[ntp:questions] How to keep Linux server in Chicago and Mumbai in sync to within 5 microseconds

Chuck Swiger cswiger at mac.com
Fri Jan 14 00:13:39 UTC 2011


On Jan 13, 2011, at 3:08 PM, David Woolley wrote:
> Chuck Swiger wrote:
>> Understanding that they are using quadrature modulation with a given
>> rate (approx 1.2e6) gives a close approximation of the necessary timing precision:
> 
> Not obviously.

Okay.  I'm not going to insist that something is obvious if people disagree....  :-)

> If a timing constraint exists I guess it will be associated with limiting the amount of processing needed to re-synch on a handover.

Rather precise CDMA timing constraints exist.  They are well documented in the relevant standards which have already been mentioned, as well as by subsequent standards like CDMA2000 and things like the US FCC E911 location initiatives.

> [ ... ]
> [Very long line rewrapped. It was a long line, not a paragraph according to email standards.]

On a good day, my MUA sends "Content-type: text/plain; format=flowed" and should contain line breaks following the 80-character-per-line Usenet conventions, which modern MUAs might well reassemble based upon the user's window size.  If it is being re-interpreted after transmission by MTAs, mailing-list MIME filters, or similar, well, that lies beyond my control.

>>  http://en.wikipedia.org/wiki/Phase-shift_keying
> 
> All that QPSK really tells us is that baud rate is half the chipping rate.

Yes, but a ~1.2e6 hertz signal rate, modulo the factor of two from QAM yielding ~620 KHz, implies microsecond-level timing.

>> 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.
> 
> CDMA has a lot more to do with an arcane branch of mathematics than physics.

I suppose, but I was thinking of basic "T = 1 / f"-- ie, the period is the reciprocal of frequency.

Regards,
-- 
-Chuck




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