[ntp:questions] how does jitter and round trip time affect the accuracy of the local clock?

David Woolley david at ex.djwhome.demon.invalid
Sun Jun 26 20:05:59 UTC 2011

chipper wrote:
> If the ntp client needed to be withing .5 milliseconds of the server,how 

Is the server keeping perfect time?

> could I tell if the jitter and delay were to
> much to support this kind of accuracy?

What percentage of the time can you tolerate the time being out by more 
than 500 microseconds?  Do you have perfectly symmetric network delays? 
  If the jitter is of the order of 500 microseconds, and your delays are 
perfectly symmetric, and there is no clock wander (in particular, the 
temperature is tightly controlled), the error will exceed 500 
microseconds, a small but significant amount of the time.  The smaller 
the jitter, the less the time, but it will probably never go to zero.

If the delay is less than 1ms, and the server clock is perfect, it is 
unlikely that the error will exceed 500 microseconds, although there may 
be some overshoots.  Most people operate with delay >> desired accuracy, 
in which case you are at the mercy of the network delay asymmetry.

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