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Hi,
can someone please detail the carrier sense mechanism in 802.11 for me?

In particular:-
1) Does a device(AP or client)actually try and detect the presence of a valid 802.11 signal, or does it just try and detect some RF energy in it's channel (this may be present due to some noise or other device transmitting at the frequency, but not necessarily an 802.11 device)

2)What would the minimum detected signal strength be for a device to decide that the channel is free? Is there also a need to have a minimum SNR as well as just a minimum signal strength before deciding that the channel is free.

Answers to the above will help me to work out one of my concerns around understanding a scenario where if I have two APs right next to each other, say AP A. on channel 1 and AP B. on channel 11. I have seen a graph in a book showing that at a few inches away from the radio on channel 1, when it is transmitting it's signal could be spilling into channel 11 at around -70dbm. This would mean that the radio on channel 11 would not be able to hear any clients which were far away enough to have a signal around this level. Also, I assume that the radio on channel 11 would not be able to transmit while the radio on channel 1 was transmitting, as it would deem that channel 11 is busy???

The book goes on to recommend at least 5ft separation between radios. which is fair enough.




Reagrds,
Dianuj