[ntp:questions] Adjusting PPS offset

Maarten Wiltink maarten at kittensandcats.net
Mon Sep 5 07:01:45 UTC 2011

"unruh" <unruh at wormhole.physics.ubc.ca> wrote in message
news:slrnj67c9d.rn0.unruh at wormhole.physics.ubc.ca...

> [...] If your PPS is off by 2ms, throw
> it away immediately. It is completely useless. In general PPS is off
> by less than 1 us and probably 100ns or so.

"In general" PPS will be off by anything up to half a second in either
direction, distributed uniformly.

A Caesium oscillator is the steadiest frequency source known to man,
but the phase is completely random. That does not make it useless. You
simply have to determine the phase, and can then trust it will remain
constant to a quite unreasonable degree (1e-13, this week?).

> [...] And 2ms for a bunch of network servers is bad. It seems that
> there is something along the route out of your system which is
> delaying the packets packets really badly but consistantly.

Two milliseconds is bad? I live in the real world, with outbound links
that are saturated for most of the day, and jitter five times that,
and two milliseconds is a _good_ day on my network. It's also plenty
good _enough_ for me, thank you very much, so I won't be digging into
traffic prioritisation and all that to make it better.

Maarten Wiltink

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