Date: Fri, 17 Mar 2006 14:00:28 -0800 (PST) From: Kan Cai <kcai@cs.ubc.ca> To: freebsd-mobile@freebsd.org, freebsd-net@freebsd.org Subject: Wireless sensing function is missing? Message-ID: <Pine.LNX.4.60.0603171355470.9710@lore.cs.ubc.ca>
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Greetings all:
Sorry if this is reposting, didn't see my first email got through.
I have been doing some 802.11 wireless experiments, and the reults seem
to suggest that wireless sensing function is missing. In theory, a
wireless device should back off when it senses some signals in the
airspace, even if these signals are too weak to capture, just to avoid
potential packet drops due to collisions.
The experiment I did is to measure the UDP sending and recving rates by
putting two wireless nodes on the verge of transmission range, and then
putting rectangle garbage bins stuffed with books in front of one wireless
node to absorb the signal. Here are the results:
# of garbage bins Sending Rate Recving Rate
1 4.77Mbps 3.32Mbps
2 5.83Mbps 2.26Mbps
3 7.73Mbps 0.48Mbps
4 8.18Mbps 0.00Mbps
The other wireless node is sending UDP packet at maximum rate 8.2Mbps.
Even though the impact of interference weakens as I put more garbage
cans, the sum of the sending rate and receving rate as shown is about
8.1-8.2Mbps, which suggests that it is not affect by those sensing
signals. Otherwise, it would back off its transmissions until the network
is idle. It can still receive some packets sent by another node, which
indicates that it should be able to sense most of the signals although it
might not be able to receive them.
I wonder if this is true that sensing function is missing? If yes, is it
supposed to be implemented in the driver or net80211 layer? Thanks in
advance!
cheers,
--ken
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