Re: Jitter versus polling interval

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.
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