When a computer sends a message to another one, the electric signal is present the whole network, but only the computer whith the destination address contained on the message will be able to read it.

Ethernet access control is distributed because, unlike some other network hardware, there is no central authority granting access to transmission. The Ethernet access scheme is called Carrier Sense Multiple Access with Collission Detect (CSMA/CD). This means, multiple machines can access the Ethernet simultaneously and each machine determines whether the ether is idle by sensing whether a carrier signal is present. When a host interface has a packet to transmit, it listens to the ether to see if a message is being transmitted. When no transmition is sensed, the host starts transmitting. The signal arrives almost immediately to all other computers in the net.

When a computer begins transmission, the signal does not reach all parts of the network simultaneusly. Instead it travels along the cable at about 80% of the speed of light. Thus, it is possible for two computers to both sense that the network is idle and begin transmission simultaneously. When the two electrical signals cross they become scrambled, such that neither is meaningful. Such incidents are called collisions.

An Ethernet handles collisions in an ingenious fashion. Each transceiver monitors the cable while it is transmitting to see if a foreign signal interferes with its transmission. This is called collision detect (CD). When a collision is detected, the host interface aborts trnsmission, waits for the activity to subside and then tries again. The delay time for each computer is is first calculated at random. If at the second try it fails, the new delay time is two times the original, four times for the third collision and so on.