[ntp:questions] Confused about pool rotation

Terje Mathisen "terje.mathisen at tmsw.no" at ntp.org
Thu Feb 7 07:17:01 UTC 2013

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>   Cell towers are already synchronized with stratum one
>    servers and / or GPS,  and usually contain stable oscillators
>     e.g. rubidium, cesium, ...

Most (GSM/3G/4G) cell towers make do with OCXOs/TCXOs, but very good 
ones, rated at a few ppb/day in drift rate.

I.e. pretty much perfect compared to what NTP usually has to work with.

>    Cell towers can't do seamless handoffs for moving cell phones
>     to adjacent towers without close synchronization.

The interesting part here is that the actual requirement is for 
_frequency_ synchronization, not absolute time sync!

Better FLL corresponds directly with better hand-off performance at 
speed: Driving at highways speeds require something like 50 ppb afair (I 
was the chief architect for a national cell phone network for a couple 
of years), a bullet train requires 20 ppb in order to handle the Doppler 


- <Terje.Mathisen at tmsw.no>
"almost all programming can be viewed as an exercise in caching"

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