Carrier sense multiple access with collision avoidance

Encyclopedia : C : CA : CAR : Carrier sense multiple access with collision avoidance

In computer networking, Carrier Sense Multiple Access With Collision Avoidance (CSMA/CA) is a medium access control protocol in which:

a carrier sensing scheme is used,
a data station that intends to transmit sends a jam signal
after waiting a sufficient time for all stations to receive the jam signal, the data station transmits a frame
while transmitting, if the data station detects a jam signal from another station, it stops transmitting for a random time and then tries again.

CSMA/CA is a modification of pure Carrier Sense Multiple Access (CSMA). Please visit this article for a complete description of the basic protocol.

Collision avoidance is used to improve the performance of CSMA by attempting to reserve the network for a single transmitter. This is the function of the "jamming signal" in CSMA/CA. The performance improvement is achieved by reducing the probability of collision and retry. Extra overhead is added due to the jamming signal wait time, so other techniques give better performance. Collision avoidance is particularly useful in media such as radio, where reliable collision detection is not possible.

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).

Compare use of the jamming signal in Carrier sense multiple access with collision detection (CSMA/CD), which uses another technique to improve CSMA performance.

Source: derived in part from Federal Standard 1037C

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