Re: [ntp:questions] NTP absolute accuracy?
David J Taylor wrote: David J Taylor david-tay...@blueyonder.not-this-bit.nor-this.co.uk.invalid wrote in message news:%xqgm.126$ym4...@text.news.virginmedia.com... Unruh wrote in message news:34jgm.50898$ph1.36...@edtnps82... [] Note that chrony will give you a factor of 2 or three improvement over ntp in the errors, assuming that the roundtrip is equally split on Linux or BSD. True but it doesnt support AutoKEY operations so it has no value in provable time settings which are now key requirements in many commercial settings. For those without wide-bandwidth academic connections - those folks on cable or ADSL - how good is an equal split round trip assumption? David David - This doesnt take into account the issues of peering agreements and backhauling which add a non-deterministic type of latency to the time-setting process as does the peering agreements between the various carriers who's networks the time-service providers systems operate in. It also takes into account system I/O latency's and the issues of time-provider or infrastructure loading. Also most parties using cable networks are probably running home gear with SNTP enablement in most instances. Its unlikely that a NTP client exists in most sub-$100 infrastructure devices. I think the answer is that no one knows and it could be as little as a few milliseconds or it could be more and we wont know. It also depends on whether the time server those client systems are tied to are located on the Open Internet or are operated by the Cable Providers for their clients use. Then the secondary non-deterministic impulse and the asymmetric routing delays are mitigated or eliminated so again - it depends on how NTP is being used and whether its a real NTP instance or just a SNTP instance (as it exists today) being used. Thanks, David, David and Jan. A few milliseconds is what I had expected, so if you are on a consumer line, what implications does that have for unruh's comment? Cheers, David ___ questions mailing list questions@lists.ntp.org https://lists.ntp.org/mailman/listinfo/questions No virus found in this incoming message. Checked by AVG - www.avg.com Version: 8.5.423 / Virus Database: 270.14.39/2470 - Release Date: 10/30/09 15:18:00 ___ questions mailing list questions@lists.ntp.org https://lists.ntp.org/mailman/listinfo/questions
Re: [ntp:questions] NTP absolute accuracy?
RedGrittyBrick wrote: Unruh wrote: David J Taylor david-tay...@blueyonder.not-this-bit.nor-this.co.uk.invalid writes: At source, it's recently been within about 10 microseconds: Sorry, at 10usec, the distance away of the transmitter must be less than 3 km. At *source*, the distance to the transmitter must surely be a lot less than 3 km :-) According to the National Physics Laboratory Every UTC second is marked by an 'off' preceded by at least 500 ms of carrier, and this second marker is transmitted with an accuracy better than ±1 ms. from which I conclude it's never going to to be more accurate than a few ms at *receiver*. Yea - every TAI second is what they mean. UTC is a calculated value which is published after the fact and sets the TOD at a particular instant some 25 to 30 days previous. From that point onward until the next UTC fixing per schedule T the time is measured in incremental TAI seconds per the convention of the Meter's definition of the Second. Todd Someone has compared a KHz radio time signal from DCF with the GHz radio time signals from GPS: http://www.hopf-time.com/en/dcf-gps.htm. I imagine MSF accuracy is similar to DCF. No virus found in this incoming message. Checked by AVG - www.avg.com Version: 8.5.423 / Virus Database: 270.14.44/2475 - Release Date: 11/01/09 19:39:00 ___ questions mailing list questions@lists.ntp.org https://lists.ntp.org/mailman/listinfo/questions
Re: [ntp:questions] NTP absolute accuracy?
David Woolley da...@ex.djwhome.demon.invalid writes: David Malone wrote: The Rugby unit was built by Ian Dowse, and is described here: http://www.maths.tcd.ie/~dwmalone/time/rugby.html This is a simple AM detector for the slow code (I don't know if the fast code is still transmitted). It doesn't phase lock onto the carrier. Yep - It just delivers the pulses to DCD on a serial port. The driver uses the PPS API and averages the second marks over the minute before delivering the dates to NTP via the SHM driver. I do have some software radio stuff that looks at various long wave signals. Where I am, we can hear MSF, DCF, BBC 4 and TDF, all of which carry time signals. I have some plots of the phase here: http://www.maths.tcd.ie/~dwmalone/phase.png You can clearly see the phase-modulated timecodes on top of BBC 4 and TDF. Josh Tobin did some work with me to write software decoders for these over the summer, and had the TDF signal working as an NTP refclock. We haven't cleaned the code up yet, but hope to soon. David. ___ questions mailing list questions@lists.ntp.org https://lists.ntp.org/mailman/listinfo/questions
Re: [ntp:questions] NTP absolute accuracy?
John Hasler wrote: Bill Unruh writes: The bandwidth of whatever marks it has to be pretty narrow, or the transmitter would interfer with everything around it. Ie, that means a large time uncertainty. The 60KHz WWVB signal is synchronously amplitude-modulated at a 1Hz rate. The exact cycle on which the amplitude changes can be locked in by a digital phase-locked loop. A VCXO can be phase-locked to the carrier to within a small fraction of a cycle. Thus with a strong signal the PPS error should be dominated by variations in propagation delay. In practice accuracies of +-100usec are reportedly achievable, but my very casual research indicate that even the fancy receivers don't bother to include the hardware necessary to do so. Too bad NIST can't find the money for the PRNG phase modulation DCF77 has. Perhaps the NAVSTAR satellites and GPS provide more/better results than a VLF broadcast. ___ questions mailing list questions@lists.ntp.org https://lists.ntp.org/mailman/listinfo/questions
Re: [ntp:questions] NTP absolute accuracy?
Unruh wrote: David J Taylor david-tay...@blueyonder.not-this-bit.nor-this.co.uk.invalid writes: Unruh wrote in message news:iebhm.50125$db2.41...@edtnps83... [] MSF? See: http://www.npl.co.uk/science-technology/time-frequency/time/products-and-services/msf-radio-time-signal UK 60KHz radio time signal. Ah, OK. Its accuracy is probably not much better than a few msec I assume. GPS is a few usec. Since the network gives accuracies of better than a few ms, MSF is not a good way of testing the accuracy of network set clocks. Hi I expected to get away with it from trying upstairs: hrs % offset jitter % offset jitter % offset jitter (ms) (ms)(ms) (ms)(ms) (ms) 100 50 0.185 0.086 95 0.642 0.255 99.5 1.111 0.488 frame aerial 70 50 0.362 0.172 95 0.802 0.370 99.5 1.057 0.495 ferrite-rod With PPS driver added hrs % offset jitter % offset jitter % offset jitter (ms) (ms)(ms) (ms)(ms) (ms) 26 50 0.165 0.070 95 0.621 0.246 99.5 0.673 0.333 frame aerial The % refers to logging at 30min intervals so 50% of offsets were at or under 165us. Having moved the receiver with ferrite-rod downstairs it's not so good, I'm not yet setup for PPS driver. hrs % offset jitter % offset jitter % offset jitter (ms) (ms)(ms) (ms)(ms) (ms) 2650 0.561 1.095 1.344 For same system using ntpd and internet from before using MSF: hrs % offset jitter % offset jitter % offset jitter (ms) (ms)(ms) (ms)(ms) (ms) 9650 1.535 0.467 95 4.404 1.672 99.5 4.850 4.889 the other two servers from internet are better. 9650 0.642 0.610 95 3.726 2.258 99.5 4.959 2.916 9650 0.507 0.520 95 1.329 0.811 99.5 2.588 2.030 I'll try swapping to frame aerial and PPS module tonight but I think I'm in a leaky faraday cage with wire mesh under the plaster on the walls so for anything better will have to wait until I've finished the units that will be mounted on mast at rear of house. David ___ questions mailing list questions@lists.ntp.org https://lists.ntp.org/mailman/listinfo/questions
Re: [ntp:questions] NTP absolute accuracy?
