[ntp:questions] Jitter versus polling interval

David Woolley david at ex.djwhome.demon.co.uk.invalid
Sun Mar 1 12:35:01 UTC 2009


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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