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CSMA/CA vs CSMA/CD


Wireless LANs use Carrier Sense Multiple Access with Collision Avoidance (CSMA/CA) instead of Carrier Sense Multiple Access with Collision Detection (CSMA/CD) used by Ethernet LANs. Collision detection is not possible because a sending station cannot receive at the same time it transmit and, therefore, cannot detect a collision. Instead, the Request to Send and Clear to Send protocols are used to avoid collisions.

In CSMA, a station wishing to transmit has to first listen to the channel for a predetermined amount of time so as to check for any activity on the channel. If the channel is sensed "idle" then the station is permitted to transmit. If the channel is sensed as "busy" the station has to defer its transmission. This is the essence of both CSMA/CA and CSMA/CD. In CSMA/CA, once the channel is clear, a station sends a signal telling all other stations not to transmit, and then sends its packet.

Wireless LANs use a different frame format than wired Ethernet LANs. Additional information for wireless LANs is provided in the Layer 2 frame header.

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