I know what the differential voltage looks like. If you measure across Tx+ and Tx- it goes from -1V to 0V to 1V and so on, but how are these voltage levels actually transmitted?
Does Tx+ go from 0V - 1V and Tx- from -1V - 0V, or are both from 0V - 1V or something completely different?
On the wire, as the interface is transformer isolated, there is no other voltage where to reference anything except for the two wires themselves that work as a pair. Thus at first it may not be even useful to discuss about voltages, but simply think about current instead : either no current flows, or it flows into one direction in the loop, or flows into the other direction in the loop.
If we do talk about voltages, for example we can take the average of both wires as the common voltage reference signal, such as the point of Ethernet transformer center tap, then if TX+ wire goes positive, the TX- will have an equal negative voltage, and vice versa, as the wires are a balanced and symmetrical differential pair, terminated into equal impedances.
That is how the wires work, but how it works on the chip side of the transformer is a completely different question, and depends on the chip, as there are multiple ways how the chip can drive the waveforms into Ethernet isolation transformer.
