[ntp:questions] WiFi & NTP.

Guy gdelavil at ens.insa-rennes.fr
Fri Aug 24 09:21:47 UTC 2007


Dear all,

I am a student from a French engineering school. I am currently
working on a synchronization over WiFi project that could apply to
embedded devices.

For the moment, I have only reached a precision of 5 ms.

(Whereas I get a precision of a few hundreds of microseconds with an
Ethernet connection.)

Nowadays the WiFi tends to be more and more integrated "transparently"
into the other networks. And it might be interesting to consider and
optimize the synchronization over a WiFi connection using NTP!

So I was wondering if some of you had already tried to optimize a
synchronization process over a wireless connection?

If so, your feedback and suggestions are welcomed ;->


Anyway, here is what I have done so far:

. When it comes to channel congestion, I have already run a series of
tests. I launched NTP in unicast (client/server) mode and at the same
time I streamed data between the two devices. I got an average offset
of 65 ms (computed as the average of the offset absolute values). So
that's where some QoS came in handy to prioritize the NTP packets and
give them the same priority as Network Control packets. Doing so, and
still streaming data, I got an average offset of 15 ms. Without
traffic on the connection and with the QoS rules, that's when I reach
a precision of 5 ms.

. I have also tried to select manually the WiFi channel so to avoid
using channel 1 like everyone but it didn't seem to have much effect.

. Concerning the RTS/CTS messages, the threshold was set by default to
2347. The NTP packets I send and receive only are 76 bytes long. I
changed the threshold value to 1 (just to make sure the value is
smaller than the actual NTP packet size and because 0 might disable
something) to see if there are any effects. And it seems to improve
slightly the synchronization.

. A comment I read concerning the RTS/CTS delay drew my attention to
some parameters of the WiFi driver such as the size of the congestion
window, the srl, and lrl, I should use complementarily to my QoS
rules. But I don't know exactly how?

. By the way, I wanted to point out that in my situation the RMS
jitter hardly goes below one millisecond. But as I don't really know
what this value means (I have nothing to compare it to) I don't know
whether it is important or not.


Guy




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