Note: Many topics at this site are reduced versions of the text in "The Encyclopedia of Networking and Telecommunications." Search results will not be as extensive as a search of the book's CD-ROM.

Ethernet networks use a collision-sensing protocol called CSMA/CD (carrier sense multiple access/collision detection). The protocol allows multiple devices connected to a shared network cable to use that cable by taking turns accessing it. The basic strategy goes like this:

1. A computer listens on the cable to see if another computer is transmitting, which is indicated by a voltage change on the cable. If busy, the computer waits and listens.


2. When the cable is not busy, a computer attempts to transmit.


3. Another computer may attempt to transmit at the same time, which causes a collision.


4. Both computers that attempted to transmit must back off, wait, and then attempt to transmit again.

Computers on the network detect collisions by looking for abnormally changing voltages. Signals from multiple systems overlap and distort one another. Overlapping signals will push the voltage above the allowable limit. This is detected by attached computers, which reject the corrupted frames (called runts).