Data Rate & Modulation:
✔ 400 Gbps (4x 100G PAM4 lanes).
✔ Uses PAM4 (4-level Pulse Amplitude Modulation) for higher efficiency than NRZ.

Form Factor:
✔ QSFP-DD (Quad Small Form-Factor Pluggable Double Density) – Backward compatible with QSFP56/QSFP28 when used at lower speeds.
✔ Supports 2x 200G or 8x 50G breakout configurations (depending on vendor).

Optical Interface:
✔ SR4 (Short Range 4-lane) – Uses 4 parallel multimode fibers (MMF) (OM4 recommended).
✔ MTP/MPO-12 connector (8 fibers used for Tx/Rx, 4 unused).

Wavelength & Distance:
✔ 850nm per lane (4x 850nm VCSEL lasers).
✔ Up to 100m on OM4 MMF (70m on OM3, 150m possible on OM5).

Power Consumption:
✔ Typically < 10W, optimized for high-density deployments.

Compliance & Standards:
✔ IEEE 802.3cm (400G Ethernet over MMF).
✔ QSFP-DD MSA compliant.
✔ Supports DDM/DOM (Digital Diagnostic Monitoring) for real-time health checks.

Advantages

✅ 2x bandwidth of 200G in a slightly larger form factor (QSFP-DD).
✅ Lower per-bit power consumption than dual 200G solutions.
✅ PAM4 efficiency doubles data rate per lane vs. NRZ.
✅ Future-proof for AI/ML, 5G, and cloud-scale workloads.

✔ Hyperscale data centers (spine-leaf architectures).
✔ AI/ML clusters, HPC, and cloud networking.
✔ 400G switches/routers (e.g., Cisco Nexus 9236C, Arista 7060X4, Juniper QFX).