[ntp:questions] Offset in ms vs Frequency in PPM

Michael L. Semon mlsemon33 at verizon.net
Wed Mar 7 03:32:02 UTC 2007


edouard funke wrote:
> Hi,
> 
> I would like to know if there is any relation between variations of
> the offset in millisecond and variations of the frequency in PPM in
> loopstats file ?
> 
> Edouard

Yes.  Offset is the difference between the OS software clock and the 
time sources.  ntpd changes the frequency of that software clock to 
correct that offset, and that frequency is recorded as the PPM value in 
the loopstats file.

The next time you drive on a highway, try to stay exactly six car 
lengths behind the car in front of you, and do it for five minutes.  You 
will have a better idea of what ntpd is trying to do and how it does it. 
  Think of your speed as the frequency (PPM).  The distance between the 
two cars is the offset.  The distance between the two cars will change 
from one second to the next second, and that change is the jitter.

The two fields after frequency (PPM) in the loopstats file are quite 
handy, too.  It is good to have low values for RMS jitter and Allan 
deviation, and ntpd has to be running for a while to achieve those values.

Michael








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