[ntp:questions] NTP Frequency value clarification

Martin Burnicki martin.burnicki at burnicki.net
Tue Mar 22 08:43:03 UTC 2016


Hello Kashif,

Kashif Mumtaz Tahir wrote:
> Hello folks ,
> 
> I just want to ask one question which might be novice.      We are
> maintaining  NTP server and monitoring its   Offset, Jitter and  Frequency
> values for its health check.
> 
> Offset and Jitter we know very well but want to clarify about  Frequency
> value.  What actually it is ? and what is impact of increase/decrease in its
> value on NTP server.   

The system time on your mainboard is derived from a quartz on the
mainboard which generates a clock signal which is more or less off its
nominal frequency, so your undisciplined system time would drift more or
less depending on how much your quartz is off.

This mean native frequency offset depends on the individual quartz, so
this varies from one mainboard to the next, even if the board is the
same type from the same manufacturer.

In addition, the real frequency offset varies with the ambient
temperature of the quartz. The temperature can vary inside the PC
housing, depending on whether the CPU is idle or busy, and also
depending on variations of the room temperature, e.g. when the air
condition kicks in.

The NTP daemon tries to measure the frequency offset and applies a
correction to get the real frequency as close as possible to the nominal
frequency. The mean drift correction value depends on the native
frequency offset of the individual quartz, and the variations over time
depend on the stability of the quartz when the temperature changes.

When the quartz frequency changes due to temperature changes then the
system time starts to drift, and ntpd needs to adjust the frequency
correction to compensate this, so the time offsets and frequency drift
are somewhat related.

For example, see
http://people.ntp.org/burnicki/sample-loopstats/loopstats.2016w11.pdf

Regards,

Martin



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