Re: [ntp:questions] NTP constant delay ?
On 2012-12-26, Ran Shalit ransha...@gmail.com wrote: Hello, I intend to use SNTP protocol with 2 machines connected by LAN. According to decumentatio about SNTP it sys that SNTP provides accuracy typically within 100 ms. Not sure where this nonesense was written. I would like to ask please what can we expect in our configuration will the delay be constant within 100msec, meaning that we cn measure No. A statement like that means it will vary from + or- 100ms. But ntp is capable of much better behaviour than that. it constant delay, or will it drift such that in one meanute is will Yes. be delay X, and in other meanutes totally other delay within 100msec. If the delay is constant, I assume that we can always delete this delay constant X and get a time synchronization around zero. Is that a right assumptuion ? Thank you very much, Ran ___ questions mailing list questions@lists.ntp.org http://lists.ntp.org/listinfo/questions
Re: [ntp:questions] NTP constant delay ?
Ran Shalit wrote: I intend to use SNTP protocol with 2 machines connected by LAN. According to decumentatio about SNTP it sys that SNTP provides accuracy typically within 100 ms. That's pessimistic and is based, in part, on SNTP not defining how one gets from the time measurements to the local clock settings. or how often one makes them. I would like to ask please what can we expect in our configuration will the delay be constant within 100msec, meaning that we cn measure it constant delay, or will it drift such that in one meanute is will be delay X, and in other meanutes totally other delay within 100msec. On a LAN, the measurement error will vary, but not by anything like 100ms. Over a WAN, it is possible for it to vary by up to a second. If the delay is constant, I assume that we can always delete this delay constant X and get a time synchronization around zero. Is that a right assumptuion ? Even SNTP corrects for round trip delay, so the remaining delay errors will be due to asymmetry. The systematic part of errors will affect a full NTP system as well, and people don't normally worry about them. The biggest problem in correcting them is measuring them. If you have some means to measure them, you would be better off using that for time synchronisation. In any case, if you want better handling of random variations, there is really no reason not to use a full NTP implementation. Errors from typical SNTP systems arise from: - stepping the time to match even a bad measurement; - not correcting for clock drift between measurments; - not being able to detect and ignore a source with a faulty clock. If you correct for all of these, you have something that is pretty close to being a full NTP. One other thing. If you are looking for hard limits on the error, the only absolute guarantees that can be given are much larger than errors than the typical ones. ___ questions mailing list questions@lists.ntp.org http://lists.ntp.org/listinfo/questions
Re: [ntp:questions] NTP constant delay ?
On 27/12/2012 10:11, Ran Shalit wrote: [] Hi, SNPT performance with talks about accuracy of 100msec are shown in the following white paper: http://www.cisco.com/en/US/tech/tk869/tk769/technologies_white_paper09186a0080117070.shtml In out configuration we are talking about 2 machines, one using Linux SNTP client, the other I do not know, but assume it is RT Operating system. both machines are connected through ethernet. It seems that if we use SNTP (as is required right now), we can't assure more accuracy then 100msec (according to that document), which might be probematic with our video sync requirements which are talking about less then 40msec. I appreciate any comments or ides, Regards, Ran It seems that you will be limited by what the RTOS offers. Your Linux system will usually run full NTP and be able to offer time over SNTP to the RTOS system. Check whether the RTOS can run NTP, and if so, use it. If not, run SNTP and try to keep the updates as frequent as possible, perhaps every 60 seconds. I am suggesting here that the RTOS be synced to the Linux system directly over the LAN. I suspect that the CISCO document is referring to less direct connections when it talks of 100 ms. If you are syncing both PCs to a third source, then that's different. If I understand it correctly, the danger with SNTP is that it will step the clock rather than alter the clock's rate, and this might result in lost video frames. -- Cheers, David Web: http://www.satsignal.eu ___ questions mailing list questions@lists.ntp.org http://lists.ntp.org/listinfo/questions
Re: [ntp:questions] NTP constant delay ?
On 2012-12-27, Ran Shalit ransha...@gmail.com wrote: On Thu, Dec 27, 2012 at 11:24 AM, unruh un...@invalid.ca wrote: delay be constant within 100msec, meaning Hi, SNPT performance with talks about accuracy of 100msec are shown in the following white paper: http://www.cisco.com/en/US/tech/tk869/tk769/technologies_white_paper09186a0080117070.shtml Try it. See what you get. Or put in ntpd or chrony and get a few microsecond accuracy. In out configuration we are talking about 2 machines, one using Linux SNTP client, the other I do not know, but assume it is RT Operating system. both machines are connected through ethernet. Whatever RT operating system is. It seems that if we use SNTP (as is required right now), we can't assure more accuracy then 100msec (according to that document), which And you believe all documents you read on the web? might be probematic with our video sync requirements which are talking about less then 40msec. I appreciate any comments or ides, SNTP is not a program. It is a protocol, used for a) delivering time from a stratum 0 time source, or b) receiving time on a machine which will not server it to anyone else. ntpd is the stadard ntp program. chrony is in many cases more accurate. Both will give 10s of microseconds (not milliseconds) on Linux/bsd.. systems. Only ntpd works on windows. Regards, Ran ___ questions mailing list questions@lists.ntp.org http://lists.ntp.org/listinfo/questions
Re: [ntp:questions] NTP constant delay ?
unruh wrote: ransha...@gmail.com wrote: In out configuration we are talking about 2 machines, one using Linux SNTP client, the other I do not know, but assume it is RT Operating system. Whatever RT operating system is. Good Question. is that Microsoft Windows RT OS http://windows.microsoft.com/en-US/windows/rt-welcome, or a RTOS http://en.wikipedia.org/wiki/List_of_real-time_operating_systems http://en.wikipedia.org/wiki/Real-time_operating_system -- E-Mail Sent to this address blackl...@anitech-systems.com will be added to the BlackLists. ___ questions mailing list questions@lists.ntp.org http://lists.ntp.org/listinfo/questions
