, and receiving equipment normally selects traffic from the working connection. A fault or other condition affecting the working SONET/SDH connection causes an APS operation, and traffic is then selected from the protection connection.
This type of redundancy protection tends to be implemented in a single equipment chassis. This can significantly limit the actual level of protection that is provided, in that a failure affecting one equipment installation would interrupt traffic flow.
Such a protection mechanism also provides protection only for the optical SONET/SDH connections, and not for further connections, such as access-side connections from which traffic for SONET/SDH connections originates. APS operations are driven only by optical connection faults or failures.