What is CSMA/CD?

CSMA/CD stands for Carrier-Sense Multiple Access with Collision Detection. It is a media access method which means it defines how the network places data on the cable and how it takes it off. CSMA/CD specifies how bus topologies such as Ethernet handle transmission collisions. A collision occurs when two or more computers transmit signals at the same time.

"Carrier Sense" means that each station on the LAN continually listens to (tests) the cable for the presence of a signal prior to transmitting. "Multiple Access" means that there are many computers attempting to transmit and compete for the opportunity to send data (i.e., they are in contention). "Collision Detection" means that when a collision is detected, the stations will stop transmitting and wait a random length of time before retransmitting.

CSMA/CD works best in an environment where relatively fewer, longer data frames are transmitted. This is in contrast to token passing which works best with a relatively large amount of short data frames. Because CSMA/CD works to control or manage collisions rather than prevent them, network performance can be degraded with heavy traffic. The greater the traffic, the greater the number of collisions and retransmissions.