On 2010-06-16, Marcelo Pimenta <marcelopiment...@gmail.com> wrote: > 2010/6/15 unruh <un...@wormhole.physics.ubc.ca> > >> On 2010-06-14, Marcelo Pimenta <marcelopiment...@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) 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 >> >> _______________________________________________ >> questions mailing list >> questions@lists.ntp.org >> http://lists.ntp.org/listinfo/questions >> _______________________________________________ questions mailing list questions@lists.ntp.org http://lists.ntp.org/listinfo/questions
