See CSMA/CD in Computer
(Carrier Sense Multiple Access/Collision Detection) The LAN access method used in Ethernet. When a device wants to gain access to the network, it checks to see if the network is quiet (senses the carrier). If it is not, it waits a random amount of time before retrying. If the network is quiet and two devices access the line at exactly the same time, their signals collide. When the collision is detected, they both back off and each wait a random amount of time before retrying. See CSMA/CA and Ethernet.