Media Access Control in Wireless Networks

The collision detection part of CSMA/CD is only possible if the PHY layer transceiverCSMA/CA is used, where the CA standscollision avoidance .

enables the device to listen to the medium while transmitting. This is possible on a wired

network, where invalid voltages resulting from collisions can be detected, but is not

possible for a radio transceiver since the transmitted signal would overload any attempt to

receive at the same time. In wireless networks such as 802.11, where collision detection

is not possible, a variant of CSMA/CD known as

for

Apart from the fact that collisions are not detected by the transmitting device, CSMA/CA

has some similarities with CSMA/CD. Devices sense the medium before transmitting and

wait if the medium is busy. The duration fi eld in each transmitted frame (see preceding

Table 2.6 ) enables a waiting device to predict how long the medium will be busy.

Once the medium is sensed as being idle, waiting devices compute a random time period,

called the contention period, and attempt to transmit after the contention period has

expired. This is a similar mechanism to the back off in CSMA/CD, except that here it

is designed to avoid collisions between stations that are waiting for the end of another

station’s transmitted frame rather than being a mechanism to recover after a detected

collision.