Carrier-sense multiple access with collision detection

CSMA/CD is used to improve CSMA performance by terminating transmission as soon as a collision is detected, thus shortening the time required before a retry can be attempted.

The procedure is complete when the frame is transmitted successfully or a collision is detected during transmission.

[4][5] On all other media, a carrier sensed on the receive channel while transmitting triggers a collision event.

[6] Repeaters or hubs detect collisions on their own and propagate jam signals.

[7][8] The collision recovery procedure can be likened to what happens at a dinner party, where all the guests talk to each other through a common medium (the air).

This in turn means: A station noting a collision has occurred is sending a 4 to 6 byte long pattern composed of 16 1-0 bit combinations.

As a correctly set up CSMA/CD network link should not have late collisions, the usual possible causes are full-duplex/half-duplex mismatch, exceeded Ethernet cable length limits, or defective hardware such as incorrect cabling, non-compliant number of hubs in the network, or a bad NIC.

A NIC cannot detect local collisions without attempting to send information.

The only functional difference between half and full-duplex operation in this regard is whether or not the transmit and receive pairs are permitted to be used simultaneously.

Due to interference on the medium, its data is corrupted and frame check sequence fails, requiring recovery at a higher layer, if possible.

During this period (usually 16 frames)[clarification needed], other users are denied use of the medium.

In some instances, back-off can occur for so long that some stations actually discard packets because maximum attempt limits have been reached.

The IEEE 802.3 standard, which defines all Ethernet variants, for historical reasons still bore the title "Carrier sense multiple access with collision detection (CSMA/CD) access method and physical layer specifications" until 802.3-2008, which uses new name "IEEE Standard for Ethernet".