NRZ
Beginner
The simplest way to send bits: high means 1, low means 0.
Novice
Non-return-to-zero: two signal levels, one bit per symbol. Used for lanes up to 25 Gb/s, after which links moved to PAM4.
Expert
Two-level signaling (PAM2). Nyquist frequency is half the bit rate, so a 25.78 Gb/s NRZ lane has its Nyquist at 12.9 GHz, about the same as a 53 Gb/s PAM4 lane.