Ethernet – How to 1000BaseT transmit on Rx & receive on Tx pins

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How can 1000–BaseT (Gigabit ethernet twisted copper pairs) technology transmit over the Rx pins & receive over the Tx pins?

It's baseband technology (hence the name), right? Discrete digital pulses that occupy the entire bandwidth. Not broadband analog magic, where you can have multiple signals/channels propagate a single pair.

Why couldn't 10–BaseT (10Mb/s) or 100–BaseT (100Mb/s) do the same? What changed, specifically to allow this?

I know that 1000-BaseT uses all four pairs (8 conductors) of the RJ45 connector/Cat6 cable, and the earlier technology only used 2 pairs (4 conductors). But that alone couldn't facilitate this phenomenon. There must be more to it.

Best Answer

In 1000base-T there is circuitry called a hybrid which make the signal bidirectional. It's not really transmitting and receiving the wrong way round on the receive and transmit pairs, it's transmitting and receiving on all the pairs, which the other standards dedicate to transmit and receive.

IEEE 802.3 defines them as

"1.4.220 hybrid: A circuit (implementable with active or passive components) that enables full duplex trans- mission by allowing symbols to be transmitted and received on the same wire pair at the same time. It is often used together with an echo canceller to get adequate separation of transmit and receive signals."

Wikipedia entry describes it as

In a departure from both 10BASE-T and 100BASE-TX, 1000BASE-T uses four lanes over all four cable pairs for simultaneous transmission in both directions through the use of echo cancellation with adaptive equalization called hybrid circuits. -- https://en.wikipedia.org/wiki/Gigabit_Ethernet#1000BASE-T

In this case the bidirectionality is done with "voltage level analogue magic" (as it might be called) rather than "frequency-domain analogue magic".

It certainly could have been possible with other signalling, or have half-duplex turnaround (like two-wire RS-485), but the choice was made for pairs to go only in one direction. Like most standards decisions, it's a trade-off between cost, complexity, required-time-to-implement, and political manoeuvring from the interested parties.

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