Carrier Sense Multiple Access/Collision Detect (CSMA/CD)

Posted onCategoriesComputer Networks

Carrier Sense Multiple Access/Collision Detect (CSMA/CD) is a scheme where the hosts try to detect collisions, and if they occur, stop transmitting, and then try to re-transmit at a later time. Any host on the network segment that wants to send data must check the segment to see if another host is already sending data. This is called “listening to the wire”, and this is the Carrier Sense part of CSMA/CD. All the hosts on the network segment will do this – that is the Multiple Access part.

Collision Detection, which can be referred to as a host will be able to quickly detect any collision that its data is involved in. If two hosts listen to the wire at the exact same time, determine that no one else is sending data, and then send their data onto the segment at the same time, a collision will occur.

While collision happens, is actually a change in the voltage of the signals on the wire. The sending hosts detect this voltage changes and basically would know there is a collision, and both of the senders will send a jam signal over the network. This jam signal indicates to all other hosts on the network segment that there has been a collision, and they should not send data onto the wire.

After jam signals have been sent, each of the hosts will be waiting a random amount of time before listening to the wire again. The system invokes random time to help to ensure that the two hosts do not transmit data simultaneously again.