Carrier sense multiple access with collision detection
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In computer networking, Carrier Sense Multiple Access With Collision Detection (CSMA/CD) is a network control protocol in which
- a carrier sensing scheme is used and
- 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). Please visit this article for a complete description of the basic protocol.
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.
Ethernet is the classic CSMA/CD protocol.
See also the similar Carrier sense multiple access with collision avoidance (CSMA/CA) protocol.
Very partly derived from Federal Standard 1037C
[edit] References
IEEE802.3 CSMA/CD is specified in section 4.2 (page 49-76).
Carrier Sense Multiple Access (CSMA)
Ethernet uses a refinement of ALOHA, known as Carrier Sense Multiple Access (CSMA), which improves performance when there is a higher medium utilisation. When a NIC has data to transmit, the NIC first listens to the cable (using a transceiver) to see if a carrier (signal) is being transmitted by another node. This may be achieved by monitoring whether a current is flowing in the cable (each bit corresponds to 18-20 milliAmps (mA)). The individual bits are sent by encoding them with a 10 (or 100 MHz for Fast Ethernet) clock using Manchester encoding. Data is only sent when no carrier is observed (i.e. no current present) and the physical medium is therefore idle. Any NIC which does not need to transmit, listens to see if other NICs have started to transmit information to it.
However, this alone is unable to prevent two NICs transmitting at the same time. If two NICs simultaneously try transmit, then both could see an idle physical medium (i.e. neither will see the other's carrier signal), and both will conclude that no other NIC is currently using the medium. In this case, both will then decide to transmit and a collision will occur. The collision will result in the corruption of the frame being sent, which will subsequently be discarded by the receiver since a corrupted Ethernet frame will (with a very high probability) not have a valid 32-bit MAC CRC at the end.
