CSMA/CD is an acronym for Carrier Sense Multiple Access/Collision Detection, and it is in fact a means of avoiding collisions on a LAN, but its operation is a bit more complicated.
The first point of complication is "Carrier Sense". Strictly speaking, this isn't as simple as "listening before talking", but is rather a detection of another station's intelligence already occupying the communications channel. For a wired network such as Ethernet, the distinction is not terribly important, but in a heterogeneous wireless environment, it could be. Stations using different modulation schemes are unable to demodulate each others' carriers, and so each could assume that the channel is clear, when in fact they are interfering with each other. Even stations with the same modulation at the fringe of each others' reception range may interfere fatally with each others' communication with other stations.
Most wireless systems use some form of a channel assessment algorithm based on a number of factors, usually including carrier detection and received RF level. A station typically will not transmit if its receiver detects a high RF level, whether or not it can demodulate the signal.
The second part of the complication is Collision Detection. On an Ethernet, each station listens to the channel as it transmits, and if it detects another stations signal at the same time as its own, both stations signal a collision and stop transmitting, allowing the medium to recover more quickly from a collision. RF systems aren't able to listen to the transmit channel at the same time as they are transmitting, and are therefore unable to detect collisions with other stations.
Section 9 of 802.11 specifies extensive collision avoidance mechanisms also designed to avoid the "hidden station" and "exposed station" problems associated with simple "listen before talk" protocols.
As a last aside, 802.11a is a proposal to extend 802.11 for higher-speed operation in the 5GHz ISM band by adding OFDM operation to the PHY, and 802.11b is a similar proposal for the 2.4GHz band. |
