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carrier sense multiple access (CSMA)

Carrier Sense Multiple Access (CSMA) is an access method in which several network stations have access to the transmission medium. In the CSMA process, the station wishing to transmit listens to the channel ( carrier sensing) before it transmits. A station may only transmit if the transmission medium is not already occupied by another station. If the transmission medium is occupied, the station waits until it is free and it can transmit. The station obtains information about the state of the transmission medium by listening.

There are several variants of CSMA for collision detection and avoidance: Carrier Sense Multiple Access with Collision Avoidance(CSMA/ CA), Carrier Sense Multiple Access with Collision Detection (CSMA/ CD) and Carrier Sense Multiple Access with Collision Resolution (CSMA/ CR).

The CSMA/CA method - CA for Collision Avoidance - avoids unnecessary additional collisions by ensuring that conflicting transmission requests are not processed randomly but in a priority-controlled manner, thus achieving deterministic behavior similar to the token method. The CSMA/CA method is used, among other things, in 802.11 wireless networks.

CSMA/CD access method

CSMA/CD access method

In the CSMA/CD method - CD for Collision Detection - the stations wishing to transmit detect collisions on the transmission medium, whereupon they briefly withdraw their transmission request and start a new transmission attempt after a changing delay time. In the case of CSMA/CD, this is referred to as the binary exponential backoff( BEB), and in the case of WLANs with CSMA/CA procedures, it is referred to as the contention window. A station may only transmit if the medium is not already occupied by another station. If the transmission medium is occupied, the station waits until the medium is free and it can transmit.

The CSMA/CR procedure is a multiple access procedure used in the CAN bus, which works with arbitration to avoid collisions, in which the stations have a mutual agreement on the right of access.

Informations:
Englisch: carrier sense multiple access - CSMA
Updated at: 07.02.2014
#Words: 317
Links: method, network, transmission medium, indium (In), process
Translations: DE
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