[ntp:questions] Jitter versus polling interval
David J Taylor
david-taylor at blueyonder.neither-this-bit.nor-this.co.uk
Sun Mar 1 10:37:45 UTC 2009
David Woolley wrote:
> David J Taylor wrote:
>> Does anyone know an approximate relationship between polling interval
>> and peak-to-peak jitter for a non-ref-clock system? I.e would the
>> jitter offset) go up linearly with polling interval, or as the
>> square, or what?
> Do you mean true jitter (RMS error from true time) or the jitter
> statistic produced by NTP?
Neither, directly. I mean the peak-to-peak variation in the reported
offset, as "measured" by eye on a graph like the "Daily" graph here:
> I may be completely wrong, but I thought that true jitter initially
> went down with increasing polling interval and that ntpd stopped
> increasing the polling interval before it started to go back up
> again. That's because ntpd increases the loop time constant as it
> increases, which compensates for the increased poll interval and, in
> the Millsian world, measurement jitter has a 1/f spectrum.
> In the real world, you'll need to specify the nature of the
> measurement noise.
Yes, I'm looking for an approximate value only at the moment - an order of
magnitude relationship to say whether its linear or quadratic (with number
of seconds). I'm thinking of a system where the maximum polling interval
might be set to 7, 8, 9 or 10.
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