Date: Fri, 17 Mar 2006 12:51:27 -0800 From: "Kan Cai" <kan.cai@gmail.com> To: freebsd-net@freebsd.org, freebsd-mobile@freebsd.org Subject: Wireless sensing function is missing? Message-ID: <5365ea660603171251w330dcfbds316e73c83038aa3c@mail.gmail.com>
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Greetings all:
I have been doing some 802.11 wireless experiments, and the reults seem t=
o
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, i=
t
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 b=
e
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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