Operating bidirectionally on twisted pairs is a challenge. The encoding used for 1000BASE-T is highly complex. It not only must achieve a balance of 0s and 1s on each cable, but it also must create a balanced electromagnetic condition across all four pairs.
The encoding method consists of several steps:
Each byte is scrambled.
The scrambled byte is translated to a 4-tuple of special symbols via a mapping called 8B1Q4.
Each symbol is represented on the physical medium by a voltage. Five different voltage levels are used, corresponding to an alphabet of five different symbols. The physical transmission method is called 4-dimensional 5-level pulse amplitude modulation (4D-PAM5).
More details are presented in Appendix ...