[ntp:questions] Stick to PPS, even if the prefer server fails

David Mills mills at udel.edu
Thu Mar 26 03:19:07 UTC 2009


Dave,

The NTP discipline executes a frequency correction once per second; the 
kernel executes a correction once per timer interrupt. Assume an 
intrinsic oscilator frequency error of 100 PPM. Do the math.

Dave

Dave Hart wrote:

>On Mar 25, 9:19 pm, Unruh <unruh-s... at physics.ubc.ca> wrote:
>  
>
>>I have no idea why and whether kernel PPS code is any better ( or worse)
>>than say PPS discipline using the shm PPS refclock using parallel port
>>interrupt. Ie, both can discipline
>>to about 1-2usec level.
>>    
>>
>
>
>Re-read the thread, then.  A kernel clock disciplined by PPS allows
>PPS to continue to discipline the clock when ntpd's PPS implementation
>stops doing so because of a prefer peer problem.
>
>
>  
>
>>The main problem is that the ntp model is too slow
>>reacting to temperature induced drifts.
>>    
>>
>
>
>I'm sure that's your main problem re ntpd, but it's completely
>tangential to the thread.
>
>
>  
>
>>>The complete system doesn't serve time to others. It's intention is
>>>monitoring. I'm building this as a diploma thesis for a
>>>telecommunication company. They have a large number of ntp servers
>>>with GPS receivers (called SSU) across the country. So my system only
>>>watches the offsets, that these SSUs have to assure that they are OK.
>>>      
>>>
>>They can monitor only to the msec (or 1/10 msec)  level unless the other systems are at the same place.
>>    
>>
>
>
>I think he's monitoring the self-reported offsets between the NTP
>disciplined clock and the local GPS receiver.  Network delay would not
>be a factor if so.
>
>Cheers,
>Dave Hart
>
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>  
>




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