On 2011-12-25, Charles Elliott <elliott...@verizon.net> wrote: >> You might look at the peerstats file and also look at the "roundtrip" >> time. I might be that occasionally one of the paths from wireless to >> computer gets shorter ( clearer signal?) and ntpd will then take that >> as >> a good value, and an earlier value, and try to correct for that offset- >> - >> which it does by stepping the frequency. > > > This comment raises an interesting issue. There is a large, significant, > and negative > correlation between "Delay" and "Offset." The larger the delay, the more > toward > minus infinity the offset tends. Recall that in the regression equation
That says that the noise is occuring in one of the paths, rather than the other. > Y = BX + A, B is the correlation between the variables X and Y. So if the > correlation is significant, this implies that there is a relation between > them. > I can't think of a physical relation between delay and offset, so if NTP > finds > a relation, there has to be something wrong. ntp assumes that the outgoing and incoming trips are the same time. If not, you get an offset. Thus if the return trip takes 10 min and the outgoing 1ms, ntp is going to say that the remote clock is five minutes further behind than it is. On a wireless I could easily imagine that the trip times are not the same, and that one way of the other is preferentially got a higher jitter than the other. > > > Charles Elliott > >> -----Original Message----- >> From: questions-bounces+elliott.ch=verizon....@lists.ntp.org >> [mailto:questions-bounces+elliott.ch=verizon....@lists.ntp.org] On >> Behalf Of unruh >> Sent: Saturday, December 24, 2011 1:18 PM >> To: questions@lists.ntp.org >> Subject: Re: [ntp:questions] Windows and Wi-Fi - starts well, frequency >> steps? >> >> On 2011-12-24, David J Taylor <david-tay...@blueyonder.co.uk.invalid> >> wrote: >> > Folks, >> > >> > I've recently been testing NTP 4.2.7p241 on a variety of Windows >> systems >> > with almost uniformly excellent results. For me, it's the best >> version of >> > NTP to date - thanks Dave Hart! However, it has now revealed a >> couple of >> > issues which may be fundamental to NTP, or may be artefacts of the >> Windows >> > implementation: >> > >> > - one Netbook PC worked very well on a LAN connection (about 1 ms >> steady >> > jitter). However, when moving to a Wi-Fi connection after a power- >> down >> > reboot, the reported jitter gradually built up over about a 30 minute >> > period, ending up with a 5 ms peak, later decaying to a value between >> 1.3 >> > and 2.5 ms. The offset also appeared to have spikes which because >> much >> > worse after about 30 minutes. >> >> I would expect wifi to be much worse than a lan for jitter. The signal >> has to be converted, broadcast, reconverted and then sent on down the >> lan. That all takes time, and can have aproblem with dropped bits, >> retransmission, etc. >> >> > >> > Question: would you expect the reported jitter to increase over the >> > first 30 minutes or so? >> >> Could be somone switched on a vacuum cleaner for example. >> >> > >> > - this same PC shows a frequency value which was steady around +1.7 >> ppm on >> > the LAN connection. With the Wi-Fi connection, approximately every >> 90 >> > minutes, the frequency offset takes a sudden negative step of about >> 0.4 >> > ppm, which prevents NTP reaching the +1.7 ppm value it may be aiming >> for. >> > There is nothing from NTP in the Event Log at the time of these >> jumps. >> > These jumps in frequency do correspond to spikes in the offset >> values. >> >> That is now ntp works. All it knows is the current offset, and tries to >> get rid of it by changing the frequency. >> It does not know that there is a sudden step. I does not remember the >> old offset values. >> >> You might look at the peerstats file and also look at the "roundtrip" >> time. I might be that occasionally one of the paths from wireless to >> computer gets shorter ( clearer signal?) and ntpd will then take that >> as >> a good value, and an earlier value, and try to correct for that offset- >> - >> which it does by stepping the frequency. >> >> >> >> > >> > Question: why would the frequency show such a sudden step? >> Shouldn't >> > there be some smoothing somewhere? >> > >> > - another PC working off the same Wi-Fi connection also shows steps >> in the >> > frequency, but both negative and positive steps, and not at quite the >> same >> > intervals. Comparing today's graphs, the steps are not occurring at >> the >> > same time in the two PCs. One PC is showing negative spikes in the >> > offset, the other both positive and negative. >> > >> > Question: why would Wi-Fi give rise to these offset spikes, and why >> is >> > NTP so sensitive to them? I suppose the answer to how the spikes >> arise >> > could be simply "that's how Wi-Fi is, with transmission uncertainties >> and >> > the possibility of interference. I had expected a greater variation >> to >> > the offset with Wi-Fi, but not the spikes. Perhaps NTP is sensitive >> > because I have minpoll 5 and maxpoll 5, perhaps widening the loop >> > bandwidth? >> > >> > Cheers, >> > David >> > >> >> _______________________________________________ >> 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
