Many services running on modern digital

telecommunications network A telecommunications network is a group of Node (networking), 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 ...

s require accurate synchronization for correct operation. For example, if

telephone exchange A telephone exchange, telephone switch, or central office is a central component of a telecommunications system in the public switched telephone network (PSTN) or in large enterprises. It facilitates the establishment of communication circuits ...

s are not synchronized, then

bit slip In digital transmission, bit slip is the loss or gain of a bit or bits, caused by clock driftvariations in the respective clock rates of the transmitting and receiving devices. One cause of bit slip is overflow of a receive buffer that occu ...

s 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 displacement, within a specified limited range. However, they may be mesochronous in practice. In common usage, mesochronous networks are often described as ''synchronous''.

History

Synchronization in communications was a hard problem for Alexander Bain in the development of the

teleprinter A teleprinter (teletypewriter, teletype or TTY) is an electromechanical device that can be used to send and receive typed messages through various communications channels, in both point-to-point (telecommunications), point-to-point and point- ...

Thomas Edison Thomas Alva Edison (February11, 1847October18, 1931) was an American inventor and businessman. He developed many devices in fields such as electric power generation, mass communication, sound recording, and motion pictures. These inventions, ...

achieved synchronization in his stock ticker with a clunky but effective ''unison mechanism'' to resynchronize periodically. In the teleprinter world, Howard Krum finally came up with a good decoding mechanism for async signals around 1912. Synchronization remained a problem well into the electronic era. The final solution to the synchronization problem came with the

phase-locked loop A phase-locked loop or phase lock loop (PLL) is a control system that generates an output signal whose phase is fixed relative to the phase of an input signal. Keeping the input and output phase in lockstep also implies keeping the input and ou ...

. Once available, analog TVs, modems, tape drives, VCRs, and other common devices synchronized consistently.

Components

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 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 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 enginee ...

. #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 (ITU)In the other common languages of the ITU: * * is a list of specialized agencies of the United Nations, specialized agency of the United Nations responsible for many matters related to information ...

in its recommendation G.811, by

European Telecommunications Standards Institute The European Telecommunications Standards Institute (ETSI) is an independent, not-for-profit, standardization organization operating in the field of information and communications. ETSI supports the development and testing of global technical ...

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

References