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All devices compete for the same transmission medium. Only one device can send data at a time.
In a collision domain, a collision occurs when data packets from different devices are sent to the shared medium at the same time.
Each device on the network can send data only when the network is idle. Therefore, the possibility of collisions is higher and the network efficiency is lower when there are more devices on the network.
Points that share the same information channel form a collision domain. For example, if all ports of a hub belong to the same collision domain, no port can receive and send data at the same time.