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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.

  • Did any answer help you? If so, you should accept the answer so that the question doesn't keep popping up forever, looking for an answer. Alternatively, you can provide and accept your own answer. – Ron Maupin Dec 25 '18 at 9:36
  • Thanks @Ron, I know how it works. If and when I'm satisfied an answer is fully accurate, and I can understand it; I'll declare it. If you can improve or clarify an existing answer to remove all doubt, please do. I haven't abandoned or forgotten about it though. I think it's important we don't just pick something purely for the sake of closing it. – tjt263 Dec 25 '18 at 16:45
  • OK. I was just doing year-end cleanup. Also, I would suggest that you really asked this question in the wrong place. You will probably get a better explanation on Electrical Engineering. – Ron Maupin Dec 25 '18 at 19:14
  • @Ron. I think you're right. Can you transfer it to the other site? – tjt263 Dec 27 '18 at 8:25
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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.

  • It's actually not a hybrid circuit that is used but a hybrid which is a special type of transformer. – Zac67 Oct 8 '18 at 17:04
  • So, if you had some superfast multimeters, you would detect the same traffic on all pairs? – tjt263 Oct 8 '18 at 17:13
  • Superfast multimeters, aka oscilloscopes, would show different bits on the different pairs, but yes, traffic going in both directions. – jonathanjo Oct 8 '18 at 17:17
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    @Zac thanks for picking up ambiguity, I reworded hope you agree it's clearer; also added IEEE definition – jonathanjo Oct 8 '18 at 17:21
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Why couldn't 10-BaseT (10Mb/s) or 100-BaseT (100Mb/s) do the same? What changed, specifically to allow this?

The quality of the cabling improved drastically as well as the methods for transmitting data and the receivers used to “read” the signal from the cable. These days you could retroactively modoify the 10/100BASE-T standards to use the same technologies but why would you?

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Broadband (xDSL) uses frequency-division multiplex (FDM) to separate receive and transmit direction - both use their own frequency sub-band.

Ethernet 1000BASE-T and faster use telecom hybrids to cancel out their own signal in order to receive the other end's signal. Unlike 10BASE-T or 100BASE-TX, there are no dedicated receive and transmit pairs - the data stream is split into four lanes and transmitted over all four pairs simultaneously while they are also receiving.

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

When 10BASE-T was designed in the late 1980s, running 1 Gbit/s over twisted pair cabling was hardly imaginable. There weren't even computers capable of using that speed - it was generally limited by system throughput, not the cable.

Technology advanced and for 100BASE-TX, IEEE had to think up some way to reduce the frequencies on the cable (by using MLT-3). For 1000BASE-T later on they had to increase the voltage levels (PAM-5) and split the traffic to four lanes. 10GBASE-T requires a much better cable (Cat-6A) and uses very elaborate encoding to reduce frequencies even further.

Ethernet's popularity has always been significantly powered by its low cost. Oversizing the capabilities would have seriously hurt that.

  • So, if you had some superfast multimeters, you would detect the same traffic on all pairs? – tjt263 Oct 8 '18 at 17:14
  • No. Each pair carries 1/4 of total traffic. The stream is split in four lanes, these are transmitted separately and reassembled at the link end. – Zac67 Oct 28 '18 at 11:55
  • is that what makes it faster ? – tjt263 Oct 28 '18 at 16:29
  • Like a bucket with four holes drains quicker than one. – tjt263 Oct 28 '18 at 16:32
  • Exactly. Instead of a single lane running 1000 Mbit/s (as with 1000BASE-X) you've got four lanes of only 250 Mbit/s each. (Very roughly) PAM-5 reduces that to 125 MBaud which Cat-5 can handle. – Zac67 Oct 28 '18 at 16:37

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