Wikipedia

Carrier-sense multiple access with collision avoidance

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In computer networking, CSMA-CA belongs to a class of protocols called as CSMA-CA. CSMA CA stands for: Carrier Sense Multiple Access With Collision Avoidance. In CSMA-CA, 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 the "collision avoidance" part of the protocol.

Detail

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 LANs. One of the problems of wireless LANs 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

  • GNET an early proprietary LAN protocol
  • Apple's LocalTalk implemented CSMA/CA on an electrical bus using a three-byte jamming signal.
  • 802.11 RTS/CTS implements virtual carrier sensing using short Request to Send and Clear to Send messages for WLANs (802.11 mainly relies on physical carrier sensing though).
  • IEEE 802.15 (Wireless PAN) uses CSMA/CA

See also

Carrier sense multiple access with collision detection (CSMA/CD), (used for Ethernet) takes a different approach to the same problem.

Source: derived in part from Federal Standard 1037C

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