QSFP-DD (400G / 800G)
Direct Answer
Quad Small Form-Factor Pluggable Double Density (QSFP-DD) is a high-speed transceiver form factor defined by the QSFP-DD MSA and SFF-TA-1002. It incorporates an 8-lane electrical interface via an extra row of electrical contacts, delivering 400 Gbps (8x50G PAM4) and 800 Gbps (8x100G PAM4) while maintaining mechanical backward compatibility with QSFP28 and QSFP+.
Source: QSFP-DD MSA Specification / IEEE 802.3bs / IEEE 802.3ck
Specs
| Standard / MSA | QSFP-DD MSA / SFF-TA-1002 |
| Form Factor | QSFP-DD (Double Density) |
| Electrical Channels | 8 lanes (via dual-row contact pins) |
| Max Data Rate (400G Generation) | 400 Gbps (8x50G PAM4) |
| Max Data Rate (800G Generation) | 800 Gbps (8x100G PAM4) |
| Optical Connectors | MPO/MTP-16, Dual MPO-12, Duplex LC, CS, SN, MDC |
| Electrical Modulation | PAM4 (4-level Pulse Amplitude Modulation) |
| Thermal Power Dissipation | Up to 12–25 W |
| Backward Compatibility | Accepts QSFP+, QSFP28, and QSFP56 modules |
Double Density Contact Architecture
QSFP-DD achieves an 8-lane electrical interface by adding a second row of recessed contact pads inside the module, engaging a dual-depth card edge in the host cage. When a legacy 4-lane QSFP+/QSFP28 module is inserted, it engages only the front row of contacts, allowing seamless backward compatibility without physical adapters.
PAM4 Modulation and 400G/800G Standards
QSFP-DD relies on PAM4 modulation (encoding 2 bits per electrical baud symbol). Key optics include 400G-DR4/XDR4 (parallel single-mode MPO), 400G-FR4/LR4 (duplex single-mode WDM), and high-density breakout to 2x200G or 4x100G SN/MDC connectors.