Maarten Wiltink wrote: Unruh unruh-s...@physics.ubc.ca wrote in message news:iebhm.50125$db2.41...@edtnps83... [...] The problem is that I have no idea what the accuracy of any of those items is. YOur ISP's timesever may be a stratum 7 getting time from a bunch of bozos. Or itmay be stratum 1 getting its time from a well implimented GPS clock. ntpq -p will tell you that. And ntptrace does exactly that, climbing the chain up from you to stratum 1. I used ntptrace on about 20 different public servers and have cut that list down to those that are closest and have most diverse sources. Nowadays from here, using uk pool is as good a method as any unless your server is in the pool which is why I've gone back to my previous selection, but unfortunately that list keeps getting shorter as servers are taken out of service and not replaced. David ___ questions mailing list questions@lists.ntp.org https://lists.ntp.org/mailman/listinfo/questions
Re: [ntp:questions] NTP absolute accuracy?
Unruh wrote: David J Taylor david-tay...@blueyonder.not-this-bit.nor-this.co.uk.invalid writes: At source, it's recently been within about 10 microseconds: Sorry, at 10usec, the distance away of the transmitter must be less than 3 km. At *source*, the distance to the transmitter must surely be a lot less than 3 km :-) According to the National Physics Laboratory Every UTC second is marked by an 'off' preceded by at least 500 ms of carrier, and this second marker is transmitted with an accuracy better than ±1 ms. from which I conclude it's never going to to be more accurate than a few ms at *receiver*. Someone has compared a KHz radio time signal from DCF with the GHz radio time signals from GPS: http://www.hopf-time.com/en/dcf-gps.htm. I imagine MSF accuracy is similar to DCF. -- RGB ___ questions mailing list questions@lists.ntp.org https://lists.ntp.org/mailman/listinfo/questions
Re: [ntp:questions] NTP absolute accuracy?
David J Taylor david-tay...@blueyonder.not-this-bit.nor-this.co.uk.invalid writes: Thanks, David, David and Jan. A few milliseconds is what I had expected, so if you are on a consumer line, what implications does that have for unruh's comment? I've been plotting the offset reported by ntpq -p for my Rugby receiver and a GPS-based server over DSL at: http://www.dwmalone.net/mrtg/ntp/ (Appologies if you're using IPv6, the link to the machine is a bit wonkey right now.) I take a sample every 64s from the Rugby peer, and lanczos is the other peer uses ntp's standard polling. The Rugby unit was built by Ian Dowse, and is described here: http://www.maths.tcd.ie/~dwmalone/time/rugby.html It was calibrated to be within about 1ms of the GPS unit when they were connected to machines on the same LAN. David. ___ questions mailing list questions@lists.ntp.org https://lists.ntp.org/mailman/listinfo/questions
Re: [ntp:questions] NTP absolute accuracy?
Unruh wrote: John Hasler jhas...@newsguy.com writes: Bill Unruh writes: Sorry, at 10usec, the distance away of the transmitter must be less than 3 km. 10usec at the transmitter. Also your system needs to see the start of the tone to 10usec which means that the tone would have to be about 1MHz which is a bit beyond audio. There isn't any tone. It's the UK equivalent of WWVB. How is the beginning of the second ( an hour) marked? The MSF signal carries time and date information that is transmitted in a simple binary-coded decimal format by on-off modulation of the carrier. The full sequence of information is transmitted each minute, using two data bits during every second of the minute except the first. The MSF time and date code includes the following information: * year * month * day of month * day of week * hour * minute * British Summer Time (in effect or imminent) * DUT1 (a parameter giving UT1-UTC; http://www.npl.co.uk/science-technology/time-frequency/time/products-and-services/msf-radio-time-signal The bandwidth of whatever marks it has to be pretty narrow, or the transmitter would interfer with everything around it. Ie, that means a large time uncertainty. Accuracy of the transmitted signal is continuously monitored at other locations. E.g. Teddington (~300 miles from transmitter). http://www.npl.co.uk/upload/pdf/user_guide_bulletins.pdf MJD DateUTC(NPL)-MSF(60 kHz) 12h UTC (microseconds) 55075 2009-09-01 -9.5 55076 2009-09-02 -9.7 55077 2009-09-03 -9.9 55078 2009-09-04 -9.8 55079 2009-09-05 -9.8 From http://resource.npl.co.uk/time/bulletins/msf/msfbul_09_2009.pdf Ie how fast is the carrier actually switched off and on. It cannot be instantaneous or the signal would spill out badly into neighbouring bands. Simple on-off carrier modulation is used, the rise and fall times of the carrier are determined by the combination of antenna and transmitter. ... http://www.npl.co.uk/upload/pdf/MSF_Time_Date_Code.pdf -- RGB ___ questions mailing list questions@lists.ntp.org https://lists.ntp.org/mailman/listinfo/questions
Re: [ntp:questions] NTP absolute accuracy?
David Malone wrote: The Rugby unit was built by Ian Dowse, and is described here: http://www.maths.tcd.ie/~dwmalone/time/rugby.html This is a simple AM detector for the slow code (I don't know if the fast code is still transmitted). It doesn't phase lock onto the carrier. ___ questions mailing list questions@lists.ntp.org https://lists.ntp.org/mailman/listinfo/questions
Re: [ntp:questions] NTP absolute accuracy?
David Woolley wrote: David Malone wrote: The Rugby unit was built by Ian Dowse, and is described here: http://www.maths.tcd.ie/~dwmalone/time/rugby.html This is a simple AM detector for the slow code (I don't know if the fast code is still transmitted). It doesn't phase lock onto the carrier. Fast code isn't mentioned in latest info I downloaded and someone mentioned in a reply to one of my messages that it was no longer transmitted. Maplin MSF module hasn't been available for some years either, but I swapped xtal and aerial of Conrad DCF77 module and that works here receiving MSF better than it did DCF. I'm still assembling parts for a DIY phase locked frequency reference, but as I've found, that still requires GPS or other method to calibrate to use for ntp. David ___ questions mailing list questions@lists.ntp.org https://lists.ntp.org/mailman/listinfo/questions
Re: [ntp:questions] NTP absolute accuracy?
