It is known to provide a plurality of communications channels, for example optical fiber transmission channels on which digital signals are transmitted in time division multiplexed frames, between different locations. In order to maintain transmission in the event of a fault on one of the channels, it is also known to provide a so-called protection channel via which the traffic of a faulty channel is transmitted. The routing of traffic from a faulty channel onto the protection channel is referred to as protection switching.
The SONET (Synchronous Optical Network) standard (ANSI T1.105 and Bellcore TA-TSY-000253) defines a physical interface, optical line rates known as Optical Carrier (OC) signals, a frame format, and an OAM&P protocol. The OC signals have electrical equivalents call Synchronous Transport Signals (STS). A base rate of 51.84 Mbit/s (OC-1/STS-1) is defined, with higher rates being integer multiples of the base rate. SONET also defines requirements for protection switching, a hierarchy of failure conditions to be used and a signalling scheme. The signalling uses Automatic Protection Switching (APS) bytes within the line overhead (LOH) of the SONET frame.
It is desirable to provide an optical communications system that takes advantage of the inherent efficiency of the SONET standard with respect to the use of automatic protection switching (APS) bytes defined therein for APS signalling. It is also desirable to provide a protection switching arrangement that reduces the duplication of hardware required for implementation.