[ntp:hackers] A stop-gap authenticated time service

Poul-Henning Kamp phk at phk.freebsd.dk
Sun Nov 15 09:35:51 UTC 2015


--------
In message <56484F6A.1000807 at rubidium.dyndns.org>, Magnus Danielson writes:

>> The "Allan intercept" is a rule of thumb it has no formal mathematical basis.
>
>For NTP it relates back to an article by Judah Levine on the ACTS 
>system, which got lifted into the NTP world without validating the noise 
>properly.

If I recall correctly, that article didn't leave any space for
the steering-mechnisms noise and time delay.

The PI steering used in typical PLLs introduce a very non-random
noise due to the (exponential-ish) tapering off of the P term
corrections and adds a significant time-delay, both of which must
be compensated for relative to the Allan Intercept.

I've found nobody who did the math to get from Allan Intercept to
optimal PI parameters.

-- 
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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