PAM4
PAM4 is a way of modulating signals over wires or fibers to transmit digital data.
PAM stands for Pulse Amplitude Modulation, and the 4 indicates that four voltage levels are used to encode data.
I associate PAM4 with eye diagrams which look like this:
NRZ
NRZ is another way of modulating signals that transmits half as many bits as PAM4 but is less susceptible to error.
NRZ stands for Non Return to Zero. There are only two voltage levels used to encode data which makes the analog signal easier to digitize without error correction.
NRZ’s eye diagram looks like this:
