PAM4
Beginner
A way of sending two bits at once by using four different signal levels instead of just on and off.
Novice
Four-level pulse-amplitude modulation. Each symbol takes one of four voltage (or light) levels, so it carries two bits. It doubles the data rate at the same symbol rate, but the levels are closer together, so the signal is more sensitive to noise.
Expert
Two bits per symbol; for the same peak swing each eye is one third the height of NRZ, about a 9.5 dB SNR penalty. Used from 50 Gb/s lanes upward, which is why links rely on forward error correction (RS(544,514) in Ethernet).