[ntp:questions] Adjusting PPS offset

unruh unruh at wormhole.physics.ubc.ca
Mon Sep 5 15:00:54 UTC 2011


On 2011-09-05, Maarten Wiltink <maarten at kittensandcats.net> wrote:
> "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.

We are talking about PPS from a GPS receiver, not from a caesium
oscillator. 

>
> 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.
>
> Groetjes,
> Maarten Wiltink
>
>




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