A collision domain is a logical area in a computer network where data packets can collide with one another. The term is used mainly with regard to ethernet, which is by far the most commonly used type of local area network (LAN).
A collision occurs when two or more devices attempt to send a signal along the same transmission channel at the same time. It can result in garbled, and thus useless, messages.
Collision domains can range in size from a single segment of cable to an entire network, at least a small one. They are extended by physical layer (OSI layer one) devices, such as repeaters and hubs, because such devices automatically forward all data that is sent on the cable or other media; they are broken up by data link layer and network layer (OSI layers two and three, respectively) devices, such as bridges, switches and routers.
Thus, a collision domain could also be defined as being all the network segments between a pair of bridges or other layer two devices.
The optimal size of a collision domain depends on a number of factors. Among them is the number of collisions, as a higher number of them results in a lower efficiency of the network.
Created September 30, 2005.