PAM4/PAM6/PAM8 Modulation

Overview

Pulse Amplitude Modulation (PAM) encodes multiple bits per symbol by using more than two voltage levels on a signal wire, increasing bandwidth per lane without proportionally increasing the physical baud rate. PAM4 (4 levels = 2 bits/symbol) is the established standard for 112G and 224G Ethernet and InfiniBand interconnects and is the current baseline for high-speed copper and optical data center interconnects in AI systems.

PAM6 (6 levels = ~2.58 bits/symbol) and PAM8 (8 levels = 3 bits/symbol) are candidate modulation schemes being evaluated by the industry to push per-lane copper speeds to 336G and 448G respectively, extending copper interconnect viability beyond the 224G ceiling without requiring optics. The tradeoff is reduced noise margin and increased DSP complexity. If successful, PAM6/PAM8 would allow copper CPC, cable cartridges, and backplanes to serve another one or two data lane generations before the physics-driven shift to optics becomes unavoidable.

TE Connectivity and others in the interconnect industry use the PAM4→PAM6/PAM8 progression as evidence that copper will coexist with optics in AI racks for multiple future generations rather than being displaced entirely.