Synchronization Status Messages (SSM) are supported on devices that support Synchronous Ethernet. SSM allows the synchronization distribution network to determine the quality level of the clock sourcing a specific synchronization trail and also allows a network element to select the best of multiple input synchronization trails. SSMs are defined for various transport protocols (including SONET/SDH, T1/E1, and Synchronous Ethernet), for interaction with office clocks (such as BITS or SSUs) and embedded network element clocks.
SSM allows equipment to autonomously provision and reconfigure (by reference switching) their synchronization references, while helping to avoid the creation of timing loops. These messages are particularly useful for synchronization re-configurations when timing is distributed in both directions around a ring.