[ntp:questions] Re: Post processing of NTP data...

Val Schmidt vschmidt at ccom.unh.edu
Tue Sep 27 14:53:59 UTC 2005


>>
> If you configure, as you should, a minimum of four network servers,  
> which of the four different offsets would you use?

It's a good question. Doesn't ntp actually use only one of them at  
any given time, switching between them as network latency or other  
parameters warrant. I suppose one would want to correct to the one  
being used at the moment.
>
> If you want highly accurate time, using a GPS timing receiver as a  
> hardware reference clock can give you offsets less than ten  
> microseconds.  This assumes that your hardware and O/S are good  
> enough to follow the reference clock that closely
>
> I have an old Sun Ultra 10 workstation with a 330 MHz processor  
> running Solaris 8, NTP-4.2.0 and a Motorola Oncore M12 Timing  
> receiver that synchronizes with offsets of less than 10  
> microseconds.  (My total cash outlay was $300 for the Ultra 10,  
> $100 for a Solaris 8 Media kit, and $200 for the GPS receiver.

How much of this is system-load dependent? Does your Sun do anything  
but serve time? Do you see significant excursions with other  
processes active or moderate to heavy network traffic to or from the  
box?

>
> Free BSD is another O/S that seems to be highly regarded for its  
> timekeeping abilities although I have no personal experience with it.
>
> Poul Henning Kamp has used Soekris single board computers, Free  
> BSD, and Motorola Oncore M12 receivers to achieve sub microsecond  
> accuracies.

Impressive.

>
> Windows (any version) and Linux on the Intel platform do not  
> perform very well in this application.

Is this because interrupts are dropped from time to time or other  
reasons?

>

------------------------------------------------------
Val Schmidt
CCOM/JHC
University of New Hampshire
Chase Ocean Engineering Lab
24 Colovos Road
Durham, NH 03824
e: vschmidt [AT] ccom.unh.edu
m: 614.286.3726





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