Collision

In short: A collision occurs when two devices send at the same time on the same transmission medium and the signals overlap. The data is then unusable.

In more detail: The problem exists in networks where several devices share a medium: formerly Ethernet with a bus or hub, today still with Wi-Fi.

In Depth

• Collision domain: the area in which collisions have an effect. A hub forms a single domain. A switch separates it per port, so in modern wired networks (full duplex) there are practically no collisions any more.
• Detecting: in wired Ethernet the devices listen while sending and detect the overlap (CSMA/CD). They send again after a random wait.
• Avoiding: in Wi-Fi senders cannot hear collisions, so one tries to avoid them beforehand (CSMA/CA).
• Consequences: repeated transmissions cost bandwidth and raise latency. It gets worse as the number of participants grows.
• Do not confuse: in computer science there are also hash collisions: two different inputs give the same hash value. That is an entirely different problem which good hash functions avoid.

See also: broadcast, Ethernet