CDMA/CD Carrier Sense Multiple Access Collision Detection

Carrier Sense Multiple Access Collision Detection, CSMA/CD is the method used by Ethernet and some other services to prevent two devices transmitting against each other.

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When it is possible for different devices to transmit on the same channel simultaneously, a method of resolving the issue is needed otherwise data will be lost and the channel will become congested.

Early Ethernet networks used a single cable to connect multiple devices. Here there was a real possibility that two could transmit at the same time, and therefore the mechanism called Carrier Sense Multiple Access Collision Detection, CSMA/CD was developed.

The CSMA/CD mechanism is detailed within the IEEE 802.3 standard and even though some of the latest incarnations of Ethernet do not use it, it is still incorporated in most of them.

CSMA/CD basics

Although it is normally associated with Ethernet, carrier sense multiple access schemes are used within a number of areas of data transmission.

Essentially, carrier-sense multiple access with collision detection, CSMA/CD is a media access control method used for managing the sending of data on a multiple acmes local area network.

The CSMA elements of CSMA/CD use carrier-sensing techniques and if a node requires to transmit, it listens to see if another carrier is present. If another carrier is detected, it defers its transmissions for a randomly chosen length of time, checking again to see if any nodes have a carrier present and are transmitting.

In addition to this, there is still a possibility that collisions might occur as a result of delays on the cabling or in the sensing mechanism between when the sensing ended and the transmission started. To overcome this issue, CSMA/CD uses collision detection techniques whereby a transmitting station detects collisions by sensing transmissions from other stations while it is transmitting a frame.

When this collision condition is detected, the station stops transmitting that frame, transmits a jam signal, and then waits for a random time interval before trying to resend the frame.

CSMA/CD is a modification of pure carrier-sense multiple access, CSMA. CSMA/CD is used to improve CSMA performance by terminating transmission as soon as a collision is detected, thus shortening the time required before a retry can be attempted.