[time-nuts] Need Time Help
kb8tq at n1k.org
Tue Oct 4 21:52:30 EDT 2016
1) You are a typical Ham in a home environment
2) All the servers are “out there” on the internet
3) You have any of the normal modems feeding your home
You have a very basic issue in terms of path delay. All the servers you can access
have the *same* asymmetric delay. In that case, no matter how many servers you
add to the ensemble, the situation never gets better. You are always stuck with the
(likely unknown) uplink / downlink delay difference of your modem. Exactly what
that number is depends a *lot* on the modern and the system feeding the modem.
It is *very* possible to see static delay asymmetry well beyond the 5 ms that the OP is after.
On most systems there is also a dynamic asymmetry that is related to loading. That
just makes things harder to work out …..
> On Oct 4, 2016, at 9:05 PM, Chris Albertson <albertson.chris at gmail.com> wrote:
> 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
> you problem.
> 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
>> 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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> Chris Albertson
> Redondo Beach, California
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