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CSMA/CD (carrier sense multiple access/collision detection) is the most widely used protocol (i.e., set of rules) for determining how network devices respond in the event of a collision. A collision occurs when two or more devices on a network attempt to transmit over a single data channel (e.g., a twisted pair copper wire cable or an optical fiber cable) simultaneously. It is detected by all participating devices, and, after a brief, random, and different interval of time (called a backoff delay) has elapsed for each device, the devices attempt to transmit again. If another collision occurs, the time intervals from which the random waiting times are selected are increased step-by-step in a process referred to as exponential back off. CSMA/CD is a modification of pure CSMA. Carrier sense refers to the fact that a transmitting device listens for a carrier wave (i.e., a waveform that carries signals) before attempting to transmit. That is, it first tries to detect the presence of an encoded signal from another device. If a carrier is sensed, the device waits for the transmission in progress to finish before starting its own transmission. Multiple access describes the fact that multiple devices send and receive on the medium. Transmissions by one node are generally received by all other nodes using the medium. Collision detection is used to improve CSMA performance by terminating transmission as soon as a collision is detected, and reducing the probability of a second collision on the next try. The techniques used for detecting collisions depend on the type of media: in the case of electrical wires, for example, collisions are detected by comparing the transmitted data with the received data. CSMA/CD operates at the physical layer is the bottom level in the OSI (open systems interconnection) seven layer model, which is used to standardize and simplify definitions with regard to computer networks. This layer defines all physical and electrical specifications for devices used to interface to the network, and it deals with data only in terms of raw bits (i.e., it does not recognize MAC addresses, IP addresses and packets). A major feature of CSMA/CD is that it is simple to implement. This has helped make it an international standard and an important part of the ethernet, which is the most widely deployed architecture for LANs (local area networks).
Created September 28, 2005. |