What is the difference between CSMA/CD and CSMA/CA?

CSMA/CA (Carrier Sense Multiple Access/Collision Avoidance) is the channel access mechanism used by most wireless LANs in the ISM bands. A channel access mechanism is the part of the protocol which specifies how the node uses the medium : when to listen, when to transmit...

The basic principles of CSMA/CA are listen before talk and contention. This is an asynchronous message passing mechanism (connectionless), delivering a best effort service, but no bandwidth and latency guarantee (you are still following ?). It's main advantages are that it is suited for network protocols such as TCP/IP, adapts quite well with the variable condition of traffic and is quite robust against interferences.

CSMA/CA is fundamentally different from the channel access mechanism used by cellular phone systems.

CSMA/CA is derived from CSMA/CD (Collision Detection), which is the base of Ethernet. The main difference is the collision avoidance : on a wire, the transceiver has the ability to listen while transmitting and so to detect collisions (with a wire all transmissions have approximately the same strength). But, even if a radio node could listen on the channel while transmitting, the strength of its own transmissions would mask all other signals on the air. So, the protocol can't directly detect collisions like with Ethernet and only tries to avoid them.

Under CSMA/CD, when a station has data to send, it first listens to determine whether any other station on the network is occupying the medium. If the channel is busy, the station will wait until it becomes idle before transmitting data. Since it is possible for two stations to listen at the same time and discover an idle channel, it is also possible that two stations could then transmit at the same time. When this occurs a collision will take place, and then a jamming signal is sent throughout the network in order to notify all stations of the collision. The stations will then wait for a random period of time before re-transmitting their respective frames.