CSMA/CD

Carrier Sense Multiple Access with Collision Detection(CSMA/CD)

CSMA/CD (Carrier Sense Multiple Access with Collision Detection) helps hosts to decide when to send data on shared network segments and how to detect collision if occur. Such that, in a hub networks, two device can send data at same time. This can root a collision. CSMA/CD enable device to sense wire to ensure that not any other device is currently transmitting data. But, if two device sense that the wire is clear and send data at the same time, a collision can happen. If the collision occur, data have to resend again after a random period of time.

Consider the below example:

In above topology, We have hub network. Host A is trying to commune with host B. Host A “sense” the wire and decide to send data. But, in same time, host C sends its data to host D and the collision occur. The sending device (host A and host C) detects collisions and resend again after a random period of time.

NOTE
 switches are used in networks instead of hubs, CSMA/CD is not really used. Every port on switch usually work in a full duplex mode, there are no data collision in a full duplex mode.