Duplex and Collisions

Half duplex means one side talks at a time, like a walkie-talkie; when two devices transmit together the signals collide and both must retry. Full duplex means both directions at once, like a phone call, and collisions cannot happen.

Old hubs blindly repeated every signal to every port, forcing all devices to share one half-duplex party line. Modern switches give every port its own private full-duplex link, which is one big reason they replaced hubs completely.