David Woolley wrote in message news:hcnjt3$bq...@news.eternal-september.org... [] This is a simple AM detector for the slow code (I don't know if the fast code is still transmitted). It doesn't phase lock onto the carrier. The fast code stopped many years ago. October 1998 according to: http://www.npl.co.uk/science-technology/time-frequency/time/faqs/what-is-the-msf-fast-code-%28faq-time%29 http://www.creative-science.org.uk/nplmsf.html Cheers, David ___ questions mailing list questions@lists.ntp.org https://lists.ntp.org/mailman/listinfo/questions
Re: [ntp:questions] NTP absolute accuracy?
Bill Unruh writes: The bandwidth of whatever marks it has to be pretty narrow, or the transmitter would interfer with everything around it. Ie, that means a large time uncertainty. The 60KHz WWVB signal is synchronously amplitude-modulated at a 1Hz rate. The exact cycle on which the amplitude changes can be locked in by a digital phase-locked loop. A VCXO can be phase-locked to the carrier to within a small fraction of a cycle. Thus with a strong signal the PPS error should be dominated by variations in propagation delay. In practice accuracies of +-100usec are reportedly achievable, but my very casual research indicate that even the fancy receivers don't bother to include the hardware necessary to do so. Too bad NIST can't find the money for the PRNG phase modulation DCF77 has. -- John Hasler jhas...@newsguy.com Dancing Horse Hill Elmwood, WI USA ___ questions mailing list questions@lists.ntp.org https://lists.ntp.org/mailman/listinfo/questions
Re: [ntp:questions] NTP absolute accuracy?
John Hasler jhas...@newsguy.com writes: Bill Unruh writes: The bandwidth of whatever marks it has to be pretty narrow, or the transmitter would interfer with everything around it. Ie, that means a large time uncertainty. The 60KHz WWVB signal is synchronously amplitude-modulated at a 1Hz rate. The exact cycle on which the amplitude changes can be locked in by a digital phase-locked loop. A VCXO can be phase-locked to the carrier to within a small fraction of a cycle. Thus with a strong signal the PPS error should be dominated by variations in propagation delay. In practice accuracies of +-100usec are reportedly achievable, but my very casual research indicate that even the fancy receivers don't bother to include the hardware necessary to do so. Too bad NIST can't find the money for the PRNG phase modulation DCF77 has. This is of course all made totally redundant by the GPS time delivery-- even makes up for the transmission delay. It begins to look like like Morse code in the days of cell phones. Quaint, but not really useful. Now, perhaps if the whole GPS system gets knocked out by the Chinese, those using such ancient techniques will be kings of the time hill, but I suspect we all will have other problems then. -- John Hasler jhas...@newsguy.com Dancing Horse Hill Elmwood, WI USA ___ questions mailing list questions@lists.ntp.org https://lists.ntp.org/mailman/listinfo/questions
Re: [ntp:questions] NTP absolute accuracy?
Bill Unruh writes: This is of course all made totally redundant by the GPS time delivery-- even makes up for the transmission delay. It begins to look like like Morse code in the days of cell phones. Quaint, but not really useful. One could say the same about NTP. -- John Hasler jhas...@newsguy.com Dancing Horse Hill Elmwood, WI USA ___ questions mailing list questions@lists.ntp.org https://lists.ntp.org/mailman/listinfo/questions
Re: [ntp:questions] NTP absolute accuracy?
Unruh wrote: John Hasler jhas...@newsguy.com writes: The 60KHz WWVB signal is synchronously amplitude-modulated at a 1Hz rate. The exact cycle on which the amplitude changes can be locked in by a digital phase-locked loop. A VCXO can be phase-locked to the carrier to within a small fraction of a cycle. Thus with a strong signal the PPS error should be dominated by variations in propagation delay. In practice accuracies of +-100usec are reportedly achievable, but my very casual research indicate that even the fancy receivers don't bother to include the hardware necessary to do so. Too bad NIST can't find the money for the PRNG phase modulation DCF77 has. This is of course all made totally redundant by the GPS time delivery-- even makes up for the transmission delay. It begins to look like like Morse code in the days of cell phones. Quaint, but not really useful. MSF is good for weather forecasting at the moment :-) http://www.lordynet.me.uk/pub/ntp/me6000g_ntp-day.png (green=+ve/blue=-ve) With DCF this was almost each night that signal was lost for maybe thirty minutes then same again early morning. This is switch between groundwave and skywave. I thought I was close enough to transmitter at about 160km from Anthorn (MSF) vs 1000km from Frankfurt (DCF), but obviously not. David ___ questions mailing list questions@lists.ntp.org https://lists.ntp.org/mailman/listinfo/questions
Re: [ntp:questions] NTP absolute accuracy?
Unruh unruh-s...@physics.ubc.ca wrote in message news:fgmhm.50325$db2.6...@edtnps83... [] This is of course all made totally redundant by the GPS time delivery-- even makes up for the transmission delay. It begins to look like like Morse code in the days of cell phones. Quaint, but not really useful. Not really useful? Hardly! The LF radio signals reach where GPS cannot, and there are perhaps millions of devices and users out there who rely on LF radio to keep clocks and watches displaying the correct time. It's not redundant just yet, and even more accurate than it was a few years back. Even LORAN may be making a comeback, I believe. David ___ questions mailing list questions@lists.ntp.org https://lists.ntp.org/mailman/listinfo/questions
Re: [ntp:questions] NTP absolute accuracy?
Unruh wrote in message news:rl5hm.50089$db2.40...@edtnps83... [] As I tried to emphasise, if the round trip is not symmetric, then neither ntp not chrony can compensate for that lack of symmetry, and the absolute time will be out. If occasionally it has an assymetric round trip, then ntp will probably elimate it, and chrony may or may not, depending on how you set up chrony, but typically chrony will be more sensitive to that asymmetry. Eb, say on exactly every 5th query, an extra .3ms is added to the return trip, and lets say that the minimum round trip is .15ms plus or minus .1 ntp will always throw out those fifth cases. but will have an error around .05 ms. chrony will not, and without them would have an error of .02ms but with them would have an error of about .05ms, just as would ntp. Ie, in this case the chrony advantage would be more or less erased by those asymmetric round trips. There is no definative answer. It depends on the situation. Many thanks - that's clearer. David ___ questions mailing list questions@lists.ntp.org https://lists.ntp.org/mailman/listinfo/questions
Re: [ntp:questions] NTP absolute accuracy?
