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.
Importance Synchronization is as important as power at the cell site. 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.
The importance of GPS-derived timing 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.
How GPS-derived timing can fail GPS is sensitive to jamming and interference because the signal levels are so low and can easily be swamped by other sources, that can be accidental or deliberate. Also since GPS depends on line of sight signals it can be disrupted by Urban canyon effects, making GPS only available to some locations at certain times of the day, for example. A GPS outage however is not initially an issue because clocks can go into holdover, allowing the interference to be alleviated as much as the stability of the oscillator providing holdover will allow. The more stable the oscillator, the longer the system can operate without GPS.
Defining holdover In Synchronization in telecommunications applications holdover is defined by ETSI as:
An operating condition of a clock which has lost its controlling input and is using stored data, acquired while in locked operation, to control its output. The stored data are used to control phase and frequency variations, allowing the locked condition to be reproduced within specifications. Holdover begins when the clock output no longer reflects the influence of a connected external reference, or transition from it. Holdover terminates when the output of the clock reverts to locked mode condition.
One can regard holdover then as a measure of accuracy or error acquired by a clock when there is no controlling external reference to correct for any errors. MIL-PRF-55310 defines Clock Accuracy as:
T ( t ) = T 0 + ∫ 0 t R ( t ) d t + ϵ ( t ) = T 0 + ( R 0 t + 1 2 A t 2 + . . . ) + ∫ 0 t E t ( t ) d t + ϵ ( t ) {\displaystyle T(t)=T_{0}+\int _{0}^{t}R(t)\,dt\ +\epsilon (t)=T_{0}+(R_{0}t+{\frac {1}{2}}At^{2}+...)+\int _{0}^{t}E_{t}(t)\,dt+\epsilon (t)}
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