CSMA/CD

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CSMA/CD

(computer science)
A method of controlling multiaccess computer networks in which each station on the network senses traffic and waits for it to clear before sending a message, and two devices that try to send concurrent messages must both step back and try again. Abbreviation for carrier-sense multiple access with collision detection.

CSMA/CD

Carrier Sense Multiple Access / Collision Detect.

The low level network arbitration protocol used on Ethernet. Nodes wait for quiet on the net before starting to transmit and listen while they are transmitting. If two nodes transmit at once the data gets corrupted. The nodes detect this and continue to transmit for a certain length of time to ensure that all nodes detect the collision. The transmitting nodes then wait for a random time before attempting to transmit again thus minimising the chance of another collision. The ability to detect collision during transmission reduces the amount of bandwidth wasted on collisions compared with simple ALOHA broadcasting.

CSMA/CD

(Carrier Sense Multiple Access/Collision Detection) The transmission method used in Ethernet networks. When Ethernet was designed in the 1970s, it was a shared medium. At any moment, only one frame from one station was transmitting in one direction (half duplex). See 10Base5 and 10Base2.

With CSMA/CD, if the network is busy when a station wants to transmit (carrier sense), the station waits a random number of microseconds before trying again. However, if two stations coincidentally transmit their frames at exactly the same time, their signals will collide. Both stations detect the collision and back off a random duration before retrying.

Today, collisions have been mostly eliminated, because shared Ethernet gave way to full-duplex, point-to-point channels between sender and receiver (see switched Ethernet). However, CSMA/CD provides compatibility for older shared Ethernet hubs that may still be in place. Ethernet is a data link protocol, and CSMA/CD is a MAC layer protocol (see MAC layer). See data link protocol, Ethernet and CSMA/CA.