David Lord wrote: How do you get the time difference between your GPS and system time? Include the GPS in your ntp.conf, but mark it with noselect on the server line. ___ questions mailing list questions@lists.ntp.org https://lists.ntp.org/mailman/listinfo/questions
Re: [ntp:questions] NTP absolute accuracy?
David Lord sn...@lordynet.org writes: Unruh wrote: David Lord sn...@lordynet.org writes: David J Taylor wrote: Unruh unruh-s...@physics.ubc.ca wrote in message news:qg_gm.50009$db2.46...@edtnps83... David J Taylor david-tay...@blueyonder.not-this-bit.nor-this.co.uk.invalid writes: David J Taylor david-tay...@blueyonder.not-this-bit.nor-this.co.uk.invalid wrote in message news:%xqgm.126$ym4...@text.news.virginmedia.com... Unruh wrote in message news:34jgm.50898$ph1.36...@edtnps82... [] Note that chrony will give you a factor of 2 or three improvement over ntp in the errors, assuming that the roundtrip is equally split on Linux or BSD. For those without wide-bandwidth academic connections - those folks on cable or ADSL - how good is an equal split round trip assumption? David Thanks, David, David and Jan. A few milliseconds is what I had expected, so if you are on a consumer line, what implications does that have for unruh's comment? Cheers, David Which comment? The one I quoted: Note that chrony will give you a factor of 2 or three improvement over ntp in the errors, assuming that the roundtrip is equally split on Linux or BSD. On consumer-grade circuits the assumption about equal round trip is unlikely to be valid. I hope that David Lord can make some tests, and I await the results with interest. Both fileserver using ntpd and desktop using chrony had only my local ntp servers as sources so I've restarted both with three remote sources, one common source at my isp, and alternate sources from two different sites. What I'm most interested in is the effect of temperature as it's one of two periods in year that gets largest temperature variations and I've already had to restart the servers as ntpd was unable to keep up with the higher minpoll values I had set. I'm now not quite sure how best to compare stats from chrony vs ntpd. Well, the way I did it was to attach a GPS PPS to the computer and use the offset between it and the system time as a measurement of the accuracy. (the gps was NOT used as a source for eitehr ntp or chrony). How do you get the time difference between your GPS and system time? I wrote a parallel port interrupt routine ( well adapted on) which read the PPS interrupt and time stamps it. Ideally that timestamp should be exactly on the second. It is usually off by a few tens of microseconds. That is the time difference. At the moment there only seems to be 100us between offsets and apart from this being insignificant I wouldn't know which was nearer to ntp time. ?? sure you would the kernel time is ntp time. The offsets are the difference between the time ntp gets from sending out and receiving back a packet from the remote site. This is not ntp time That reading goes through the clock filter, and then is fed into the feedback loop to disciple the computer clock. That computer clock time is the closest one can come to something called ntp time. Unlike chrony, ntp does NOT use the time readings from the remote site to do a best estimate of the true time, and then drive the kernel time toward that. It assumes at each reading that the output of the clock filter IS the best estimate of the the true time, and slowly drives the clock to minimize that. Slowly means that in the end, the time ntp approaches is a sort of mean of those readings. Chrony on the other hand uses from 3 to 60 of thepast readings to find the best estimate of what the true time now is, and finds the difference between the system clock and that true time. It then corrects for that difference both in offset and rate. It is a very different philosophy. My adsl connection has also been off already for 30 minutes after I'd set this up. One of my servers that gets time from MSF and also has both test systems as sources. Both test systems also have the common source of my isp's timeserver so I'm expecting to get something meaningful from that. MSF? The problem is that I have no idea what the accuracy of any of those items is. YOur ISP's timesever may be a stratum 7 getting time from a bunch of bozos. Or it may be stratum 1 getting its time from a well implimented GPS clock. You are interested in the difference between your computer time and true UTC. the only way you will get that is by having a local source of true time ( eg a GPS with a 1usec or so accuracy). I used a GPS 18LVM, but of course even there I am not sure that the internals of the GPS receiver do not introduce microsecond sized time errors. I have never calibrated my GPS against some better true time standard. I might be able to get GPS to both systems but not for a while. One GPS receiver should be able to be split to supply time to both machines to usec accuracy. ___ questions mailing list questions@lists.ntp.org https://lists.ntp.org/mailman/listinfo/questions
Re: [ntp:questions] NTP absolute accuracy?
Unruh unruh-s...@physics.ubc.ca wrote in message news:iebhm.50125$db2.41...@edtnps83... [...] The problem is that I have no idea what the accuracy of any of those items is. YOur ISP's timesever may be a stratum 7 getting time from a bunch of bozos. Or itmay be stratum 1 getting its time from a well implimented GPS clock. ntpq -p will tell you that. And ntptrace does exactly that, climbing the chain up from you to stratum 1. Groetjes, Maarten Wiltink ___ questions mailing list questions@lists.ntp.org https://lists.ntp.org/mailman/listinfo/questions
Re: [ntp:questions] NTP absolute accuracy?
David J Taylor david-tay...@blueyonder.not-this-bit.nor-this.co.uk.invalid writes: Unruh wrote in message news:iebhm.50125$db2.41...@edtnps83... [] MSF? See: http://www.npl.co.uk/science-technology/time-frequency/time/products-and-services/msf-radio-time-signal UK 60KHz radio time signal. Ah, OK. Its accuracy is probably not much better than a few msec I assume. GPS is a few usec. Since the network gives accuracies of better than a few ms, MSF is not a good way of testing the accuracy of network set clocks. David ___ questions mailing list questions@lists.ntp.org https://lists.ntp.org/mailman/listinfo/questions
Re: [ntp:questions] NTP absolute accuracy?
Unruh wrote in message news:vcjhm.50153$db2.9...@edtnps83... [] UK 60KHz radio time signal. Ah, OK. Its accuracy is probably not much better than a few msec I assume. GPS is a few usec. Since the network gives accuracies of better than a few ms, MSF is not a good way of testing the accuracy of network set clocks. At source, it's recently been within about 10 microseconds: http://www.npl.co.uk/science-technology/time-frequency/time/products-and-services/msf-bulletins http://resource.npl.co.uk/time/bulletins/msf/msfbul_08_2009.pdf In the field, the accuracy will depend on the receiver you use, the propagation, and the quality of signal you have. A few milliseconds is probably a fair estimate, although it would be interesting to see some measured values. Cheers, David ___ questions mailing list questions@lists.ntp.org https://lists.ntp.org/mailman/listinfo/questions
Re: [ntp:questions] NTP absolute accuracy?
David J Taylor writes: UK 60KHz radio time signal. Bill Unruh writes: Ah, OK. Its accuracy is probably not much better than a few msec I assume. Should be as good as WWVB, which is good to within 100usec. The propagation delay is highly predictable at 60KHz. These stations can also can provide extremely aaccurate frequency references. -- John Hasler jhas...@newsguy.com Dancing Horse Hill Elmwood, WI USA ___ questions mailing list questions@lists.ntp.org https://lists.ntp.org/mailman/listinfo/questions
Re: [ntp:questions] NTP absolute accuracy?
