Hi If:
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 ….. Bob > On Oct 4, 2016, at 9:05 PM, Chris Albertson <albertson.ch...@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 <att...@kinali.ch> wrote: > >> On Tue, 4 Oct 2016 15:41:58 +1100 >> Larry Hower <ho...@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 >> _______________________________________________ >> time-nuts mailing list -- time-nuts@febo.com >> To unsubscribe, go to https://www.febo.com/cgi-bin/ >> mailman/listinfo/time-nuts >> and follow the instructions there. >> > > > > -- > > Chris Albertson > Redondo Beach, California > _______________________________________________ > time-nuts mailing list -- time-nuts@febo.com > To unsubscribe, go to https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts > and follow the instructions there. _______________________________________________ time-nuts mailing list -- time-nuts@febo.com To unsubscribe, go to https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts and follow the instructions there.