What Is PAM4? Definition and Key Facts

PAM4 (4-level Pulse Amplitude Modulation) doubles bit rate per lane versus NRZ by encoding two bits per symbol — the modulation behind 50G+ electrical and optical lanes.

Glossary · Updated 2026-10-23 · 1 min read

PAM4 (4-level Pulse Amplitude Modulation) is a signaling scheme that encodes two bits per symbol using four amplitude levels, instead of the single bit per symbol of traditional NRZ. Doubling the information per symbol lets a lane run at twice the bit rate for the same symbol rate, which is how electrical lanes moved from 25G to 50G and optical lanes from 50G to 100G.

Key facts:

• Trade-off: four signal levels are closer together, cutting noise margin to roughly one third of NRZ. PAM4 links therefore depend heavily on forward error correction (FEC, typically RS-FEC).
• Where it is used: 100G QSFP28 (4×25G NRZ was the first step), then 50G-per-lane PAM4 in 200G QSFP56, 400G QSFP-DD/OSFP, and 100G-per-lane PAM4 in 800G modules.
• Why it matters: without PAM4, 400G in a pluggable form factor would require 16 lanes — physically impractical.

The cost of PAM4 is signal integrity engineering: tighter channel loss budgets, better equalization, and mandatory FEC. The benefit is bandwidth density — more bits per lane, per module, and per watt.

Related: What is QSFP-DD? · QSFP-DD vs OSFP comparison