I wrote: Should be as good as WWVB, which is good to within 100usec. But evidently many commercial receivers are only good to a ms or two. -- John Hasler jhas...@newsguy.com Dancing Horse Hill Elmwood, WI USA ___ questions mailing list questions@lists.ntp.org https://lists.ntp.org/mailman/listinfo/questions
Re: [ntp:questions] NTP absolute accuracy?
I wrote: Should be as good as WWVB, which is good to within 100usec. The David Woolley writes: Most receivers only use the slow code (including the simple hardware solutions for ntpd). I'm assuming a real receiver. With the single-chip designs intended for atomic watches you'll be lucky to get with a second. For high accuracy you would have to sync to the carrier, and possibly use the fast time codes. With WWVB I'd use an analog PLL to recover the carrier and a DPLL to get the PPS. -- John Hasler jhas...@newsguy.com Dancing Horse Hill Elmwood, WI USA ___ questions mailing list questions@lists.ntp.org https://lists.ntp.org/mailman/listinfo/questions
Re: [ntp:questions] NTP absolute accuracy?
David J Taylor david-tay...@blueyonder.not-this-bit.nor-this.co.uk.invalid writes: Unruh wrote in message news:vcjhm.50153$db2.9...@edtnps83... [] UK 60KHz radio time signal. Ah, OK. Its accuracy is probably not much better than a few msec I assume. GPS is a few usec. Since the network gives accuracies of better than a few ms, MSF is not a good way of testing the accuracy of network set clocks. At source, it's recently been within about 10 microseconds: Sorry, at 10usec, the distance away of the transmitter must be less than 3 km. Also your system needs to see the start of the tone to 10usec which means that the tone would have to be about 1MHz which is a bit beyond audio. http://www.npl.co.uk/science-technology/time-frequency/time/products-and-services/msf-bulletins http://resource.npl.co.uk/time/bulletins/msf/msfbul_08_2009.pdf In the field, the accuracy will depend on the receiver you use, the propagation, and the quality of signal you have. A few milliseconds is probably a fair estimate, although it would be interesting to see some measured values. Cheers, David ___ questions mailing list questions@lists.ntp.org https://lists.ntp.org/mailman/listinfo/questions
Re: [ntp:questions] NTP absolute accuracy?
Bill Unruh writes: Sorry, at 10usec, the distance away of the transmitter must be less than 3 km. 10usec at the transmitter. Also your system needs to see the start of the tone to 10usec which means that the tone would have to be about 1MHz which is a bit beyond audio. There isn't any tone. It's the UK equivalent of WWVB. http://resource.npl.co.uk/docs/networks/time/meeting3/martin.pdf -- John Hasler jhas...@newsguy.com Dancing Horse Hill Elmwood, WI USA ___ questions mailing list questions@lists.ntp.org https://lists.ntp.org/mailman/listinfo/questions
Re: [ntp:questions] NTP absolute accuracy?
John Hasler jhas...@newsguy.com writes: Bill Unruh writes: Sorry, at 10usec, the distance away of the transmitter must be less than 3 km. 10usec at the transmitter. Also your system needs to see the start of the tone to 10usec which means that the tone would have to be about 1MHz which is a bit beyond audio. There isn't any tone. It's the UK equivalent of WWVB. How is the beginning of the second ( an hour) marked? The bandwidth of whatever marks it has to be pretty narrow, or the transmitter would interfer with everything around it. Ie, that means a large time uncertainty. Ie how fast is the carrier actually switched off and on. It cannot be instantaneous or the signal would spill out badly into neighbouring bands. http://resource.npl.co.uk/docs/networks/time/meeting3/martin.pdf -- John Hasler jhas...@newsguy.com Dancing Horse Hill Elmwood, WI USA ___ questions mailing list questions@lists.ntp.org https://lists.ntp.org/mailman/listinfo/questions
Re: [ntp:questions] NTP absolute accuracy?
Unruh wrote: John Haslerjhas...@newsguy.com writes: There isn't any tone. It's the UK equivalent of WWVB. How is the beginning of the second ( an hour) marked? The bandwidth of whatever marks it has to be pretty narrow, or the transmitter would interfer with everything around it. Ie, that means a large time uncertainty. Not if they do the same spread spectrum modulation of the carrier as emplyed by the German DCF77 transmitter: By phase-locking a receiver to that signal, you can get down to the 10-30 us or so range. (You still have to be close enough to the transmitter so that the propagation path is perfectly predictable, otherwise you'll see systematic excursions as a function of the time of day/night.) Terje -- - Terje.Mathisen at tmsw.no almost all programming can be viewed as an exercise in caching ___ questions mailing list questions@lists.ntp.org https://lists.ntp.org/mailman/listinfo/questions
Re: [ntp:questions] NTP absolute accuracy?
Unruh wrote in message news:34jgm.50898$ph1.36...@edtnps82... [] Note that chrony will give you a factor of 2 or three improvement over ntp in the errors, assuming that the roundtrip is equally split on Linux or BSD. For those without wide-bandwidth academic connections - those folks on cable or ADSL - how good is an equal split round trip assumption? David ___ questions mailing list questions@lists.ntp.org https://lists.ntp.org/mailman/listinfo/questions
Re: [ntp:questions] NTP absolute accuracy?
David J Taylor david-tay...@blueyonder.not-this-bit.nor-this.co.uk.invalid writes: Unruh wrote in message news:34jgm.50898$ph1.36...@edtnps82... [] Note that chrony will give you a factor of 2 or three improvement over ntp in the errors, assuming that the roundtrip is equally split on Linux or BSD. For those without wide-bandwidth academic connections - those folks on cable or ADSL - how good is an equal split round trip assumption? I have a machine with Rugby reciever that also syncs (over DSL) to a machine with a GPS unit. Based on that, it looks like the asymetry is small (not more than a handful of ms, on a path of 32ms). David. ___ questions mailing list questions@lists.ntp.org https://lists.ntp.org/mailman/listinfo/questions
Re: [ntp:questions] NTP absolute accuracy?
David J Taylor wrote: Unruh wrote in message news:34jgm.50898$ph1.36...@edtnps82... [] Note that chrony will give you a factor of 2 or three improvement over ntp in the errors, assuming that the roundtrip is equally split on Linux or BSD. For those without wide-bandwidth academic connections - those folks on cable or ADSL - how good is an equal split round trip assumption? 2Mbit adsl here seems to give me mostly maximum variation of 5-6ms and for 50% its at 0.6ms or less. Remote connections to my servers shows offset mostly following what I see here but with occasional irregular blips of from 5ms to 200ms and these don't always seem to correspond to any local variation in load or network traffic. Heavy downloads at 230kB/s sometimes have no effect but other times I see latency as monitored by my ISP shoot up from 20ms to 100ms which I believe is down to congestion or throttling in BT network. David ___ questions mailing list questions@lists.ntp.org https://lists.ntp.org/mailman/listinfo/questions
Re: [ntp:questions] NTP absolute accuracy?
