Full-duplex ethernet

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While the early coaxial cable based variants of ethernet were half duplex by design, all the common variants twisted pair (10BASE-T, 100BASE-TX and 1000BASE-T) and fiber optic ethernet provide separate channels for send and receive.

To allow use of hubs and for compatibility with existing variants of ethernet they were initially implemented in a half duplex manner with the tranceiver (usually by this point integrated into the device) detecting a collision if an attempt was made to transmit and receive simultaneously and looping back data to the host so it could hear itself transmit (as it would on a shared medium).

However if both ends of the link are not hubs and the hardware supports it the two channels can be split and used to make a full duplex link. Unfortunately if autonegotiation is enabled on one end and forced full duplex the other the end with autonegotiation will detect the link as half duplex causing large numbers of errors.