Synchronization in telecommunications
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Many services running on modern digital telecommunications networks require accurate synchronization for correct operation. For example, If switches do not operate with the same rate clocks then slips will occur degrading performance. Telecommunications networks rely on the use of highly accurate Primary Reference Clocks which are distributed network wide using Synchronization Links and Synchronization Supply Units.
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[edit] Components of a telecommunications synchronization network
[edit] Primary Reference Clock
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 10ยนยน. To get this performance systems based on atomic clocks or receivers using GPS are normally used..
[edit] Synchronization Supply Unit
Synchronization Supply Units are used to ensure reliable synchronisation distribution. They have a number of key functions:
- They filter the synchronisation signal they receive to remove the higher frequency phase noise,
- They provide distribution by providing a scalable number of outputs to synchronise other local equipment
- 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.
[edit] 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 and by European Telecommunications Standards Institute in its standard EN 300 462-1-1
- 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.
[edit] See also
- PDH, SDH
- GPS
- Caesium standard
- ANSI T1.101
- International Telecommunication Union
- European Telecommunications Standards Institute
[edit] References
- Synchronization of Digital Telecommunications Networks, Stefano Bregni, Wiley, ISBN 0-471-61550-1
