Synchronization in telecommunications  Many services running on modern digital telecommunications networks require accurate synchronization for correct operation. For example, if telephone exchanges are not synchronized, 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". 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". 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 clocks or GPS disciplined oscillators are normally used. Synchronization supply units (SSU) are used to ensure reliable synchronisation distribution. They have a number of key functions:  In telecoms networks two key parameters are used for measurement of synchronisation performance. These parameters are defined by the International Telecommunication Union 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.