In a synchronous system such as SONET, the average frequency of all clocks in the system will be the same (synchronous) or nearly the same (plesiochronous). Every clock can be traced back to a highly stable reference supply. Thus, the STS–1 rate remains at a nominal 51.84 Mbps, allowing many synchronous STS–1 signals to be stacked together when multiplexed without any bit-stuffing. Thus, the STS–1s are easily accessed at a higher STS–N rate.

Low-speed synchronous virtual tributary (VT) signals are also simple to interleave and transport at higher rates. At low speeds, DS–1s are transported by synchronous VT–1.5 signals at a constant rate of 1.728 Mbps. Single-step multiplexing up to STS–1 requires no bit stuffing, and VTs are easily accessed.

Pointers accommodate differences in the reference source frequencies and phase wander and prevent frequency differences during synchronization failures.

Synchronization Hierarchy

Digital switches and digital cross-connect systems are commonly employed in the digital network synchronization hierarchy. The network is organized with a master-slave relationship with clocks of the higher-level nodes feeding timing signals to clocks of the lower-level nodes. All nodes can be traced up to a primary reference source, a Stratum 1 atomic clock with extremely high stability and accuracy. Less stable clocks are adequate to support the lower nodes.

Synchronizing SONET

The internal clock of a SONET terminal may derive its timing signal from a building integrated timing supply (BITS) used by switching systems and other equipment. Thus, this terminal will serve as a master for other SONET nodes, providing timing on its outgoing OC–N signal. Other SONET nodes will operate in a slave mode called loop timing with their internal clocks timed by the incoming OC–N signal. Current standards specify that a SONET network must be able to derive its timing from a Stratum 3 or higher clock.