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
