[time-nuts] Divide by five

Poul-Henning Kamp phk at phk.freebsd.dk
Mon Nov 10 03:16:56 EST 2014

In message <546070B3.1010209 at rubidium.dyndns.org>, Magnus Danielson writes:

>The NTP scatter-plot wedge is another way to present it, and finding the 
>"tip" of that wedge is really about finding the min-value of delay in 
>each direction prior to doing the two-way time-transfer equations.

The important part about the wedge diagram is that the wedge is not
filled out.  With a very large probability only one of the two packets
is impacted by extra delays.

The current NTP filter totally fails to exploit this fact and
performs quite a bit worse because of it:  A median filter is
really not a suitable way to handle that situation.

But steering back to the original topic again:  You really should
not look at NTP for clock-ensemble or clock-steering examples, the
challenges NTP copes with, are nothing like the challenges you have
with a bunch of local high-quality frequencies.

There used to be an academic paper on timing.com's home-page about
their clock-ensemble algorithm called something like "Advances in
Time-Scale Algorithms".

It's not super detailed but it was certainly a much better place
to start than NTP source or documentation

Poul-Henning Kamp       | UNIX since Zilog Zeus 3.20
phk at FreeBSD.ORG         | TCP/IP since RFC 956
FreeBSD committer       | BSD since 4.3-tahoe    
Never attribute to malice what can adequately be explained by incompetence.

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