What is the eye diagram and how do I interpret it for a high speed serial link?

What is the eye diagram and how do I interpret it for a high speed serial link? The eye diagram is the fundamental visualization tool for assessing the signal quality of a digital link, created by overlapping all possible bit transitions on a single display: (1) How it is created: the oscilloscope triggers on the data clock (or recovers the clock from the data). Each bit period is overlaid on the same display. The result is an "eye"-shaped pattern where all possible transitions (0→1, 1→0, 0→0, 1→1) are superimposed. For NRZ (2-level) signaling: one eye opening. For PAM4 (4-level) signaling: three eye openings stacked vertically. (2) Key parameters: eye height: the vertical opening of the eye at the sampling point. Represents the voltage margin available for the receiver to distinguish between 0 and 1. Larger eye height = more noise margin. Eye width: the horizontal opening of the eye. Represents the timing margin available for the receiver to sample the data correctly. Larger eye width = more jitter margin. Eye mask: a specification-defined region inside the eye that must be clear of any signal traces. If any trace enters the mask: the signal fails the specification. Common masks: IEEE 802.3 (Ethernet), USB4, PCIe Gen5/6. Jitter: the horizontal spread of the transitions at the crossing point. Total jitter = deterministic jitter (DJ) + random jitter (RJ). DJ: caused by ISI (inter-symbol interference), duty cycle distortion, and periodic jitter. RJ: caused by thermal noise, phase noise, and random processes. Rise/fall time: the time for the signal to transition between 20% and 80% of the amplitude. Faster transitions = wider eye opening (steeper zero crossings). (3) What degrades the eye: channel loss (insertion loss): attenuates high-frequency components, slowing edge rates and reducing eye height. This is the dominant impairment for long channels (> 10 cm at 25 Gbps). Impedance mismatches (reflections): create ghost transitions that close the eye horizontally and vertically. Crosstalk: noise from adjacent signals adds random and deterministic noise, reducing eye height. Jitter: random and deterministic timing variations reduce eye width.