Autonegotiation is a requirement for 1000BASE-T implementations as minimally the clock source for the link has to be negotiated, as one endpoint must be master and the other endpoint must be slave.
1000BASE-T uses four cable pairs for simultaneous transmission in both directions through the use of echo cancellation with adaptive equalization. Line coding is five-level pulse-amplitude modulation (PAM-5).
Since autonegotiation takes place on only two pairs, if two 1000BASE-T interfaces are connected through a cable with only two pairs, the interfaces will complete negotiation and choose gigabit as the best common operating mode, but the link will never come up because all four pairs are required for data communications.
Each 1000BASE-T network segment is recommended to be a maximum length of 100 meters and must use Category 5 cable or better.
Automatic MDI/MDI-X configuration is specified as an optional feature in the standard that is commonly implemented. This feature makes it safe to incorrectly mix straight-through and crossover-cables, plus mix MDI and MDI-X devices.
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