Clock synchronisation

Clock synchronization is a topic in computer science and engineering that aims to coordinate otherwise independent clocks. Even when initially set accurately, real clocks will differ after some amount of time due to clock drift, caused by clocks counting time at slightly different rates. There are several problems that occur as a result of clock rate differences and several solutions, some being more appropriate than others in certain contexts.

In serial communication, clock synchronization can refer to clock recovery which achieves frequency synchronization, as opposed to full phase synchronization. Such clock synchronization is used in synchronization in telecommunications and automatic baud rate detection.

Plesiochronous or isochronous operation refers to a system with frequency synchronization and loose constraints on phase synchronization.

Synchronous operation implies a tighter synchronization based on time perhaps in addition to frequency.

As a result of the difficulties managing time at smaller scale, there are problems associated with clock skew that take on more complexity in a distributed system in which several computers will need to realize the same global time.

For instance, in Unix systems the make command is used to compile new or modified code without the need to recompile unchanged code. The make command uses the clock of the machine it runs on to determine which source files need to be recompiled. If the sources reside on a separate file server and the two machines have unsynchronized clocks, the make program might not produce the correct results.

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