[time-nuts] Need Time Help

Attila Kinali attila at kinali.ch
Tue Oct 4 17:34:29 EDT 2016

On Tue, 4 Oct 2016 15:41:58 +1100
Larry Hower <hower at hower.net> wrote:

> Ultimately we want sub-millisecond accuracy.

If you want to go that way, you will have to leave windows as
this operating system does not offer the facilities to get down
to such a low level....Unless you calibrate the whole path by injecting
a time pulse into the signal path like Jim Lux and TvB suggested

With linux you can get systems synchronized to better than 1ms by
using a PTP server in the local network or by directly using PPS.
This should get you in the order of better than 100µs probaly 20-30µs.

BTW: A word of advice against using NTP servers over the internet
for accurate time distribution. I recently set-up two NTP servers
to be used as stratum 2 servers (server A and B). Both synchronize
to the same stratum 1 server (server S), but are at different ISPs
and thus use different paths. NTP on both A and B reports the following
values (current snapshot, values are representative):

Link	delay	offset	jitter
A-S	4.205    0.020   0.081
B-S	2.112    0.039   0.079
A-B 	0.606   -0.877   3.192 (as reported by A)

I.e. even though A and B use the same server S as reference, the 
time difference between both servers is 800-900µs. I am not sure
where this path asymmetry comes from, but my guess would be on
the connectivity of A (there are two groups of stratum 2 it syncs
to and one of them shows the same ~900µs offset). I also do not
know why the jitter between A and B is so large even though the
delay is pretty low (seems to be a weirdness at a router inbetween).

			Attila Kinali

It is upon moral qualities that a society is ultimately founded. All 
the prosperity and technological sophistication in the world is of no 
use without that foundation.
                 -- Miss Matheson, The Diamond Age, Neil Stephenson

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