Holdover in synchronization applications: Difference between revisions

Revision as of 15:34, 12 September 2011

Revision as of 15:43, 12 September 2011

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GPS as a source of timing is a key component in not just [[Synchronization in telecommunications]] but to critical infrastructure in general. <ref>http://www.dtic.mil/cgi-bin/GetTRDoc?AD=ADA503921</ref> Of the 18 Critical Resource and Key infrastructure (CIKR<ref>http://training.fema.gov/EMIWeb/IS/IS860a/CIKR/sectorMenu.htm</ref>)sectors, 15 use GPS derived timing to function correctly. <ref>http://www.swpc.noaa.gov/sww/SWW_2011_Presentations/Wed_830/GPS-PNTTimingStudy-SpaceWeather4-27.pptx</ref>

A key application for GPS derivedin timing[[Synchronization 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.<ref>http://www.gmat.unsw.edu.au/snap/publications/khan&dempster2007b.pdf</ref> In these applications holdover is used in base stations to ensure continued operation while GPS is unavailable and to reduce the costs associated with repairs <ref>http://www.eetimes.com/design/communications-design/4213947/Understanding-the-concepts-of-synchronization-and-holdover</ref>

Some of the most stringent requirements come from the newer generation of wireless base stations, where phase accuracy targets as low as 1us need to be maintained for correct operation. <ref>http://www.telecom-sync.com/pdf/2008/Day1/WCDMA_and_LTE_Synchronisation_Aspects_(Stefano_Rufini,_Ericsson).pdf</ref> However the need for accurate timing has been an integral part of the history of wireless communication systems, and it has been suggested that the search for reliable and cost effective timing soluctions was spurred on by the need for CDMA to compete with lower cost solutions. <ref>http://www.4timing.com/SyncGPS.pdf</ref>

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]]ref>http://www.eetimes.com/design/communications-design/4213947/Understanding-the-concepts-of-synchronization-and-holdover</ref>

Search for reliable timing based on something other than $3k Rubidium spurred on by the need for CDMA to compete. <ref>http://www.4timing.com/SyncGPS.pdf</ref>

As mentioned above there are key applications for GPS derived timing in other critical infrastructure applications. One notable application where timing accuracy (and the means to maintain it through holdover) is of importance is in the use of [[Synchrophasor|Synchrophasors]] in the power industry to detect line faults. <ref>http://tf.nist.gov/general/pdf/2193.pdf</ref>

Holdover is important for E911 for example. Most critical telecom applications require precise time and frequency and synchronization to operate properly such as VoIP, video streaming, TDM (time division multiplex) services, voice switching, mobile services, and any LBS (___location based services, with E911 being the most important).<ref>http://www.eetimes.com/design/communications-design/4213947/Understanding-the-concepts-of-synchronization-and-holdover</ref>

1us for synchrophasors <ref>http://tf.nist.gov/general/pdf/2193.pdf</ref>

==How GPS Derived Timing Can Fail==