A collision domain is a section of a network where data packets can collide with one another when being sent on a shared medium or through repeaters, particularly when using early versions of Ethernet. A network collision occurs when more than one device attempts to send a packet on a network segment at the same time. Collisions are resolved using carrier sense multiple access with collision detection (CSMA-CD) in which the competing packets are discarded and re-sent one at a time. This becomes a source of inefficiency in the network.
Only one device in the collision domain may transmit at any one time, and the other devices in the domain listen to the network in order to avoid data collisions. Because only one device may be transmitting at any one time, total network bandwidth is shared among all devices. Collisions also decrease network efficiency on a collision domain; if two devices transmit simultaneously, a collision occurs, and both devices must retransmit at a later time.
Collision domains are found in a hub environment where each host segment connects to a hub that represents only one collision domain and only one broadcast domain. Collision domains are also found in wireless networks such as Wi-Fi.
Modern wired networks use a network switch to eliminate collisions. By connecting each device directly to a port on the switch, either each port on a switch becomes its own collision domain (in the case of half duplex links) or the possibility of collisions is eliminated entirely in the case of full duplex links.
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