[ntp:questions] Jitter versus polling interval
David J Taylor
david-taylor at blueyonder.neither-this-bit.nor-this.co.uk
Sun Mar 1 12:55:06 UTC 2009
David Woolley wrote:
> David J Taylor wrote:
>> 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:
> For a Millsian network, the peak to peak value would be unbounded, so
> one needs to consider RMS, or assume a non-Millsian constraint, such
> as bounded peak to peak measurement error.
> Thinking more about it, I think there will be no nett effect from the
> measurement error, so the only contribution will be from oscillator
> frequency. It looks like that is assumed to vary as the square root
> of poll interval.
> So, I would say, for small poll intervals, asymptotic to constant and
> for large poll intervals, asymptotic to the 1.5th power) of the
> interval. ntpd attempts not to choose poll intervals that take it
> beyond the transition between these two, so maybe sub-linear for all
> of the normal operation range. If you get something significantly
> different, you may not have a Millsian network, and ntpd may not be
> the right tool for you.
> If you want the real answer, you will need to ask Dave Mills, but he
> will probably just point you to his mathematical analysis, and I
> suspect you are asking here because you don't feel competent to use
> those, or rich enough to buy the book.
A figure of the 1.5th power is fine for me. As I only have two "by eye"
measurements, I can neither say that's right or wrong. It's certainly not
The book is on my Amazon wanted list, but I'm not sure how much I will
appreciate if it's mainly maths. One of my criteria for judging image
processing papers was "doe it contain images?"!
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