Link training & adaptation

Link training is the handshake-and-tune ritual two SerDes chips perform before they trust the channel between them. Every board trace, connector and cable is a little different and drifts with temperature, so the link can't ship with fixed settings — instead, before real data flows, the two ends exchange known patterns and adjust until the eye is as open as it can be. It's like two musicians tuning to each other before a concert: play a note, listen, nudge the peg, repeat, until they're locked in.

Concretely, the receiver measures its eye and tells the transmitter (over a back-channel) to raise or lower each FFE tap, while it simultaneously adapts its own CTLE peaking and DFE coefficients and locks its CDR. Protocols like PCIe, Ethernet (Clause 72/73/93) and USB define exact training sequences and equalization request messages. Once the eye meets the target BER margin, the link declares 'trained', switches to live data, and keeps adapting slowly in the background to chase temperature and voltage drift.