[ntp:questions] Re: NTP stepping issue

Philip Homburg philip at pch.home.cs.vu.nl
Sat Oct 23 21:36:59 UTC 2004


In article <cldtmo$gde$1 at dewey.udel.edu>,
David L. Mills <mills at udel.edu> wrote:
>It is 
>useful primarily at long poll intervals where errors are dominated by 
>the intrinsic frequency stability (wander) of the clock oscillator. At 
>shorter poll intervals the errors are dominated by phase errors due to 
>network and operating system latencies. The trick is to combine then in 
>an intelligent hybrid loop, as described on the NTP project page.

The strange thing is that the 500 ppm / 128 ms limit keeps popping up. 
I can understand strange limits closed-source / broken operating systems.
But somehow that limit is also present on open source operating systems.

>There is a genuine opportunity to test your theories. See the NTP 
>simulator, which is included in the software distribution. The best 
>place to experiment is in the ntp_loopfilter.c file. The simulator can 
>generate ramps, constant offsets and various combinations of random 
>phase and frequency errors. You can also record real-world data and feed 
>that to the simulator.

The problem is of course time. The main experiment I want to do is 
black box testing of NTP implementations: create a reference clock with
a known distortion, feed it to the ntp implementation that is to be tested 
and then poll that implementation to compare the filtered time to
the input signal.

As far as I know, such a blackbox test setup does not exist. 

Maybe one day I can find the time to create such a test environment, but
I'd rather have somebody else (who also knows enough about statistics to
do a proper analysis of the raw results) build it.


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