David J Taylor wrote: For those without wide-bandwidth academic connections - those folks on cable or ADSL - how good is an equal split round trip assumption? I calculated this for my case once (3840 kbit/s down and 512 kbit/s up). The difference in transmission time due only to the different bitrates was about 1.2ms in my case. This is however only the best case, because it assumes that the uplink is not saturated. If it is, and if you are able to prioritise your upstream NTP packets over all other traffic, then you need to add half of the transmission latency of the average upstream packet to the above 1.2ms. To give you some idea, assuming that the uplink is saturated due to a file upload (i.e. MTU-sized packets), this brings the difference in latency to 14.4ms (including the above 1.2ms). If you cannot control upstream priority queuing, then the upstream latency depends on the total size of your upstream queue, which may add several MTU-sized packet transmission times to the difference (in my case, 26.5ms times three plus 1.2ms). This is then really the worst case, because it assumes that the downlink is not saturated so that there is no queueing on the ISP's side. Cheers, Jan ___ questions mailing list questions@lists.ntp.org https://lists.ntp.org/mailman/listinfo/questions
Re: [ntp:questions] NTP absolute accuracy?
David J Taylor david-tay...@blueyonder.not-this-bit.nor-this.co.uk.invalid wrote in message news:%xqgm.126$ym4...@text.news.virginmedia.com... Unruh wrote in message news:34jgm.50898$ph1.36...@edtnps82... [] Note that chrony will give you a factor of 2 or three improvement over ntp in the errors, assuming that the roundtrip is equally split on Linux or BSD. For those without wide-bandwidth academic connections - those folks on cable or ADSL - how good is an equal split round trip assumption? David Thanks, David, David and Jan. A few milliseconds is what I had expected, so if you are on a consumer line, what implications does that have for unruh's comment? Cheers, David ___ questions mailing list questions@lists.ntp.org https://lists.ntp.org/mailman/listinfo/questions
Re: [ntp:questions] NTP absolute accuracy?
David J Taylor wrote: David J Taylor david-tay...@blueyonder.not-this-bit.nor-this.co.uk.invalid wrote in message news:%xqgm.126$ym4...@text.news.virginmedia.com... Unruh wrote in message news:34jgm.50898$ph1.36...@edtnps82... [] Note that chrony will give you a factor of 2 or three improvement over ntp in the errors, assuming that the roundtrip is equally split on Linux or BSD. For those without wide-bandwidth academic connections - those folks on cable or ADSL - how good is an equal split round trip assumption? David Thanks, David, David and Jan. A few milliseconds is what I had expected, so if you are on a consumer line, what implications does that have for unruh's comment? I'd go for equal split being as good an approximation as any. Mostly uplinks aren't loaded too much here but I've had altq running for a few years, which increased my upload from maybe 13kB/s to near 26kB/s and at same time massive latency on saturated upload link was mostly eliminated. NTP, DNS etc are also prioritised so possibly my experience isn't typical. I might leave one of desktops running chrony for an extended period with same sources as used by one of my local servers and see how they compare. David ___ questions mailing list questions@lists.ntp.org https://lists.ntp.org/mailman/listinfo/questions
Re: [ntp:questions] NTP absolute accuracy?
Jan Ceuleers wrote: This is however only the best case, because it assumes that the uplink is not saturated. If it is, and if you are able to prioritise your I suspect most severe asymmetry cases actually happen because the downlink is saturated. That's is exacerbated by the fact that it is more difficult to get the ISP to prioritise, than to do it yourself. ADSL has a higher download speed because most users receive much more than they transmit. ___ questions mailing list questions@lists.ntp.org https://lists.ntp.org/mailman/listinfo/questions
Re: [ntp:questions] NTP absolute accuracy?
David J Taylor writes: On consumer-grade circuits the assumption about equal round trip is unlikely to be valid. Why would you expect the asymmetry to be worse there than elsewhere? -- John Hasler jhas...@newsguy.com Dancing Horse Hill Elmwood, WI USA ___ questions mailing list questions@lists.ntp.org https://lists.ntp.org/mailman/listinfo/questions
Re: [ntp:questions] NTP absolute accuracy?
Unruh unruh-s...@physics.ubc.ca wrote in message news:qg_gm.50009$db2.46...@edtnps83... David J Taylor david-tay...@blueyonder.not-this-bit.nor-this.co.uk.invalid writes: David J Taylor david-tay...@blueyonder.not-this-bit.nor-this.co.uk.invalid wrote in message news:%xqgm.126$ym4...@text.news.virginmedia.com... Unruh wrote in message news:34jgm.50898$ph1.36...@edtnps82... [] Note that chrony will give you a factor of 2 or three improvement over ntp in the errors, assuming that the roundtrip is equally split on Linux or BSD. For those without wide-bandwidth academic connections - those folks on cable or ADSL - how good is an equal split round trip assumption? David Thanks, David, David and Jan. A few milliseconds is what I had expected, so if you are on a consumer line, what implications does that have for unruh's comment? Cheers, David Which comment? The one I quoted: Note that chrony will give you a factor of 2 or three improvement over ntp in the errors, assuming that the roundtrip is equally split on Linux or BSD. On consumer-grade circuits the assumption about equal round trip is unlikely to be valid. I hope that David Lord can make some tests, and I await the results with interest. Cheers, David ___ questions mailing list questions@lists.ntp.org https://lists.ntp.org/mailman/listinfo/questions
Re: [ntp:questions] NTP absolute accuracy?
