Comment by dvas

I think a quick 2-minute read on the changes around each generation gen1 -> gen4 example from 2016 will make it a bit clearer [0].

Things like packet encoding etc. Then a quick look at the signalling change of NRZ vs PAM4 in later generations.

Gen1 -> Gen5 used NRZ, PAM4 is used in PCIe6.0.

[0] Understanding Bandwidth: Back to Basics, Richard Solomon, 2016: https://www.synopsys.com/blogs/chip-design/pcie-gen1-speed-b...