Holdover in synchronization applications

Two independent clocks, once synchronized, will walk away from one another without limit. To have them display the same time it would be necessary to re-synchronize them at regular intervals. The period between synchronizations is referred to as holdover and performance under holdover relies on the quality of the reference oscillator, the PLL design, and the correction mechanisms employed.

The quote above suggests that one can think of holdover in synchronization applications as analogous to running on backup power.

Modern wireless communication systems require at least knowledge of frequency and often knowledge of phase as well in order to work correctly. Base stations need to know what time it is, and they usually get this knowledge from the outside world somehow (from a GPS Time and Frequency receiver, or from a synchronization source somewhere in the network they are connected to).

But if the connection to the reference is lost then the base station will be on its own to establish what time it is. The base station needs a way to establish accurate frequency and phase (to know what time it is) using internal (or local) resources, and that’s where the function of holdover becomes important.

A key application for GPS in telecommunications is to provide synchronization in wireless basestations. Base stations depend on timing to operate correctly, particularly for the handoff that occurs when a user moves from one cell to another. In these applications holdover is used in base stations to ensure continued operation while GPS is unavailable and to reduce the costs associated with emergency repairs, since holdover allows the site to continue to function correctly until maintenance can be performed at a convenient time.

Some of the most stringent requirements come from the newer generation of wireless base stations, where phase accuracy targets as low as 1μs need to be maintained for correct operation. However the need for accurate timing has been an integral part of the history of wireless communication systems as well as wireline, and it has been suggested that the search for reliable and cost effective timing solutions was spurred on by the need for CDMA to compete with lower cost solutions.

Within the base station, besides standard functions, accurate timing and the means to maintain it through holdover is vitally important for services such as E911

GPS as a source of timing is a key component in not just Synchronization in telecommunications but to critical infrastructure in general. Of the 18 Critical Resource and Key infrastructure (CIKR)sectors, 15 use GPS derived timing to function correctly. One notable application where highly accurate timing accuracy (and the means to maintain it through holdover) is of importance is in the use of Synchrophasors in the power industry to detect line faults. Another is in Low latency trading applications in capital markets.

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Wikipedia