Many services running on modern digital
telecommunications network
A telecommunications network is a group of nodes interconnected by telecommunications links that are used to exchange messages between the nodes. The links may use a variety of technologies based on the methodologies of circuit switching, message ...
s require accurate synchronization for correct operation. For example, if
telephone exchange
A telephone exchange, telephone switch, or central office is a telecommunications system used in the public switched telephone network (PSTN) or in large enterprises. It interconnects telephone subscriber lines or virtual circuits of digital syst ...
s are not
synchronized, then
bit 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.
Ideally, clocks in a telecommunications network are synchronous, controlled to run at identical rates, or at the same mean rate with a fixed relative
phase
Phase or phases may refer to:
Science
*State of matter, or phase, one of the distinct forms in which matter can exist
*Phase (matter), a region of space throughout which all physical properties are essentially uniform
* Phase space, a mathematic ...
displacement, within a specified limited range. However, they may be
mesochronous in practice. In common usage, mesochronous networks are often described as ''synchronous''.
Components
Primary reference clock (PRC)
Modern telecommunications networks use highly accurate primary
master clock
Master or masters may refer to:
Ranks or titles
* Ascended master, a term used in the Theosophical religious tradition to refer to spiritually enlightened beings who in past incarnations were ordinary humans
*Grandmaster (chess), National Master ...
s that must meet the international standards requirement for long term frequency accuracy better than 1 part in 10
11. To get this performance,
atomic clock
An atomic clock is a clock that measures time by monitoring the resonant frequency of atoms. It is based on atoms having different energy levels. Electron states in an atom are associated with different energy levels, and in transitions betwee ...
s or
GPS disciplined oscillator A GPS clock, or GPS disciplined oscillator (GPSDO), is a combination of a GPS receiver and a high-quality, stable oscillator such as a quartz or rubidium oscillator whose output is controlled to agree with the signals broadcast by GPS or other GNS ...
s are normally used.
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
In signal processing, phase noise is the frequency-domain representation of random fluctuations in the phase of a waveform, corresponding to time-domain deviations from perfect periodicity (jitter). Generally speaking, radio-frequency engineers ...
.
#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
The International Telecommunication Union is a specialized agency of the United Nations responsible for many matters related to information and communication technologies. It was established on 17 May 1865 as the International Telegraph Unio ...
in its recommendation
G.811, by
European Telecommunications Standards Institute
The European Telecommunications Standards Institute (ETSI) is an independent, not-for-profit, standardization organization in the field of Information and communications technology, information and communications. ETSI supports the developmen ...
in its standard
EN 300 462-1-1
En or EN may refer to:
Businesses
* Bouygues (stock symbol EN)
* Esquimalt and Nanaimo Railway (reporting mark EN, but now known as Southern Railway of Vancouver Island)
* Euronews, a news television and internet channel
Language and writing
* E ...
, 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.
GR-1244 - Clocks for the Synchronized Network: Telcordia
/ref>
* Maximum time interval error
Maximum time interval error (MTIE) is the maximum error committed by a clock under test in measuring a time interval for a given period of time. It is used to specify clock stability requirements in telecommunications standards. MTIE measurements ...
(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.
See also
* PDH, SDH and SONET
* Caesium standard
The caesium standard is a primary frequency standard in which the Absorption (electromagnetic radiation), photon absorption by transitions between the two hyperfine level, hyperfine ground states of caesium-133 atoms is used to control the output ...
* Synchronous network
Many services running on modern digital telecommunications networks require accurate synchronization for correct operation. For example, if telephone exchanges are not synchronized, then bit slips will occur and degrade performance. Telecommunic ...
* Isochronous signal
In telecommunication, an isochronous signal is a signal in which the time interval separating any two significant instants is equal to the unit interval
In mathematics, the unit interval is the closed interval , that is, the set of all real n ...
* Mesochronous network
In telecommunication, a mesochronous network is a network in which the clocks run with the same frequency but unknown phases. Compare synchronous network.
See also
*Synchronization in telecommunications
*Isochronous signal
*Plesiochronous system
...
* Plesiochronous system
In telecommunications, a plesiochronous system is one where different parts of the system are almost, but not quite, perfectly synchronised. According to ITU-T standards, a pair of signals are plesiochronous if their significant instants occur at ...
* Asynchronous communication
In telecommunications, asynchronous communication is transmission of data, generally without the use of an external clock signal, where data can be transmitted intermittently rather than in a steady stream. Any timing required to recover data f ...
* Phase-locked loop
A phase-locked loop or phase lock loop (PLL) is a control system that generates an output signal whose phase is related to the phase of an input signal. There are several different types; the simplest is an electronic circuit consisting of a ...
References
*
* {{cite book , title=Synchronization of Digital Telecommunications Networks , first=Stefano , last=Bregni , publisher=Wiley , year=2002 , isbn=0-471-61550-1
Synchronization
Data transmission
Network architecture