Synchronization in telecommunications
Encyclopedia
Many services running on modern digital telecommunications network
s require accurate synchronization
for correct operation. For example, if switch
es 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.
s that must meet the international standards requirement for long term frequency accuracy better than 1 part in 10¹¹. To get this performance, atomic clock
s or GPS are normally used.
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.
Examples of a system which measures these values is the Sync Test Set and Syncwatch.
Telecommunications network
A telecommunications network is a collection of terminals, links and nodes which connect together to enable telecommunication between users of the terminals. Networks may use circuit switching or message switching. Each terminal in the network must have a unique address so messages or connections...
s require accurate synchronization
Synchronization
Synchronization is timekeeping which requires the coordination of events to operate a system in unison. The familiar conductor of an orchestra serves to keep the orchestra in time....
for correct operation. For example, if switch
Switch
In electronics, a switch is an electrical component that can break an electrical circuit, interrupting the current or diverting it from one conductor to another....
es 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.
Primary Reference Clock (PRC)
Modern telecommunications networks use highly accurate primary reference clockReference clock
A reference clock may refer to the following:*A master clock used as a timekeeping standard to regulate or compare the accuracy of other clocks*In electronics and computing, the clock signal used to synchronise and schedule operations...
s that must meet the international standards requirement for long term frequency accuracy better than 1 part in 10¹¹. To get this performance, atomic clock
Atomic clock
An atomic clock is a clock that uses an electronic transition frequency in the microwave, optical, or ultraviolet region of the electromagnetic spectrum of atoms as a frequency standard for its timekeeping element...
s 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:- They filter the synchronisation signal they receive to remove the higher frequency phase noisePhase noisePhase noise is the frequency domain representation of rapid, short-term, random fluctuations in the phase of a waveform, caused by time domain instabilities...
, - 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 ModeHoldover in synchronization applications“Synchronization is as important as power at the cell site.” The quote above suggests that we can think of holdover in synchronization applications as analogous to running on backup power....
.
Quality metrics
In telecoms networks two key parameters are used for measurement of synchronisation performance. These parameters are defined by the International Telecommunication UnionInternational Telecommunication Union
The International Telecommunication Union is the specialized agency of the United Nations which is responsible for information and communication technologies...
in its recommendation G.811
G.811
G.811 is a recommendation from the ITU Telecommunication Standardization Sector . It specifies requirements for primary reference clock devices in synchronization networks....
, by European Telecommunications Standards Institute
European Telecommunications Standards Institute
The European Telecommunications Standards Institute is an independent, non-profit, standardization organization in the telecommunications industry in Europe, with worldwide projection...
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.
- TDEVTime deviationTDEV is a metric often used to determine an aspect of the quality of timing signals in telecommunication applications and is a statistical analysis of the phase stability of a signal over a given period of time...
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
- PDHPlesiochronous Digital HierarchyThe Plesiochronous Digital Hierarchy is a technology used in telecommunications networks to transport large quantities of data over digital transport equipment such as fibre optic and microwave radio systems...
, SDH and SONET - Caesium standardCaesium standardA caesium standard or caesium atomic clock is a primary frequency standard in which electronic transitions between the two hyperfine ground states of caesium-133 atoms are used to control the output frequency. They are one of the most accurate types of atomic clock...