[ntp:questions] Best solution for millisecond accuracy

Danny Mayer mayer at ntp.isc.org
Sun Mar 25 17:48:27 UTC 2007


Hal Murray wrote:
>> However, for a high percentile of system times being within 1ms of the
>> reference, he probably should be using PPS timing with equal length 
>> cables between the timing source and all the machines.
> 
> Don't worry about the cable length until you get down to the microseconds.
> 
> The speed of light is 1 ft per nanosecond.  That's in vacuum.  It's slower
> in cable.  1/2 or 3/4 is a good guess.
> 
> So 50 ft of cable is 100 ns or 1/10 of a microsecond or
> 1/10000 of a millisecond.
> 
> 

The length of the cable is almost never the problem. The real problem is
asymmetry in the the time needed to send and receive the packets which
is almost impossible to estimate. If you know the asymmetry every time
you can get a very good estimate of the delay and therefore you could
get a very good results for you clock.

Danny




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