Comment by eigen

> 1000Base-T uses two pairs per direction, actually. It's full duplex. Each port sees two TX and two RX pair.

you may be thinking of 1000Base-TX (TIA‐854) which uses 2 pairs in each direction, similar to 100Base-TX (IEEE 802.3u). whereas 1000Base-T (IEEE 802.3ab) uses all 4 pairs in both directions.

basically, the -TX are dual simplex with a set of wires for each direction and -T are full-duplex with the same wires used in both directions at the same time.