[ntp:questions] Slow convergence of NTP with GPS/PPS

Richard B. Gilbert rgilbert88 at comcast.net
Sat Oct 25 16:50:58 UTC 2008


Unruh wrote:
> "Richard B. Gilbert" <rgilbert88 at comcast.net> writes:
> 
>> Hal Murray wrote:
>>>> It's the poll interval of ntpd.  Ntpq does not poll!  The poll interval 
>>>> varies between 2^MINPOLL and 2^MAXPOLL.  You have set MINPOLL=MAXPOLL=4 
>>>> giving a poll interval of 2^4 or 16 seconds.  This is usually the 
>>>> correct choice for a GPS receiver.
>>> Why do you say that?  
> 
>> Because the GPS time signal is extremely accurate!
> 
>>> Or let me ask it another way, how would you
>>> decide what the right polling interval is?
>>>
> 
>> NTPD uses much longer poll intervals over the internet or even over a 
>> local network because of the variable delays introduced by the network. 
>>  No two packets are guaranteed the same path between two points unless 
>> there IS only one path.  In addition to the variations in travel time 
>> due to differing paths, there are also variable queuing delays!
> 
> No. The longer poll intervals are mainly about keeping packets off the servers. In
> principle it is always better to poll more. (in practice with the ntp
> model, this is only partially true-- you want the 8 times the poll interval
> to be close tothe Allan minimum if the noise model really is exponential
> phase and 1/f drift noise. ( on most modern networks not the greatest
> assumption-- day to night temp variations are probably more important for
> the frequency noise). 

ntp presents a very light load on the servers unless you have some 
idiots polling at two second intervals (other than an initial burst at 
startup).

The longer poll intervals allow ntpd to measure small errors very 
accurately.




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