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

Unruh unruh-spam at physics.ubc.ca
Sat Oct 25 06:08:52 UTC 2008

"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). 

>Ntpd does not know how long any particular packet was delayed, it only 
>knows what the typical delay is.  The longer polling intervals smooth 
>out the errors caused by variable delays.

No, it knows exactly now long the delay is. It just does not know if that
delay occured outbound, inbound or both. (I think you are thinking about
the broadcast model)

>See Dave's book for a rigorous mathematical explanation.  Or, if your 
>math is as poor as mine, you'll just have to take Dave's word for it.

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