2.1 Legacy Media Types
2.1.2 CSMA/CD
Carrier sense multiple access collision detect (CSMA/CD) describes the Ethernet access method. In Ethernet, multiple access is the terminology for many stations attaching to the same cable and having the opportunity to transmit. No station has any priority over any other station. However, the stations do need to take turns, as defined by the access algorithm.

Carrier sense refers to the process of listening before speaking. The Ethernet device wishing to communicate looks for energy on the media (an electrical carrier). If a carrier exists, the cable is in use and the device must wait to transmit. Many Ethernet devices maintain a counter of how many times they have to defer a transmission. Some devices call the counter a deferral or back-off counter. If the deferral counter exceeds a threshold value of 15 retries, the device attempting to transmit assumes that it will never get access to the cable to transmit the packet. In this situation, the source device discards the frame. This might happen if there are too many devices on the network, implying that there is not enough bandwidth available.

When two or more devices, on the same segment, attempt to transmit at the same time, a collision occurs. The devices that were transmitting can sense that a collision has occurred because the power level on the cable exceeds a certain mark. When stations detect that a collision has occurred, the participants generate a collision enforcement signal. The enforcement signal lasts as long as the smallest frame size. In the case of Ethernet, that equates to 64 bytes. This ensures that all stations know about the collision and that no other station attempts to transmit during the collision event. If a station experiences too many consecutive collisions, the station stops attempting to transmit the frame. Some workstations display an error message to the user; the exact message differs from platform to platform, but every workstation attempts to convey to the user that it was unable to send data for one reason or another.