[ntp:questions] Re: NTP stepping issue
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:
>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.
This Monk had first gone wrong when it was [...] cross-connected to a video
recorder that was watching eleven TV channels simultaneously, [...] The video
recorder only had to watch them, of course. It didn't have to believe them all
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