|  | 4.3 | Basics of Signals and Noise in Communications Systems
| 4.3.9 | Collision
A collision occurs when two bits from two different communicating computers are on a shared-medium at the same time. In the case of copper media, the voltages of the two binary signals are added, and cause a third voltage level. This voltage variation is not allowed in a binary system, which only understands two voltage levels. The bits are corrupted "destroyed". Some technologies, such as Ethernet, deal with a certain quantity of collisions to negotiate whose turn it is to transmit on the shared media when communicating between hosts. In some instances collisions are a natural part of the functioning of a network. However, excessive collisions can slow the network down or bring it to a halt. Therefore, a lot of network design goes into minimizing and localizing collisions. There are many ways to deal with collisions. One way is to detect them and simply have a set of rules for dealing with them when they occur, as in Ethernet. Another way is to try to prevent collisions by only allowing one computer on a shared media environment to transmit at a time. This requires that a computer have a special bit pattern called a token to transmit, as in token -ring and FDDI.