CSMA/CD (carrier sense multiple access/collision detection)

In ETHERNET, CSMA is used on a tapped coaxial cable to which all the communicating devices are connected. On the coaxial cable, in addition to sensing carrier, it is possible for the transceivers to detect collisions. 

This variation of CSMA is referred to as carrier sense multiple access with collision detection (CSMA-CD).

CSMA/CD (carrier sense multiple access/collision detection) is a MAC (media access control) protocol. It defines how network devices respond when two devices attempt to use a data channel simultaneously and encounter a data collision. The CSMA/CD rules define how long the device should wait if a collision occurs.

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.

The collision detection technology detects collisions by sensing transmissions from other stations. On detection of a collision, the station stops transmitting, sends a jam signal, and then waits for a random time interval before retransmission.

The algorithm of CSMA/CD is:

• When a frame is ready, the transmitting station checks whether the channel is idle or busy.
• If the channel is busy, the station waits until the channel becomes idle.
• If the channel is idle, the station starts transmitting and continually monitors the channel to detect collision.
• If a collision is detected, the station starts the collision resolution algorithm.
• The station resets the retransmission counters and completes frame transmission.

The algorithm of Collision Resolution is:

• The station continues transmission of the current frame for a specified time along with a jam signal, to ensure that all the other stations detect collision.
• The station increments the retransmission counters.
• If the maximum number of retransmission attempts is reached, then the station aborts transmission.
• Otherwise, the station waits for a backoff period which is generally a function of the number of collisions and restart main algorithm.

This algorithm detects collisions but it does not reduce the number of collisions. It is not appropriate for large networks; performance degrades exponentially when more stations are added.