What happens when a collision occurs between two computers on a LAN?

The LAN protocol commonly in use today, ethernet, was designed to expect computers to send data at any time and included rules for detecting when the line was already in use and for handling collisions. When two computers send data packets, the electronics involved also allows the computers to detect the other transmission. When this occurs, the transmitting computers wait a random amount of time and try to send again. The random wait time hopefully meant that each computer waited a different amount of time so that when they tried transmitting again there would not be a collision.

The original ethernet implementation had coaxial cables with multiple computers attached to the same cable. The modern implementation of ethernet has each device on a separate cable with the cables going to a box called a hub or switch. A hub served as a junction where all the data packets passed through and then went to each of the connecting cables.

A switch however kept track of what device was at the end of each cable and managed traffic, directing the data only to the cable leading to the intended destination. This virtually eliminated the collision problem over ethernet cables.