(
Carrier
Sense
Multiple
Access/
Collision
Detection) The transmission method used in Ethernet networks. When Ethernet was designed in the 1970s, it was a shared medium. At any moment, only one frame from one station was transmitting in one direction (half duplex). See
10Base5 and
10Base2.
With CSMA/CD, if the network is busy when a station wants to transmit (carrier sense), the station waits a random number of microseconds before trying again. However, if two stations coincidentally transmit their frames at exactly the same time, their signals will collide. Both stations detect the collision and back off a random duration before retrying.
Today, collisions have been mostly eliminated, because shared Ethernet gave way to full-duplex, point-to-point channels between sender and receiver (see
switched Ethernet). However, CSMA/CD provides compatibility for older shared Ethernet hubs that may still be in place. Ethernet is a data link protocol, and CSMA/CD is a MAC layer protocol (see
MAC layer). See
data link protocol,
Ethernet and
CSMA/CA.