CSMA/CD - Computer Definition
The most common medium access control (MAC) protocol used in bus networks, including 802.3 (Ethernet). The transmitting Ethernet station sends a data frame in both directions of the bus. Each transceiver of each station in the path of the frame reads the address in the frame header. If the address matches, the transceiver provides the frame to the target device. If the address does not match, the transceiver forwards the frame to the next transceiver. If any node detects a data collision, that station sends a brief jamming signal over a subcarrier frequency to advise all stations of the collision. All devices then back off the network. If the network is running the Nonpersistent CSMA protocol, each station then calculates a random time interval before monitoring the network again, and attempting a retransmission. See also 802.3, bus, CSMA, CSMA/CA, Ethernet, frame, MAC, Nonpersistent CSMA, subcarrier, and transceiver.
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(Carrier Sense Multiple Access/Collision Detection) The transmission method used in Ethernet networks. When Ethernet was designed in the 1970s, it was a shared medium. At any moment, only one frame from one station was transmitting in one direction (half duplex). See 10Base5 and 10Base2. With CSMA/CD, if the network is busy when a station wants to transmit (carrier sense), the station waits a random number of microseconds before trying again. However, if two stations coincidentally transmit their frames at exactly the same time, their signals will collide. Both stations detect the collision and back off a random duration before retrying. Today, collisions have been mostly eliminated, because shared Ethernet gave way to full-duplex, point-to-point channels between sender and receiver (see switched Ethernet). However, CSMA/CD provides compatibility for older shared Ethernet hubs that may still be in place. Ethernet is a data link protocol, and CSMA/CD is a MAC layer protocol (see MAC layer). See data link protocol, Ethernet and CSMA/CA.
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