The basic idea behind CSMA/CD is that a station needs to be able to receive while transmitting to detect collision. When there is a collision the station receives two signal: its own signal and the signal transmitted by a second station. To distinguish between the situation when there is a collision and when there is no collision, the received signals must be significantly different. The signal must add a significant amount of energy to the one created by the first one.in a wired network, the received signal has almost same energy as the sent signal because of the amplifiers which amplifies the energy between the sender and the receiver. but in a wireless network, much of the sent energy is lost in transmission. The received signal adds only 5% to 10% additional energy which is not useful for effective collision detection. So CSMA/CD does not work for WLAN.  

Since the collision cannot be detected in WLAN by CSMA/CD, so the collision must be avoided. To avoid collision CSMA/CA is used.

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