A wire is not a wire at gigabit speeds. A 1-meter PCB trace or a 3-meter copper cable acts like a low-pass filter: high-frequency edges get rounded, energy spreads into adjacent bit periods, and what arrives at the receiver looks less like a square wave and more like a soggy lump. That smearing is called inter-symbol interference (ISI), and it's the reason a 10 Gbps link that works at 1 meter falls apart at 5. Equalization is the family of tricks hardware uses to undo this damage.

There are three flavors you'll meet:

• FFE (Feed-Forward Equalization) — sits in the transmitter. It pre-distorts the signal by boosting the edges and de-emphasizing the steady-state level. If you're about to send 1 1 1 0, the TX sends the first 1 at full swing, the next two at reduced swing, then drives hard low. The channel's roll-off "fixes" the pre-distortion back into a clean eye at the RX.
• CTLE (Continuous-Time Linear Equalizer) — sits in the receiver front end. It's an analog filter with a peaking response (a zero around the Nyquist frequency) that boosts the high-frequency content the channel attenuated. Cheap, low-power, no clock needed — but it boosts noise along with signal.
• DFE (Decision-Feedback Equalizer) — also in the RX, but after the slicer. It takes the bit you just decided, multiplies it by tap coefficients, and subtracts the predicted ISI from the next incoming sample. Because it operates on already-decided bits, it cancels ISI without amplifying noise — its killer advantage over CTLE.

Real-world example: A PCIe Gen 4 link runs at 16 GT/s over an FR-4 motherboard trace that attenuates roughly −25 dB at 8 GHz Nyquist. Raw, the eye is completely closed. The spec mandates a 3-tap TX FFE (pre-cursor, main, post-cursor), a CTLE in the RX with selectable peaking from 0 to 12 dB, and typically a 1- to 4-tap DFE. Link training negotiates all these coefficients at boot — the RX tells the TX "boost pre-cursor by 2 steps" via a back-channel until the BER hits 10⁻¹². Without equalization, PCIe Gen 4 would not exist.

Rule of thumb: Channel loss budget at Nyquist roughly determines what you need. Under 10 dB: CTLE alone is fine. 10–20 dB: add TX FFE. 20–35 dB: you need DFE. Over 35 dB: switch to PAM4, optical, or a shorter cable — equalization can't perform miracles.

The catch with DFE: error propagation. One wrong bit decision feeds back as wrong ISI cancellation, which can flip the next bit. Modern designs cap DFE at 4–6 taps and rely on FEC downstream to mop up bursts.