David J Taylor david-tay...@blueyonder.not-this-bit.nor-this.co.uk.invalid writes: Unruh unruh-s...@physics.ubc.ca wrote in message news:qg_gm.50009$db2.46...@edtnps83... David J Taylor david-tay...@blueyonder.not-this-bit.nor-this.co.uk.invalid writes: David J Taylor david-tay...@blueyonder.not-this-bit.nor-this.co.uk.invalid wrote in message news:%xqgm.126$ym4...@text.news.virginmedia.com... Unruh wrote in message news:34jgm.50898$ph1.36...@edtnps82... [] Note that chrony will give you a factor of 2 or three improvement over ntp in the errors, assuming that the roundtrip is equally split on Linux or BSD. For those without wide-bandwidth academic connections - those folks on cable or ADSL - how good is an equal split round trip assumption? David Thanks, David, David and Jan. A few milliseconds is what I had expected, so if you are on a consumer line, what implications does that have for unruh's comment? Cheers, David Which comment? The one I quoted: Note that chrony will give you a factor of 2 or three improvement over ntp in the errors, assuming that the roundtrip is equally split on Linux or BSD. On consumer-grade circuits the assumption about equal round trip is unlikely to be valid. As I said, chrony does a linear regression on the last n offsets. If those n offsets are all displaced by 1 sec because the outbound packets all suffer a 1 sec delay while the inbound ones do not, then the absolute time will be out by 1 sec. This is true for chrony and for ntp, and there is nothing that can be done about it. If the packets occasionally suffer a 1 sec delay but usually are the same as an inbound, then ntp will probably eliminate those via the clockfilter. Chrony can also be setup to eliminate packets whose roundtrip times are x times the lowest value found for the roundtrip, where x is settable in /etc/chrony.conf. which will also eliminate those 1 second guys. (ntp's algorithm is stupid because it eliminates 80 % of the packets, even if the excess delays are say 10% of the total delay, and keeps them even if the delay is 100 times the minimum if all of the last 8 have been in that boat. chrony's algorithm uses the minimum value of the delay over long time scales, so that if random symmetric delay noise increases many valid packets can also be thrown out. ) Asymmetric delays are something that all time systems have trouble with, and at some level there is nothing that can be done about it. Given that problem, one then needs to look at chrony and ntp's use of the data that is actually collected and ask which makes the best use of that data to estimate the rate and offset of the local clock. I think that it is clear that chrony does a far better job. Whether there are situations in which it does a worse job, I do not know, but have not been able to think of any, but that may say more about me than about chrony or ntp. ntp has had the advantage of a long history of testing in a very wide variety of situations. chrony has been out there for about 10 years, but has not been as extensively tested in adverse situations. It used to be that chrony also had the disadvantage that it did not handle leap seconds. (Ie once very few years, it would suddenly find itself out by a second and have to slew the clock for the next hour or less to correct that-- if not warned by the leapsecond flag, ntp would either step the clock (with the concomitant problems for filesystem timing), quit entirely, or take many many hours to correct the problem by slewing. ie the uncorrected leapsecond was far less of a problem for chrony than for ntp) It also did not handle any refclocks. Both of these problems have now been corrected by M Lichvar in the git repository. (chrony.tuxfamily.org) The other problem is that chrony runs only on Linux or BSD. It does NOT run on Windows. That would require a pretty extensive rewriting. Curnow got started on this ( and kept chrony very modular to try to make this easy) but it is still a big task. I hope that David Lord can make some tests, and I await the results with interest. Cheers, David ___ questions mailing list questions@lists.ntp.org https://lists.ntp.org/mailman/listinfo/questions
Re: [ntp:questions] NTP absolute accuracy?
David J Taylor writes: On consumer-grade circuits the assumption about equal round trip is unlikely to be valid. Why would you expect the asymmetry to be worse there than elsewhere? -- John Hasler It's just an impression I had - that consumers get the worst latency and most overloaded network equipment, and no direct connection to an ultra-high-speed academic network. But I would love to learn the actual figures. David ___ questions mailing list questions@lists.ntp.org https://lists.ntp.org/mailman/listinfo/questions
Re: [ntp:questions] NTP absolute accuracy?
Unruh wrote in message news:kx%gm.50022$db2.12...@edtnps83... [] As I said, chrony does a linear regression on the last n offsets. [] Bill, I think we have seen these remarks about chrony before. What I was wanting was a yes/no answer (although numbers would be nice) to the question: - is the 2-3 times improvement in the errors affected by the assumption of equal round trip? Perhaps there is no definitive answer? Cheers, David ___ questions mailing list questions@lists.ntp.org https://lists.ntp.org/mailman/listinfo/questions
Re: [ntp:questions] NTP absolute accuracy?
David J Taylor writes: It's just an impression I had - that consumers get the worst latency and most overloaded network equipment... Perhaps, but it doesn't follow that they suffer a great deal of asymmetry. ...and no direct connection to an ultra-high-speed academic network. Of course not, and probably no bleeding-edge switches with packet buffering, NICs with interrupt coalescing, or experimental routers, either. As far as I know round trip symmetry is not a design goal for any part of the Net. On the other hand, deep packet inspection _could_ introduce asymmetry. Seems to me that the dumber the network between me and the server the more likely I am to see symmetric latentcy. -- John Hasler jhas...@newsguy.com Dancing Horse Hill Elmwood, WI USA ___ questions mailing list questions@lists.ntp.org https://lists.ntp.org/mailman/listinfo/questions
Re: [ntp:questions] NTP absolute accuracy?
David J Taylor wrote: Unruh unruh-s...@physics.ubc.ca wrote in message news:qg_gm.50009$db2.46...@edtnps83... David J Taylor david-tay...@blueyonder.not-this-bit.nor-this.co.uk.invalid writes: David J Taylor david-tay...@blueyonder.not-this-bit.nor-this.co.uk.invalid wrote in message news:%xqgm.126$ym4...@text.news.virginmedia.com... Unruh wrote in message news:34jgm.50898$ph1.36...@edtnps82... [] Note that chrony will give you a factor of 2 or three improvement over ntp in the errors, assuming that the roundtrip is equally split on Linux or BSD. For those without wide-bandwidth academic connections - those folks on cable or ADSL - how good is an equal split round trip assumption? David Thanks, David, David and Jan. A few milliseconds is what I had expected, so if you are on a consumer line, what implications does that have for unruh's comment? Cheers, David Which comment? The one I quoted: Note that chrony will give you a factor of 2 or three improvement over ntp in the errors, assuming that the roundtrip is equally split on Linux or BSD. On consumer-grade circuits the assumption about equal round trip is unlikely to be valid. I hope that David Lord can make some tests, and I await the results with interest. Both fileserver using ntpd and desktop using chrony had only my local ntp servers as sources so I've restarted both with three remote sources, one common source at my isp, and alternate sources from two different sites. What I'm most interested in is the effect of temperature as it's one of two periods in year that gets largest temperature variations and I've already had to restart the servers as ntpd was unable to keep up with the higher minpoll values I had set. I'm now not quite sure how best to compare stats from chrony vs ntpd. David ___ questions mailing list questions@lists.ntp.org https://lists.ntp.org/mailman/listinfo/questions
Re: [ntp:questions] NTP absolute accuracy?
