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{{Use dmy dates|date=September 2022}}
''“Synchronization is as important as power at the cell site.”''<ref>{{cite paper|url=http://www.juniper.net/us/en/local/pdf/whitepapers/2000400-en.pdf |title=Synchronization Deployment Considerations for IP RAN Backhaul Operators |publisher=[[Juniper Networks]] |date=2011 |accessdate=2012-10-21}}</ref>▼
Two independent clocks, once synchronized, will walk away from one another without limit.<ref name="smartclock">{{cite
==Importance==
The quote above suggests that we can think of '''holdover in synchronization applications''' as analogous to running on backup power.▼
▲
▲The quote above suggests that
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). ▼
▲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.▼
▲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.
▲Two independent clocks, once synchronized, will walk away from one another without limit.<ref>{{cite paper|url=http://tf.nist.gov/general/pdf/988.pdf |title=Smart Clock: A New Time |publisher=[[IEEE]] |date=1992-12-06 |accessdate=2012-10-21}}</ref> To have them display the same time it would be necessary to re-synchronize them at regular intervals. The periods between synchronizations is referred to as '''Holdover''' and performance under Holdover relies on the quality of the reference oscillator (usually an [[OCXO]]), the PLL design and the correction mechanisms employed.<ref name="analog1">{{cite web|url=http://www.analog.com/static/imported-files/application_notes/AN-1002.pdf |title=AN-1002 (Rev. 0) |format=PDF |date= |accessdate=2012-09-28}}</ref>
==The
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.<ref name="autogenerated1">{{cite
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.<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 name="eetimes1"/>
GPS as a source of timing is a key component in not just [[Synchronization in telecommunications]] but to critical infrastructure in general.<ref>{{cite
==How GPS-derived
GPS is sensitive to jamming and interference because the signal
A GPS outage however is not initially an issue because clocks can go into holdover,<ref>{{cite web |url=http://www.syncuniversity.org/drsync/q45.php |title=Sync University |publisher=Sync University |date=2004-12-15 |
==Defining
In [[Synchronization in telecommunications]] applications holdover is defined by [[European Telecommunications Standards Institute|ETSI]] as:
<blockquote>
</blockquote>
MIL-PRF-55310<ref>
<math>
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Where <math>T_0</math> is the synchronization error at <math>t = 0</math>; <math>R(t)</math> is the fractional frequency difference between two clocks under comparison; <math>\epsilon(t)</math> is the error due to random noise; <math>R_0</math> is <math>R(t)</math> at <math>t=0</math>; <math>A</math> is the linear aging rate and <math>E_1(t)</math> is the frequency difference due to environmental effects.
Similarly ITU G.810<ref>{{cite web|author=tsbmail |url=http://www.itu.int/rec/T-REC-G.810-199608-I |title=G.810 : Definitions and terminology for synchronization networks |publisher=Itu.int |access-date
<math>x(t) = x_0 + y_0t + \frac{D}{2}t^2 + \frac{\phi(t)}{2\pi\nu_{nom}}</math>
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Where <math>x(t)</math> is the time error; <math>x_0</math> is the time error at <math>t=0</math>; <math>y_0</math> is the fractional frequency error at <math>t=0</math>; <math>D</math> is the linear fractional frequency drift rate; <math>\phi(t)</math> is the random phase deviation component and <math>\nu_{nom}</math> is the nominal frequency.
==Implementing
In applications that require
[[NIST]] defines a Disciplined Oscillator as:
<blockquote>
''An oscillator whose output frequency is continuously steered (often through the use of a [[phase locked loop]]) to agree with an external reference. For example, a GPS disciplined oscillator (GPSDO) usually consists of a quartz or rubidium oscillator whose output frequency is continuously steered to agree with signals broadcast by the GPS satellites.''<ref>{{cite web |url=http://tf.nist.gov/general/enc-d.htm |title=Time and Frequency from A to Z |publisher=Tf.nist.gov |access-date=2012-09-28 |
</blockquote>
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[[File:GPSDO.png|thumb|left|A Modern [[GPSDO]]]]
Amongst the building blocks of a GPS Time and Frequency solution the oscillator is a key component<ref name="autogenerated2"/> and typically they are built around an Oven Controlled Crystal Oscillator ([[Crystal oven|OCXO]]) or a [[Rubidium standard|Rubidium based clock]]. The dominant factors influencing the quality of the reference oscillator are taken to be aging and temperature stability. However, depending upon the construction of the oscillator, barometric pressure and relative humidity can have at least as strong an influence on the stability of the quartz oscillator.{{
The
Once the barriers of aging and environmental effects are removed the only theoretical limitation to holdover performance in such a GPSDO is irregularity or noise in the drift rate, which is quantified using a metric like [[Allan deviation]] or [[Time deviation]].<ref>[http://www.leapsecond.com/pages/adev/adev-why.htm Leap second]</ref>{{Unreliable source?|date=October 2012}}
The complexity in trying to predict the effects on Holdover due to systematic effects like aging and temperature stability and stochastic influences like [[Random walk|Random Walk]] noise has resulted in tailor-made ''Holdover Oscillator'' solutions being introduced in the market.<ref>{{cite web|url=http://www.vectron.com/products/modules/MD-023.pdf |title=MD-023: Extended Holdover Crystal Oscillator |publisher=Vectron International |date=June 2011 |
==See also==
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* [http://www.endruntechnologies.com/pdf/OscOptions.pdf Disciplined Oscillator Options]
[[Category:Synchronization]]
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