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 > [email protected] | TCP/IP since RFC 956 > FreeBSD committer | BSD since 4.3-tahoe > Never attribute to malice what can adequately be explained by > incompetence. > > _______________________________________________ > time-nuts mailing list -- [email protected] > To unsubscribe, go to > https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts > and follow the instructions there. > -- "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 17850 Six Mile Road POB 134 Huson, MT, 59846 VOX 406-626-4304 www.lightningforensics.com www.sixmilesystems.com _______________________________________________ time-nuts mailing list -- [email protected] To unsubscribe, go to https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts and follow the instructions there.
