[time-nuts] Discipline an oscillator with NTP?

[Don Latham]

Poul and others?
As usual, I suddenly had a thought ripple across my otherwise placid
cortex, and have forgotten if there was a previous answer on this
thread. What does a NTP server look like on the network? There are now
several small net appliances/eval boards available. I run a program
called NMEAtime on my Windoze computers that currently get info from a
net NTP server. I also have some simple net appliances. So, using my GPS
10 MHZ or 1 sec signals, or Rb 10 MHZ, how might I generate a local NTP
server?
I hope I don't put this awkwardly; what is the protocol? What does the
supplicant client see and how does it ask for service?
Best to all of you,
Don


Poul-Henning Kamp
> In message <022101cc4970$b24a4b80$16dee280$@com>, "Jose Camara" writes:
>
>>After
>>one year of NTP queries, assume you have a 100ms jitter on the network
>> time,
>>you could at most tweak your oscillator, based on past performance, to
>> 6E-9.
>
> It is a lot more complicated than that, we need to talk allan-deviation
> here, not scalar numbers.
>
> The main problem here is that the 'default' NTPd software is not really
> written for something like this, and has attributes which makes it
> truly sucky for the task.
>
> If you want to do this, you want to write your own software and you
> want to give it an entirely different modus operandi.
>
> As with all oscillator discplining, what you are looking for is
> the so called "allan intercept" where the two sources allan deviation
> cross.
>
> With a NTP reference, its location varies depending on stochastic
> network properties, which depends what's between the server and you.
>
> If you control the network topology (as in: Can make sure there is
> no other traffic), you're fine, normal PLL style stuff works.
>
> If you don't control the network topology, the RTT between you and
> the server becomes a BIG problem, because the fundamental NTP
> assumption that it is symmetric is almost always wrong.
>
> You can average over long tau's, but then your ISP upgrades their
> routed and a systematic change in RTT screws your integrator over
> for several weeks.
>
> Alternatively, if your LO is stable enough (=Rb/Cs), you can operate
> on first derivative of the RTT, which turns the routed upgrade into
> a single spiky sample, but the cost is an overall higher noise in
> your error signal.
>
> --
> Poul-Henning Kamp       | UNIX since Zilog Zeus 3.20
> phk at FreeBSD.ORG         | TCP/IP since RFC 956
> FreeBSD committer       | BSD since 4.3-tahoe
> Never attribute to malice what can adequately be explained by
> incompetence.
>
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-- 
"Neither the voice of authority nor the weight of reason and argument
are as significant as experiment, for thence comes quiet to the mind."
R. Bacon
"If you don't know what it is, don't poke it."
Ghost in the Shell


Dr. Don Latham AJ7LL
Six Mile Systems LLP
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