[time-nuts] Need Time Help
kb8tq at n1k.org
Wed Oct 5 07:14:30 EDT 2016
If you buy a GPS receiver and get it set up for timing …. just use it. Then there is no
need for NTP at all….
> On Oct 5, 2016, at 12:33 AM, Chris Albertson <albertson.chris at gmail.com> wrote:
> On Tue, Oct 4, 2016 at 6:52 PM, Bob Camp <kb8tq at n1k.org> wrote:
>> 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
>> 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
>> that number is depends a *lot* on the modern and the system feeding the
>> 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 …..
> But this is easy to measure, you buy a GPS receiver and use that as an 8th
> or 6th reference clock along with the 5 or 7 Internet servers then you look
> at the difference between GPS and the Internet servers. The Internet
> servers do much better than you'd think. Not as good as GPS but really
> good. Why?
> Because NTP normally never actually sets you clock to match a server's
> clock. It adjusts the RATE of your clock so the it cruises on the middle
> of the pack of vetted servers. It adjusts your clock over a long period to
> it has the benefit of averaging out lots of random behavior. The result
> is that you can keep within a few milliseconds of the GPS even with tens of
> millisecond of delay in the communication path.
> For people new to NTP, the algorithm has it's hands the system clocks
> "fast/Slow? lever and never normally moves the clocks hands directly
> And all those Internet servers do NOT have the same asymmetric delay. Well
> they might but that would be a pathological case. Typically delays really
> are more symmetric than not (one way trip really is 1/2 the round trip)
> The clocks that don't meet this assumption are found and removed from the
> pool. It works because the dells are NOT the same but random Ans like I
> said, it is easy enough to measure. You can see that peer offsets are all
> over the map
> Also your modem is likely not causing an asymmetric delay. That modern
> wetter is is the old phone kind or a fiber optic system only takes you to
> the fist router. The modern likely has the same time of flight in both
> directions. The delay is cause by a queue some place of packets that are
> aggregated from many users. These are random but sort of predictable. A
> packet going across a continent or will see more of this then going to a
> nearby server
>>> On Oct 4, 2016, at 9:05 PM, Chris Albertson <albertson.chris at gmail.com>
>>> The problem, I think with your Internet sync's NTP servers is you are
>>> using one server S. The most common practice is to use 3 to 5 with 5
>>> 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
>>> 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
>>> 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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>>> and follow the instructions there.
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> Chris Albertson
> Redondo Beach, California
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