[ntp:questions] How to keep Linux server in Chicago and Mumbai in sync to within 5 microseconds
Chuck Swiger
cswiger at mac.com
Thu Jan 13 18:14:50 UTC 2011
On Jan 12, 2011, at 11:37 PM, unruh wrote:
>>> In presumption lies error. The CDMA spec requires their cell towers to be sync'ed to better than 10 microseconds, which is why the Praesis units mentioned originally in the thread specify that level of precision:
>>
>> http://en.wikipedia.org/wiki/IS-95
>
> Nothing here that talks of 10 microseconds
The wikipedia article is a reference describing the CDMA spec, not the spec itself. Even so, it does make mention of time synchronization:
"In the forward direction, radio signals are transmitted by base stations (BTS's). Every BTS is synchronized with a GPS receiver so transmissions are tightly controlled in time. All forward transmissions are QPSK with a chip rate of 1,228,800 per second. Each signal is spread with a Walsh code of length 64 and a pseudo-random noise code (PN code) of length 215, yielding a PN roll-over period of 80/3 ms."
Understanding that they are using quadrature modulation with a given rate (approx 1.2e6) gives a close approximation of the necessary timing precision:
http://en.wikipedia.org/wiki/Phase-shift_keying
Your email address implies that you have a background in physics; surely you can follow the references provided for yourself, if you really care to confirm the details.
Regards,
--
-Chuck
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