[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:
> 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 ?
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.
More information about the questions