CDMA/CD stands for Carrier Sense Multiple Access with Collision Detection, and is a network control protocol in computer networking. It is a protocol in which the carrier-sensing scheme is applied, and also comes into play when a transmitting data station detects another signal when transmitting a frame, in which case it will stop transmitting the frame and transmit the jam signal instead, and then wait for a random time interval before trying to transmit the frame again. It works in many ways like polite human beings having a conversation where they don’t interrupt each other. Each person ought to listen for an opportunity when no one else is talking before beginning to talk (represented by the term Carrier Sense). Further, once an opportunity arises and everyone is silent, each person has an equal chance to say something (represented by the term Multiple Access). If two individuals start talking at once, they should detect that and both of them will stop talking (this part being represented by the term Collision Detection). Accordingly, in Ethernet parlance, each interface should wait until there is no signal in the channel before it starts transmitting. In the event another interface is transmitting, there will be a signal in the channel, and this signal is called Carrier. The process where all interfaces must wait until there is no carrier is called carrier sense. All interfaces have an equal chance to send frames into the network, and none of them has a higher priority than any other; this is what multiple access is is all about.
The frames take some time to travel from one end of the Ethernet system to the other, and so the first bits of the frame do not reach the other end the moment they are released. This leaves open the possibility for more than one frame to be released into the system because two or more interfaces might have taken the opportunity when the network carrier was free and sent their frames simultaneously. In such an event, the Ethernet system detects the collision of signals and stops the transmission; the signals are resent. This mechanism is called collision detect. Technically, when the interface detects a collision (it does this by detecting more current than what it is generating, something like 24 mA for the coaxial Ethernet), the transmission is halted and the interface sends a 32-bit jam sequence. This sequence ensures that any other node that may be receiving this frame will receive a jam signal, and this leads to the other receiver discarding the frame owing to a cyclic redundancy check (CRC) error.
If the interface were to start retransmitting the frames immediately following a collision, yet another collision would almost certainly ensue. The following procedure is meant to minimize the probability of a collision. Ethernet uses what is known as the back-off period; each node selects a random number and multiplies it by a slot time (minimum frame period being 51.2 µs) and waits for this period before any retransmission is carried out.
For more information on CSMA/CD, read What is CSMA/CD?
