Carrier-sense multiple access with collision avoidance: Difference between revisions

Revision as of 10:24, 31 October 2006

Revision as of 01:31, 15 November 2006

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CSMA/CA is a modification of pure [[Carrier Sense Multiple Access]] ('''CSMA'''). Collision avoidance is used to improve the performance of '''CSMA''' by attempting to be "less greedy" on the channel. If the channel is sensed busy before transmission then the transmission is deferred for a "random" interval. This reduces the probability of collisions on the channel.

CSMA-CA is used where [[CSMA-CD]] cannot be implemented due to the nature of the channel. CSMA-CA is used in [[802.11]] based [[wireless LAN]]s. One of the problems of [[wireless LAN]]s is that it is not possible to listen whilst sending, therefore collision detection is not possible. Another reason is the hidden terminals problem, whereby a node A, in range of the receiver R, is not in range of the sender S, and therefore cannot know that S is transmitting to R.

With collision avoidance, sender S sends short a '''Request to Send''' packet (RTS) containing the address of the receiver and the length of the packet to be sent. The intended receiver R, if it is within range, replies with another short packet, '''Clear to Send''' (CTS). These two packets alert anyone within range of either the sender, the receiver, or both, to keep quiet for the duration of the main packet which is transmitted immediately after the original sender S gets the CTS. If S does not get a CTS response, it waits a while before sending another RTS.

==Usage==