> 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 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. 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
