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 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	Synchronization 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.

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^[1] and GR-1244.^[2]

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.

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

References

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

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