What Is QSFP-DD? Definition and Key Facts
QSFP-DD (Quad Small Form-factor Pluggable Double Density) is a 400G pluggable transceiver format with eight 50 Gb/s electrical lanes, backward compatible with QSFP28.
QSFP-DD (Quad Small Form-factor Pluggable – Double Density) is a pluggable optical transceiver form factor defined by the QSFP-DD MSA. The "double density" refers to its eight electrical lanes — twice the four lanes of a QSFP28 — each carrying 50 Gb/s PAM4 signaling for a total of 400 Gb/s.
Key facts:
- Dimensions: 18.35 mm wide, 89.4 mm long, 8.5 mm thick — close to a standard QSFP28.
- Density: up to 36 ports per 1U switch panel.
- Backward compatibility: hosts accept QSFP+, QSFP28, and QSFP56 modules.
- Power: typical 400G modules run at 6–12 W; the format supports up to roughly 15 W.
- 800G path: the QSFP-DD800 variant reaches 800 Gb/s with 100 Gb/s per lane.
Common modules include 400G DR4, FR4, LR4, and SR8. Because it preserves existing QSFP racks, QSFP-DD is the most widely deployed 400G form factor in enterprise and cloud data centers.
Related: What is OSFP? · What is PAM4? · QSFP-DD vs OSFP comparison
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