CSMA/CD Protocol

CSMA/CD protocol is a set of rules determining how network devices respond when two devices attempted to use a channel simultaneously.
each interface must wait until there is no signal on the channel, then it can beginning transmitting. All other decides on the segment must wait until the carrier stop before trying to transmit, this is called Carrier Sense.

The multiple access part of the protocol define an equal priority for every interface on the channel to send data. no one gets a higher priority than anyone else.

When multiple devices trying to transmit at the same time, they are able to detect this error, this is called Collision Detection. When a collision occur each node will  calculate and wait random backoff period based on number of collisions.

Simulation

CSMA/CD Simulation  shows two devices that share the same media segment. Click on the button of one device to transmit a data. Watch the data while it is propagating through the segment, before the data reaches to the other side, the node is still in an idle state, assuming the segment is free. During this period of time, click the transmit button to create a collision. When a collision occur both ends continue the transmission until minimum packet time is reached (jam signal) they increment retransmission counter and wait random backoff period before retransmitting.