Synchronization in telecommunications

Many services running on modern digital telecommunications networks require accurate synchronization for correct operation. For example, if switches do not operate with the same clock rates, then slips will occur and degrade performance. Telecommunication networks rely on the use of highly accurate primary reference clocks which are distributed network wide using synchronization links and synchronization supply units.

Components of a telecommunications synchronization network

Primary Reference Clock (PRC)

Modern telecommunications networks use highly accurate primary reference clocks that must meet the international standards requirement for long term frequency accuracy better than 1 part in 10-¹¹. To get this performance, atomic clocks or GPS are normally used.

Synchronization Supply Unit (SSU)

Synchronization supply units are used to ensure reliable synchronisation distribution. They have a number of key functions:

1. They filter the synchronisation signal they receive to remove the higher frequency phase noise,
2. They provide distribution by providing a scalable number of outputs to synchronise other local equipment
3. They provide a capability to carry on producing a high quality output even when their input reference is lost, this is referred to as Holdover Mode.

Quality metrics

In telecoms networks two key parameters are used for measurement of synchronisation performance. These parameters are defined by the International Telecommunication Union in its recommendation G.811, by European Telecommunications Standards Institute in its standard EN 300 462-1-1, by the ANSI Synchronization Interface Standard T1.101 defines profiles for clock accuracy at each stratum level, and by Telecordia/Bellcore standards GR-253 and GR-1244.

• MTIE is a measure of the worst case phase variation of a signal with respect to a perfect signal over a given period of time.
• TDEV is a statistical analysis of the phase stability of a signal over a given period of time.

Examples of a system which measures these values is the Sync Test Set and Syncwatch.

See also

• PDH, SDH and SONET
• Caesium standard

References

• Synchronization of Digital Telecommunications Networks, Stefano Bregni, Wiley, ISBN 0-471-61550-1