Synchronous Optical Networking - Synchronization

Synchronization

Clock sources used for synchronization in telecommunications networks are rated by quality, commonly called a stratum. Typically, a network element uses the highest quality stratum available to it, which can be determined by monitoring the synchronization status messages (SSM) of selected clock sources.

Synchronization sources available to a network element are:

Local external timing

This is generated by an atomic Caesium clock or a satellite-derived clock by a device in the same central office as the network element. The interface is often a DS1, with sync-status messages supplied by the clock and placed into the DS1 overhead.

Line-derived timing

A network element can choose (or be configured) to derive its timing from the line-level, by monitoring the S1 sync-status bytes to ensure quality.

Holdover

As a last resort, in the absence of higher quality timing, a network element can go into a holdover mode until higher-quality external timing becomes available again. In this mode, the network element uses its own timing circuits as a reference.