Carrier-sense multiple access with collision detection
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Carrier Sense Multiple Access With Collision Detection (CSMA/CD), in computer networking, is a network control protocol in which
- a carrier sensing scheme is used.
- a transmitting data station that detects another signal while transmitting a frame, stops transmitting that frame, transmits a jam signal, and then waits for a random time interval (known as "backoff delay" and determined using the truncated binary exponential backoff algorithm) before trying to send that frame again.
CSMA/CD is a modification of pure Carrier Sense Multiple Access (CSMA).
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 retry.
Methods for collision detection are media dependent, but on an electrical bus such as Ethernet, collisions can be detected by comparing transmitted data with received data.If they differ, another transmitter is overlaying the first transmitter's signal (a collision), and transmission terminates immediately. A jam signal is sent which will cause all transmitters to back off by random intervals, reducing the probability of a collision when the first retry is attempted. CSMA/CD is a layer 2 protocol in the OSI model.
Even when it has nothing to transmit, the CSMA/CD MAC sublayer monitors the physical medium for traffic by watching the carrierSense signal provided by the PLS (Physical layer signals to MAC layer). Whenever the medium is busy,the CSMA/CD MAC defers to the passing frame by delaying any pending transmission of its own. After the last bit of the passing frame (that is, when carrierSense changes from true to false), the CSMA/CD MAC continues to proper transmission.
Collisions are detected by monitoring the collisionDetect signal provided by the Physical Layer. When a collision is detected during a frame transmission, the transmission is not terminated immediately. Instead, the transmission continues until additional bits specified by jamSize have been transmitted (counting from the time collisionDetect went on). This collision enforcement or jam guarantees that the duration of the collision is sufficient to ensure its detection by all transmitting stations on the network.
It shall be emphasized that the description of the MAC layer in a computer language is in no way intended to imply that procedures shall be implemented as a program executed by a computer. The implementation may consist of any appropriate technology including hardware, firmware,software, or any combination.For example a NIC (Network Interface card) may contain hardware for complete implementation of Physical and MAC layers, hence it takes layer three packets from the Operating System and performs the rest of activity described above on it own using its own hardware, Or in another scenario the NIC can be a dumb device leaving the MAC layer intelligence to the operating system , Here NIC just gives the proper signal using its hardware to the operating system which does all the intelligent functions of MAC layer Reference[IEEE Std 802.3TM-2002 (Revision of IEEE Std 802.3, 2000 Edition Part 3)]
Ethernet is the classic CSMA/CD protocol. However, CSMA/CD is no longer used in the 10 Gigabit Ethernet specifications, due to the requirement of switches replacing all hubs and repeaters. Similarly, while CSMA/CD operation (half duplex) is defined in the Gigabit Ethernet specifications, few implementations support it and in practice it is nonexistent. Also, in Full Duplex Ethernet, collisions are impossible since data is transmitted and received on different wires, and each segment is connected directly to a switch. Therefore, CSMA/CD is not used on Full Duplex Ethernet networks.
See also
- Carrier sense multiple access with collision avoidance (CSMA/CA)
- Carrier Sense Multiple Access
- Token Ring
- Ethernet
- Collision (telecommunications)
By Josh B
References
This article incorporates public domain material from Federal Standard 1037C. General Services Administration. Archived from the original on 2022-01-22.- IEEE802.3
- For a more detailed explanation see here