PAM4

This is a transmission method in which 2 bits are superimposed. In normal NRZ, there are 2 steps, Low = 0, High = 2, but there are 4 steps, 1st step = 00, 2nd step = 01, 3rd step = 10, 4th step = 11 (eye pattern If you write it, it will look like two normal eye patterns overlapped vertically). The advantage is that the Nyquist frequency is half of NRZ, so the main frequency to be considered is lower, which leads to suppression of noise and the power consumption of the device. On the other hand, when viewed from the transmission line, there is also the difficulty of becoming more sensitive to variations in transmission quality at the frequency in question, in exchange for the lower frequencies to be concerned about. The connector must also have stable quality and high performance at the relevant frequency.

In addition to PAM4, there is also a transmission system called PAM5, which is used in 1000BASET and adds a clock stage to PAM4, and PAM16, which is used in 10GBaseT, etc. and vertically overlaps 4 bits.