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 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 master clocks that must meet the international standards requirement for long term frequency accuracy better than 1 part in 1011. To get this performance, atomic clocks or GPS disciplined oscillators are normally used.
Sync status messages
|DS1 ESF||S1 byte||ANSI/SONET Abbreviation||ANSI/SONET Comments||SDH Abbreviation||SDH Comments|
|00000100 11111111||0001||ST1 / PRS||Stratum 1 / Primary Ref Source|
|00000010 11111111||0010||PRC / G.811||Primary Reference Clock|
|00001000 11111111||0000||STU||Stratum Traceability Unknown||QU / UNK||Quality Unknown|
|00001100 11111111||0111||ST2||Stratum 2||G.812 Type II|
|01111000 11111111||0100||TNC||Transit Node Clock||SSU-A / G.812 Type V||also referred to as any of the following: G.812T / SSU-T / transit|
|01111100 11111111||1101||ST3E||Stratum 3E||G.812 Type III|
|unknown||1000||SSU-B / G.812 type VI||also referred to as any of the following: G.812L / SSU-L / local|
|00010000 11111111||1010||ST3||Stratum 3||G.812 Type IV|
|00100010 11111111||1011||SEC / G.813 option 1||Synchronization Equipment Timing Source(SETS)|
|00100010 11111111||1100||SMC||SONET Minimum Clock||G.813 option 2|
|00101000 11111111||not used||ST4E||Stratum 4E|
|00101000 11111111||not used||ST4||Stratum 4|
|00110000 11111111||1111||DUS||Don't Use for Sync||DUS||Don't Use for Sync|
Synchronization Supply Unit
Synchronization supply units (SSU) 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.
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.
- Maximum time interval error (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.
- Time deviation (TDEV) is a statistical analysis of the phase stability of a signal over a given period of time.
- PDH, SDH and SONET
- Caesium standard
- Synchronous network
- Isochronous signal
- Mesochronous network
- Plesiochronous system
- Asynchronous communication
- Synchronization of Digital Telecommunications Networks, Stefano Bregni, Wiley, ISBN 0-471-61550-1