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.
Question: would you expect the reported jitter to increase over the
first 30 minutes or so?
- 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.
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
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