(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). 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.
Token Ring is a data link technology for local area networks (LANs). It operates at layer 2 of the OSI model. Unlike all other standard forms of LAN interconnects, Token Ring maintains one or more common data frames that continuously circulates through the network. These frames are shared by all connected devices on the network as follows: a frame (packet) arrives at the next device in the ring sequence, that device checks whether the frame contains a message addressed to it. If so, the device removes the message from the frame. If not, the frame is empty (called a token frame), the device holding the frame decides whether to send a message. If so, it inserts message data into the token frame and issues it back onto the LAN. If not, the device releases the token frame for the next device in sequence to pick up and the above steps are repeated continuously for all devices in the token ring.