[rayiner]

128/130b encoding is used to help embed a clock signal in the data stream. After every 128 bits of data, the receiver will see a preamble of 01 or 10. That voltage transition at predictable intervals helps the receiver keep the clock in sync.

Intersymbol interference likewise applies to pretty much any high-speed digital transmissions. At high frequencies, you have to worry about things like the signal reflecting off the other end of the circuit trace, which creates inter-symbol interference.

QAM is a modulation technique. It specifies how symbols of one or more bits are represented as analog waves on the wire. PCI-E does not use QAM. It simply represents 0s and 1s high and low voltage swings--what's called Pulse Amplitude Modulation. Other modulation techniques encode symbols by varying the amplitude and phase (or both) of a carrier wave. QAM works by modulating the amplitude of two carrier waves 90 degrees out of phase (e.g. sine and cosine) based on a data signal and summing them together: https://www.techtarget.com/searchnetworking/definition/QAM

[_3u10]

Oh cool. Thanks for that. So basically it uses similar terminology because it’s modulated it’s just that QAM is a different kind of modulation?

[rayiner]

It’s similar terminology because it’s dealing with sending digital signals at high speeds using analog signals. “Modulation” just means the scheme by which sequences of bits are represented in analog signals. There necessarily has to be some sort of modulation. But the terminology is more related to common issues that arise from the receiving end reading the small, rapidly changing voltages and recovering the bit stream. Regardless of the modulation you face similar problems with keeping the clock in sync etc.