Ethernet 802.3: Carrier Sense Multiple Access with Collision Detection (CSMA/CD)This protocol is commonly used in bus (Ethernet) implementations.
Multiple access refers to the fact that in bus systems, each station has access to the common cable.
Carrier sense refers to the fact that each station listens to see if no other station is transmitting before sending data.
Collision detection refers to the principle of listening to see if other stations are transmitting whilst we are transmitting.
In bus systems, all stations have access to the same cable medium. It is therefore possible that a station may already be transmitting when another station wants to transmit. Rule 1 is that a station must listen to determine if another station is transmitting before initiating a transmission. If the network is busy, then the station must back off and wait a random interval before trying again.
Rule 2 is that a station which is transmitting must monitor the network to see if another station has begun transmission. This is a collision, and if this occurs, both stations must back off and retry after a random time interval. As it takes a finite time for signals to travel down the cable, it is possible for more than one station to think that the network is free and both grab it at the same time.
CSMA/CD models what happens in the real world. People involved in group conversation tend to obey much the same behavior.
