[ntp:questions] Re: HowTo calibrate system clock frequency using NTP

David Woolley david at djwhome.demon.co.uk
Fri Feb 3 07:32:51 UTC 2006

In article <bjaab3-qff.ln1 at osl016lin.hda.hydro.com>,
Terje Mathisen <terje.mathisen at hda.hydro.com> wrote:

> Afaik ntp.drift normally carries about an hour's worth of history.

The complete control loop for ntpd has an infinite impulse response,
so the ntp.drift value has an infinite history.  The period that 
accounts for most of the contribution to the value depends on an 
adaptive algorithm that starts by making the response fast and slows
it down as confidence increases, reducing it again if confidence drops.
This is related to the poll interval, but not in a simple way.

Nick McClaren, has, in the past, proposed the use of finite impulse
response processing, but using a statistical fit, rather than the 
current, to a first approximation, linear feedback loop.  That's 
basically having it continually do linear regressions.

(The big problems seem to be that the response is still too slow when
initially acquiring lock and the frequency response time is reduced too
much after a subsequent time step (lost interrupt, server hop, or people
breaking the clock to test the ability to track the time).)

Incidentally, I find a simple average is good enough to get 30 second
a year accuracy in an air conditioned environment, providing you do it
over about a week.

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