CSMA/CD (Carrier Sense, Multiple Access/ Collision Detection) is used to determine which computer should use the shared media at any given moment. Carrier Sense means that each machine (also called a node) that is on the network, checks the cable before sending a data packet. If one node is using the network and another node wants to send something, then is will listen to for any traffic on the cable. If it hears any traffic on the cable then it will wait until the cable is free, when there is no more traffic then the node sends the packet on through.
CSMA/CA (Carrier Sense, Multiple Access/ Collision Avoidance) is used for wireless networks has a feature that CSMA/CD doesn’t have. If the node has checked the media (which is the radio frequency it will use since it is wireless and is not using cables) to see if it were available it will send out a packet as a warning to the other nodes to tell them that it is about to send some data out. If the receiving nod receives that first packet, then it will send a confirmation packet saying it received the first packet. After this the first node will forward the actual data packet.
So CSMA/CD is used for a network that is hard wired using an Ethernet cable and the CSMA/CA is used for a network that is operating wirelessly.
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