Pushing the limits: NRZ vs PAM4

High-speed serial designers have been working on ever-faster serial data streams to enable super-fast transmissions of information. By "information", I generally mean long cat videos and the occasional bitcoin transaction. The prevailing signalling technique used today is to just send either a binary 1 or a 0 in serial fashion down the channel, commonly referred to as Non-Return to Zero (NRZ). As speeds have increased, NRZ is starting to run out of gas and the industry is searching for an alternative that can go even faster. Someone suggested that we can just use 4 levels of logic (instead of 2) and get twice the effective data rate. Brilliant! This 4-level signaling is called 4-level Pulse Amplitude Modulation, or simply PAM4.

It is interesting to watch this kind of technology development struggle to get off the ground. Just when you think the legacy technology is going to be tossed onto the scrap heap, someone figures out how to squeeze a bit more performance out of it. At the same time, the new technology turns out to be a bit more difficult than anticipated and may exhibit some birthing pains. This is Classic Technology Adoption. With that in mind, I came across two fascinating articles by Ransom Stephens. The first one asks the heretical question: Is PAM4 really necessary? In other words, Does the PAM4 Emperor Have No Clothes? This was followed by the second article with an alternative (and perhaps more balanced view): Where PAM4 fits: The Rick Eads interview.

Both of these articles are a good read; not only for the technical insight they provide but for better understanding of how engineers are pushing the limits of high-speed serial. All this, so our cat videos can load faster.