Wireless sensing function is missing?

[Kan Cai]

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