[time-nuts] Need Time Help
albertson.chris at gmail.com
Tue Oct 4 21:05:16 EDT 2016
The problem, I think with your Internet sync's NTP servers is you are only
using one server S. The most common practice is to use 3 to 5 with 5 being
about the right number. If you get Ntp enough Internet servers to work
with it can detect problem like asymmetric path lengths which I'm sure is
NTP solved the problem that stumped a few people back in the 1970's of how
to sync two clocks when there is a long delay and not constant in there
communications path. (Of course the problem is simple if the delay is
known and well measured) But the solution required the the average path
delay is the same going in each direction. worse no software can't know
there is an asymmetric delay. Well not unless it is using a few servers.
NTP basically finds then ignores the "problem servers".
PTP solves the problem by requiring that all the network hardware has
special time stamp ability that is designed to work with PTP. This
hardware is rare unless the user provides it. So PTP can't really work on
the public Internet.
You CAN do very well, to just a few Millisecond using NTP sync'ing to
Internet servers, but pick 5 of them or even 7. and make sure they are
dispersed and not all at the same place.
On Tue, Oct 4, 2016 at 2:34 PM, Attila Kinali <attila at kinali.ch> wrote:
> 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
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