[ntp:questions] Re: Question about min/max poll and an interesting plot showing maxpoll's effect

John Ackermann N8UR jra at febo.com
Thu Apr 20 22:26:06 UTC 2006


Richard B. Gilbert said the following on 04/20/2006 06:02 PM:

> Anyway, I'm a little surprised that shorter is better.  I have two
> machines as clients of my stratum 1 (GPS) server.  All three are Sun
> workstations; the server is an Ultra 10 440 MHz, one of the clients is
> also an Ultra 10 440 MHz the other is an Ultra 5 360 MHz.  All have been
> running for several days.  The Ultra 5 is polling at 64 second intervals
> with an estimated error of 11056 us while the Ultra 10 is polling at 256
> second intervals with an estimated error of 500 us.

My thinking is that during the time between samples, ntpd can only
adjust the frequency based on the drift assumption it has, which doesn't
take into account thermal or other factors.  So, with a long polling
interval you have quite a long time for the local clock to drift and as
a result you end up with larger errors that need to be corrected.  With
a shorter sample rate, the frequency control can be much tighter.

Remember that in my situation, the servers that I'm looking at are <1ms
away and are all running with highly accurate 1pps signals.  So, there's
very little noise on the signal.  Out in the real world, things are
quite different...

John



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