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Carrier Sense Multiple Access/Collision Detection (CSMA/CD)

CSMA/CD works by each node checking that the media is not being used before placing its message to the line. The node will continue to listen throughout the transmission and if a collision occurs then sending station will stop transmission and put a jamming signal on the line to warn all nodes to hold back. The transmitting nodes will apply a 'backoff algorithm' to generate a random time before they try to transmit again.

CSMA/CD is referred to as a contention method. This is because computers on the network contend, or compete, for an opportunity to send data.

Remember that if two or more computers should happen to send data at exactly the same time there will be a data collision. In this instance the two computers involved stop transmitting for a random time period and then attempt to transmit again. Each computer will determine its own waiting period; this reduces the likelihood that the computers will once again transmit at the same time.

It is important to note that it is the collision detection capability that imposes a distance limitation on CSMA/CD. Due to the effects of attenuation the collision detection mechanism is not effective beyond 2500 metres (1.5 miles). Segments cannot sense signals beyond that distance. They might, therefore, not be aware that a computer at the far end of a large network is transmitting. Should more than one computer transmit data onto the network at the same time a data collision will take place that will corrupt the data.

Next: Carrier Sense Multiple Access/Collision Avoidance (CSMA/CA)