# How does “bidirectional” transmission on gigabit ethernet work 2?

I was looking for an answer to the question how the 1000BASE-T transmission works and I came up with this post How does "bidirectional" transmission on gigabit ethernet work? but still a few issues it is not clear to me. If I understand your answers correctly, if on the two sides of the wire you receive a signal, say +1, each recipient will receive a +2 signal. The question remains: what happens if we send two signals say +1, -1 and on the other hand a signal with a value of 0 will be given in the same pair. Is this possible? If so, will the 0 signal meet the +1 signal and then it will meet with the -1 signal, so will the value we get will be 0? From the answers there, we get +1, but I do not know why. I decided to ask a new question because I have too little reputation to add a comment :(

• The accepted answer on the question you linked to answers this quite well: Echo cancellation. You know what you sent so you can subtract it from what you receive. Jul 10, 2018 at 21:41
• Yes, I just do not understand one thing. Why do we calculate the first signal that someone sent to us by subtracting the first signal we received from the first signal we sent? We should take into account all the signals we sent, because the signal we received received them in turn and each of them had an influence on him. Shouldn't we? Jul 11, 2018 at 8:21
• * I was trying to answer this question, I tried to figure out how it can work to fit the linked example. I thought that there might be only one impulse sent by us in a wire at a given moment. So if we have two impulses to send, the second one will be sent the moment the first one reaches the target. Is it true? Until now, I thought that we could give impulses one after the other. Jul 11, 2018 at 8:21
• It is a train of pulses, a continuous thing. Jul 11, 2018 at 15:00

This is a lot easier to understand if you're familiar with the rope wave metaphor:

In (a) the hand has waved up and down and a wave has started to travel along the rope. In (b) it's moved again and sent a second wave, as if corresponding to sending (+1, +1). You could imagine moving the hand down to send a -1 wave.

Multiple impulses can and will be in flight along the wire at the same time. Including reflections of your transmission from the other end.

• Except that the line is terminated at the far end, so that total far end reflections should not happen in theory. Except that in the real world, there are imperfections along the line, so that reflections do occur after all... and are trained during link establishment, and should be canceled along with the local echo...
– frr
Oct 2, 2020 at 6:39

The technique used is called echo-cancellation. It uses the idea that you know what you are sending, so you can subtract it from what you are receiving.

This paper goes into quite a lot of detail.