[ntp:questions] SNTP with 1ms of precision?
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
> >> 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,
> >> > routing. All these hosts are syncronized with a Meinberg GPS. The
> >> 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
> > 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
> > SNTP?
> ntp relies on the outgoing and ingoing times are the same. They could be
> 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
What's the resolution of you clock? What's your SO?
> > 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
> >> > 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
> > to keep my accurancy in 1ms in 100% of time. And the second reason is
> > how many ms your clock will be wrong in 59 sec without a frame to
> > my clock again? PCs clock for example is not good, that's why I need a
> > 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
> > so I can trust in variation that I can capture.
> >> > Is it enough? The Time Accurancy could be better if I could ask GPS
> >> 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
> >> > 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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