David Lord sn...@lordynet.org writes: David J Taylor wrote: Unruh unruh-s...@physics.ubc.ca wrote in message news:qg_gm.50009$db2.46...@edtnps83... David J Taylor david-tay...@blueyonder.not-this-bit.nor-this.co.uk.invalid writes: David J Taylor david-tay...@blueyonder.not-this-bit.nor-this.co.uk.invalid wrote in message news:%xqgm.126$ym4...@text.news.virginmedia.com... Unruh wrote in message news:34jgm.50898$ph1.36...@edtnps82... [] Note that chrony will give you a factor of 2 or three improvement over ntp in the errors, assuming that the roundtrip is equally split on Linux or BSD. For those without wide-bandwidth academic connections - those folks on cable or ADSL - how good is an equal split round trip assumption? David Thanks, David, David and Jan. A few milliseconds is what I had expected, so if you are on a consumer line, what implications does that have for unruh's comment? Cheers, David Which comment? The one I quoted: Note that chrony will give you a factor of 2 or three improvement over ntp in the errors, assuming that the roundtrip is equally split on Linux or BSD. On consumer-grade circuits the assumption about equal round trip is unlikely to be valid. I hope that David Lord can make some tests, and I await the results with interest. Both fileserver using ntpd and desktop using chrony had only my local ntp servers as sources so I've restarted both with three remote sources, one common source at my isp, and alternate sources from two different sites. What I'm most interested in is the effect of temperature as it's one of two periods in year that gets largest temperature variations and I've already had to restart the servers as ntpd was unable to keep up with the higher minpoll values I had set. I'm now not quite sure how best to compare stats from chrony vs ntpd. Well, the way I did it was to attach a GPS PPS to the computer and use the offset between it and the system time as a measurement of the accuracy. (the gps was NOT used as a source for eitehr ntp or chrony). David ___ questions mailing list questions@lists.ntp.org https://lists.ntp.org/mailman/listinfo/questions
Re: [ntp:questions] NTP absolute accuracy?
David J Taylor david-tay...@blueyonder.not-this-bit.nor-this.co.uk.invalid writes: Unruh wrote in message news:kx%gm.50022$db2.12...@edtnps83... [] As I said, chrony does a linear regression on the last n offsets. [] Bill, I think we have seen these remarks about chrony before. Some have, some have not. I was just putting out there what the difference between ntp and chrony was, so people could make up their own minds. What I was wanting was a yes/no answer (although numbers would be nice) to the question: - is the 2-3 times improvement in the errors affected by the assumption of equal round trip? Perhaps there is no definitive answer? As I tried to emphasise, if the round trip is not symmetric, then neither ntp not chrony can compensate for that lack of symmetry, and the absolute time will be out. If occasionally it has an assymetric round trip, then ntp will probably elimate it, and chrony may or may not, depending on how you set up chrony, but typically chrony will be more sensitive to that asymmetry. Eb, say on exactly every 5th query, an extra .3ms is added to the return trip, and lets say that the minimum round trip is .15ms plus or minus .1 ntp will always throw out those fifth cases. but will have an error around .05 ms. chrony will not, and without them would have an error of .02ms but with them would have an error of about .05ms, just as would ntp. Ie, in this case the chrony advantage would be more or less erased by those asymmetric round trips. There is no definative answer. It depends on the situation. Cheers, David ___ questions mailing list questions@lists.ntp.org https://lists.ntp.org/mailman/listinfo/questions
Re: [ntp:questions] NTP absolute accuracy?
Unruh wrote: David Lord sn...@lordynet.org writes: David J Taylor wrote: Unruh unruh-s...@physics.ubc.ca wrote in message news:qg_gm.50009$db2.46...@edtnps83... David J Taylor david-tay...@blueyonder.not-this-bit.nor-this.co.uk.invalid writes: David J Taylor david-tay...@blueyonder.not-this-bit.nor-this.co.uk.invalid wrote in message news:%xqgm.126$ym4...@text.news.virginmedia.com... Unruh wrote in message news:34jgm.50898$ph1.36...@edtnps82... [] Note that chrony will give you a factor of 2 or three improvement over ntp in the errors, assuming that the roundtrip is equally split on Linux or BSD. For those without wide-bandwidth academic connections - those folks on cable or ADSL - how good is an equal split round trip assumption? David Thanks, David, David and Jan. A few milliseconds is what I had expected, so if you are on a consumer line, what implications does that have for unruh's comment? Cheers, David Which comment? The one I quoted: Note that chrony will give you a factor of 2 or three improvement over ntp in the errors, assuming that the roundtrip is equally split on Linux or BSD. On consumer-grade circuits the assumption about equal round trip is unlikely to be valid. I hope that David Lord can make some tests, and I await the results with interest. Both fileserver using ntpd and desktop using chrony had only my local ntp servers as sources so I've restarted both with three remote sources, one common source at my isp, and alternate sources from two different sites. What I'm most interested in is the effect of temperature as it's one of two periods in year that gets largest temperature variations and I've already had to restart the servers as ntpd was unable to keep up with the higher minpoll values I had set. I'm now not quite sure how best to compare stats from chrony vs ntpd. Well, the way I did it was to attach a GPS PPS to the computer and use the offset between it and the system time as a measurement of the accuracy. (the gps was NOT used as a source for eitehr ntp or chrony). How do you get the time difference between your GPS and system time? At the moment there only seems to be 100us between offsets and apart from this being insignificant I wouldn't know which was nearer to ntp time. My adsl connection has also been off already for 30 minutes after I'd set this up. One of my servers that gets time from MSF and also has both test systems as sources. Both test systems also have the common source of my isp's timeserver so I'm expecting to get something meaningful from that. I might be able to get GPS to both systems but not for a while. David ___ questions mailing list questions@lists.ntp.org https://lists.ntp.org/mailman/listinfo/questions
Re: [ntp:questions] NTP absolute accuracy?
Richard B. Gilbert rgilber...@comcast.net writes: Juyong Do wrote: Hi, Does anyone know any survey or paper measuring absolute time accuracy over NTP? I saw statistics about offsets, RTT, and dispersion but am not sure how well those parameters reflect or are correlated with absolute accuracy of each NTP servers and clients. To my mind, the only way to measure the absolute accuracy (difference from UTC or GPS time) would be to physically bring a GPS receiver to each NTP clients and compare the timing bases. It is certainly not feasible for a large scale survey but I guess there should have been someone who've done this before. Let me know if there is. Thanks, Juyong It depends. IF the round trip time is pretty equally split between outgoing and incoming then the ntp estimate of it own accuracy ( eg the standard deviation of the offsets) should be a pretty good estimate of the absolute accuracy. Otherwise the mean of (sup abs( offset+- roundtrip/2)) should give a worst case estimate. Yes, I have done what you suggest but my measurements may not reflect the situation your computers are in. So there is not answer to what you ask. It depends on your situation ( network traffic, etc). Note that chrony will give you a factor of 2 or three improvement over ntp in the errors, assuming that the roundtrip is equally split on Linux or BSD. This is I believe primarily because of ntp's slow behaviour with respect to rate changes caused by temp fluctuations. If you can glue a thermometer onto your timing crystal and use that to help estimate the rate changes, you will gain at least a factor of 3 over ntp's raw accuracy as well. ___ questions mailing list questions@lists.ntp.org https://lists.ntp.org/mailman/listinfo/questions
