802.11 MAC -- Why wireless network use CSMA/CA instead of CSMA/CD

CSMA/CD could, in theory, be used in the wireless network, however there are serious limitations which make it a poor choice.

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1. Half Duplex

Almost all 802.11 devices run at half duplex. Half duplex means radio can transmit and receive but it only do one of these things at the same time. So a radio can't do collision detection because while it is transmitting, it is not listening to the channel to see if there has been a collision. Even you get round this issue, e.g. using multiple radios, there are still other issues.

2. Hidden Node Problem

This is a serious problem in the wireless network. Station A can communicate with Station B. Station C can also communicate with Station B. However, Station A and C can't communicate with each other since they can't sense each other on the network, because they are out of range of each other.So if use CAMA/CD, Station A never sees the signal from competing node and be able to detect there is a collision.

Wifi_hidden_station_problem.svg.png

There are also other problems, such as wireless transceivers can't send and receive on the same channel at the same time. This is due to the fact that there is and in credible different between send power(generally around 100mw) and the receive sensitivity(commonly around 0.01 to 0.001 mw). The sending power cover up any possible chance of receiving a foreign signal, no chance of "Collision Dection". For this reason Collision Avoidance with Control Message is necessary.

On most wired networks (like Ethernet) the voltage is around 1 to 2.5v, both sending and receiving are roughly the same voltage. So if you are sending a 2.5v signal, and someone else collides with a -2.5v signal, the "Detection" parts will see a signal somewhere around 0v and know a collision occurred.