[ntp:questions] SNTP with 1ms of precision?

Marcelo Pimenta marcelopimentacs at gmail.com
Thu Jun 17 13:55:37 UTC 2010


2010/6/17 unruh <unruh at wormhole.physics.ubc.ca>

> On 2010-06-16, Marcelo Pimenta <marcelopimentacs at gmail.com> wrote:
> > 2010/6/15 unruh <unruh at wormhole.physics.ubc.ca>
> >
> >> On 2010-06-14, Marcelo Pimenta <marcelopimentacs at gmail.com> wrote:
> >> > Hi everybody!!
> >> >
> >> > My question is about Time Accuracy of NTP/SNTP protocol. I want to
> know
> >> if
> >> > is possible to have precision of 1ms(could be < 1 ms?) using SNTP in a
> >> > network with 50 hosts(is the same with 100 hosts?) using only swithes,
> no
> >> > routing. All these hosts are syncronized with a Meinberg GPS. The
> traffic
> >> is
> >> > low(~25%). Is it possible to have the same time accuracy of IRIG-B for
> >> > example?
> >>
> >> I think you mean accuracy of 1ms. Yes. it can. On a local network,
> >> 100usec is easily possible ( and depending on the routers, it can be
> >> better than that).
> >>
> >
> > Yes, you are right, accuracy of 1ms. On a local network 100usec?? Even if
> we
> > use only switches(no routers), how is that possible if I have 4 types of
> > Latency increasing about 80us? Algorithms to compensate the delay? Even
> in
> > SNTP?
>
> ntp relies on the outgoing and ingoing times are the same. They could be
> one
> year, but if they are both exactly the same ( both exactly one year)
> ntp can deliver a time accuracy of 1nsec.
> Yes, I typically get time accuracies of tens of usec on a local network.
> Even over adsl frin home to work  I get 1 msec easily ( and yes I can
> detect the accuracy by
> having a gps PPS receiver at both endw to test the accuracy to 1us)
>

NTP can deliver a time accuracy of 1nsec?? I my point of view, NTP includes
methods to estimate the round-trip path delay between the server and client
but the performance is limited by SO stack latency, it is an Application
Layer protocol.

What's the resolution of you clock? What's your SO?


>
> level)
>  >
> > Anyway, the question is about 1 ms. To get 100usec I'll use PTP.
> >
> >
> >
> >> >
> >> > The internal clock in my devices(hosts) starts with the crystal
> >> frequence. A
> >> > typical offset or frequence tolerance of the crystal is less then
> >> > 1ms/minute. The resolution is about 0.9ms and the devices acts as
> unicast
> >> > clients, asking GPS every minute.
> >>
> >> Why?  The whole purpose of ntp is to discipline your clock so that it
> >> keeps time much better than that.
> >>
> >
> > The idea to have this is not overload the network asking time every
> second
> > to keep my accurancy in 1ms in 100% of time. And the second reason is
> about
> > how many ms your clock will be wrong in 59 sec without a frame to
> discipline
> > my clock again? PCs clock for example is not good, that's why I need a
> very
> > good crystal.
> >
> >
> >> Not sure what you mean by "the resolution is about .9ms" What is your
> >> device?
> >>
> >
> > I mean that my device can show me variations of 0.9ms, is the smallest
> > possible increase of time the clock model allows. I work with protection
> > IED(intelligent electronic device), is a term used in the electric power
> > industry to describe microprocessor-based controllers of power system
> > equipment.
> >
> > If something happens in 458ms, I will get exacly 458ms. Some equipment
> have
> >  so I can trust in variation that I can capture.
> >
> >>
> >> > Is it enough? The Time Accurancy could be better if I could ask GPS
> twice
> >> a
> >> > minute? A broadcast implementation with GPS sending time every minute
> >> could
> >> > help?
> >>
> >> It depends on what you want the time for.
> >>
> >
> > I need accurancy at least of 1ms in 100% of time.
> >
> >>
> >> >
> >> > I know that these kind of issue have many variables but the main
> question
> >> > is: Is possible to have precision of 1ms(could be <1ms) with SNTP?
> >>
> >> Yes, depending on the competence of the writer of the SNTP software.
> >>
> >
> > So, the problem is not in the standard but into implementation of SNTP to
> > not loose time to correct the clock and something like that?
> >
> >
> >> >
> >> > Thank you very much
> >>
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> >>
>
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