[time-nuts] Timing over low bandwidth channels

Poul-Henning Kamp phk at phk.freebsd.dk
Thu Dec 9 10:55:16 UTC 2010


In message <20101209105031.6104C80003B at ip-64-139-1-69.sjc.megapath.net>, Hal Murray writes:

>Is there a similar sort of high level picture about sending timing info?  I'm 
>not even sure what the units are.

Basically with timing you only send one bit: "now"

The most precise way to send that bit is to use a very long PRNG
spreading code, and identify the correlator output peak using
statistical estimation on the slopes up to the peak.

DCF77 sends a 512 bit PRNG every second and in hand-run testes I have
been able to determine the peak of the correlation with precision which
is 100-500 times better than the second to second jitter on the 1200km
propagation.

-- 
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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