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

David Woolley david at ex.djwhome.demon.invalid
Wed Jan 12 08:11:14 UTC 2011


Subject: How to keep Linux server in Chicago and Mumbai in sync to 
within 5 microseconds

[Hint never assume people read the subject when reading the body.]

2.5 microseconds from true will require in depth careful system design. 
  In particular, the delay between real world events and the 
corresponding application code running is likely to have much higher 
jitter than that, due to things like interrupt latency and interrupt 
coalescing.

You can probably synchronise the clock to that sort of accuracy, but the 
timing accuracy seen by the applications is likely to be a lot worse - 
they will see the correct time at which they read the time, but not the 
correct time of the real world events that they are reacting to.

Vikas wrote:
> The vendors are trying to sell me CDMA time clocks

I presume that CDMA will need to be calibrated for the distance from the 
base station, even if the base station time is dead on.  5 microseconds 
is 1.5km of free space propagation.

CDMA will, at most, only require controlled time differences over a 
single network.  Except for convenience, there is no fundamental reason 
for North America and India to have the same time.

> http://www.endruntechnologies.com/time-frequency-reference-cdma.htm
> $1375 each.
> 
> Also I would need 2 time clocks one in Chicago and other in Mumbai.
> 
> Is there is a easier/cost effective way ?
> 
> Thanks,




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