Wi-Fi - Access Protocols

IEEE 802.11 wireless LANs use a media access control protocol called Carrier Sense Multiple Access with Colpsion Avoidance (CSMA/CA). While the name is similar to Ethernet s Carrier Sense Multiple Access with Colpsion Detection (CSMA/CD), the operating concept is totally different.

WiFi systems are the half duplex shared media configurations, where all stations transmit and receive on the same radio channel. The fundamental problem of a radio system is that a station cannot hear while it is sending, and hence it is impossible to detect a colpsion. Because of this, the developers of the 802.11 specifications came up with a colpsion avoidance mechanism called the Distributed Control Function (DCF).

According to DCF, a WiFi station will transmit only when the channel is clear. All transmissions are acknowledged, so if a station does not receive an acknowledgement, it assumes a colpsion occurred and retries after a random waiting interval.

The incidence of colpsions will increase as the traffic increases or in situations where mobile stations cannot hear each other.