When two computers interfere in this way, we say a collision occurs. Collisions can be detected because the voltage levels will be higher than they should be. Whenever two computers both try to signal a 1 with a high voltage, the combined resulting voltage will be the sum of the two 1's, which is out of the acceptable range. Tranceivers are equipped to detect collisions and signal their attached computers that a collision has occurred.

After two computers collide by trying to use the coax cable at the same time, they both stop transmitting and wait a random amount of time before beginning the whole process of sensing the wire and starting to transmit again. Any other transceiver that hears a garbled packet on the coax, one that resulted from a collision, ignores it and doesn't trouble its attached computer. Of course, both of the would-be senders might collide again, although this is made suitably unlikely by waiting a random interval of time that gets wider as more and more collisions occur.