Carrier Sense Multiple Access/Collision Detect (CSMA/CD):

CSMA is an improvement over ALOHA as it restricts to transmit when the channel is busy. But if two stations sense the channel simultaneously and feel it idle then both of them transmit frames resulting in a collision. A further improvement over CSMA is Carrier Sense Multiple Access with Collision Detect (CSMA/CD) in which is used to detected transmission is aborted by both the stations.

Contention Period

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