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, mes ...
s require accurate synchronization for correct operation. For example, if
telephone exchange
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 syste ...
s are not
synchronized, then
bit slips will occur and degrade performance. Telecommunication networks rely on the use of highly accurate
primary reference clock
Primary or primaries may refer to:
Arts, entertainment, and media Music Groups and labels
* Primary (band), from Australia
* Primary (musician), hip hop musician and record producer from South Korea
* Primary Music, Israeli record label
Works
...
s 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 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 betw ...
s or
GPS disciplined oscillators 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 engin ...
.
#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 G.811 ''Timing characteristics of primary reference clocks'' is a recommendation from the ITU Telecommunication Standardization Sector (ITU-T). It specifies requirements for primary reference clock devices in synchronization networks. It discusses ...
, 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. ETSI supports the development and testing of global technical standard ...
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.
GR-1244 - Clocks for the Synchronized Network: Telcordia
/ref>
* 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 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.
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
* 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 or a multiple of the unit interval. Variations in the time intervals are constrained within spe ...
* Mesochronous network
* Plesiochronous system
* 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