Hal

We use this approach to automatically manage measurements.

There are a few more issues - the relative drift between the two clocks can be
as high as 200ppm, though typically 50-75ppm is what we observe, but this drift
is monotonic.

Also NTP can make changes at one (or both) ends - they show up as distinct
direction changes in the drift.

This gives a limit (or a measurement error function you have to work within).

Neil

> On 21 Nov 2017, at 19:08, Hal Murray <hmurray@megapathdsl.net> wrote:
> 
>> Right, no idea how Windows drivers behave. But odds are that the bottleneck
>> is at the client, since that often has worse antennas than the AP. If you're
>> in a position to take packet captures at both clients and AP you may be able
>> to figure it out; may require tightly synchronised clocks to do properly,
>> though.
> 
> It should be reasonable to synchronize the clocks at both ends well enough.
> 
> If that doesn't work and/or is inconvient, you could post process one trace
> to adjust the time stamps.  The idea is to scan both traces in parallel to
> find the minimum transit times in each direction, then adjust the time stamps
> on one end to allocate half the total time to each direction.  It would
> obviously be handy to have a few pings during a period of low traffic for
> calibration.
> 
> -------
> 
> There are two dimensions to clocks.  One is the current time.  The other is
> the frequency.  If the frequency is off, the clock will drift.  (ntpd's drift
> correction is usually stored in someplace like /var/lib/ntp/ntp.drift)
> 
> Unless you are interested in long runs, the clock will not drift far enough
> to be a serious problem, so all you have to do is get the time right before
> starting a run.
> 
> You might want to kill ntpd on the wifi end so it doesn't get confused and yank the clock around.
> 
> 
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