levels.
bit cable" :-) or more.
I think they all thought of only bit-serial transmission, not going further in the dimension.
My understanding is that one can do it better, faster, and cheaper. For example 1000BASE-T uses 4 pairs (!) of wires, IMHO a waste of wires and HW. I think one can do it with only 2 pairs if one just uses "byte-serial" transmission instead of bit-serial. If byte-serial works then why not improve it further simply by using multiple bytes per cycle. Ie. one could transmit even 32-bits in just 1 clock cycle; it all depends only on the max range levels the DAC on the sending side and the ADC on the receiving side can distinguish.