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 10¹¹. 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 Scalability is the property of a system to handle a growing amount of work by adding resources to the system. In an economic context, a scalable business model implies that a company can increase sales given increased resources. For example, a ...

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 Time deviation (TDEV), also known as \sigma_x(\tau), is the time stability of phase ''x'' versus observation interval ''τ'' of the measured clock source. The time deviation thus forms a standard deviation type of measurement to indicate the time in ...

(TDEV) is a statistical analysis of the phase stability of a signal over a given period of time.

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

Components

Primary reference clock (PRC)

Synchronization supply unit

Quality metrics

See also

References