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CSMA/CD is used in wired LANs, CSMA means that the computers on the network sense the medium if the medium is idle, the computer transmits otherwise it defers sending.CD refers to collision detection. I don’t want to write about CD because its not related to my Question.
Now in case of wireless LANs we use CSMA/CA , here CSMA refers to carrier sensing , the Question is how carrier sensing is done in case of wireless LANs? the collision avoidance is done by sending the control message to the intended receipient.

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If your wired network is still using CSMA/CD it is time to upgrade your hardware. In a full-duplex switched network collisions should be extremely rare. – Zoredache May 29 '10 at 0:52
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3 Answers

up vote 9 down vote

Basically:

It listens.

Less Basically:

The purpose of [Clear Channel Assessment (CCA)] is to detect the presence of ongoing transmissions reliably so as to enable the sensing node to decide whether to proceed with channel access. A generic link layer CCA module uses a suitable time window of the received RF signal and produces a CCA BUSY/IDLE flag as a result of suitable algorithmic processing.

Not Even Remotely Basically:

Go read Clear Channel Assessment in Energy-constrained Wideband Wireless Networks (The above quote is from this). You are better man than I if you understand all of it :-) But for example:

IEEE 802.15.4 [6] uses one of 16 nearly orthogonal 32-chip long PN sequences to represent one of 16 symbols. All packets contain a preamble consisting of 8 repetitions of the PN code corresponding to the zero symbol.

You might also want to go read 802.15.4TM-2006 , but it doesn't get into the specifics of this aspect.

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This answer deserves more than just +1, that's a great article you linked to Kyle. – Ward May 28 '10 at 18:31
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up vote 4 down vote

I think it's done by just listening to the medium and if it senses a transmission then it backs off a random time.

More info here

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That's the basic way CS works, but with wireless it's not so easy to "listen to the medium." – Ward May 28 '10 at 19:44
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up vote -1 down vote

I'm gonna guess that it's done at the physical and data-link layers - if carrier weren't there, then a node wouldn't be associated with a base station.

Why are you asking? That may get you better/more detailed answers.

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