A class of methods for sharing a data transmission medium in which hosts transmit as soon as they have data to send and then check to see whether their transmission has suffered a collision with another host's.

If a collision is detected then the data must be resent. The resending algorithm should try to minimise the chance that two hosts's data will repeatedly collide. For example, the CSMA/CD protocol used on Ethernet specifies that they should then wait for a random time before re-transmitting.

See also backoff.

This contrasts with slotted protocols and token passing.