CodingBox Documentation

QSFP-DD

QSFP Double Density (QSFP-DD) adds a second row of contacts to the QSFP connector, doubling the lanes to eight while keeping the QSFP faceplate footprint. It is the dominant 400G form factor and, as QSFP-DD800, a leading 800G one — and its cage still accepts every earlier QSFP module.

Key facts

Lanes × rate8 × 50G (PAM4) → 400G; 8 × 100G (PAM4) → 800G (QSFP-DD800)
Total400G / 800G
ConnectorMPO-12/MPO-16 (DR4/DR8/SR8), LC duplex (FR4/LR4), CS/SN duplex (2×FR4), DAC/AOC
Supply3.3 V; power classes up to ~14 W and above with enhanced cooling
Applications400GBASE-DR4/FR4/LR4/SR8, 800G-DR8/2×FR4, AI/ML fabrics, 2×200G / 4×100G / 8×50G breakout

Specifications

  • INF-8628 — QSFP-DD 8X Transceiver (Double Density): the QSFP-DD MSA hardware

specification (mechanical, electrical, thermal).

  • QSFP-DD800 — the MSA revision defining 100G-per-lane operation.
  • CMIS 5.0 — Common Management Interface Specification (management). See

CMIS.

Management interface

QSFP-DD is the form factor for which CMIS was created. The host drives a module state machine (ModuleLowPwr → ModulePwrUp → ModuleReady) and per-lane data-path states, selects an application from the module's advertised list, and reads banked pages for identity, diagnostics and configuration. Extended operations — firmware update, tuning — go through the CDB command interface. Codes are from SFF-8024.

Because so much depends on the handshake, "module stuck in low power" and "recognised but not ready" are the typical QSFP-DD failures — see CMIS issues.

Compatibility notes

  • Backward compatible cage: a QSFP-DD port accepts QSFP112, QSFP56, QSFP28 and

QSFP+ modules on four of its eight lanes.

  • Breakout options: 2×200G, 4×100G, 8×50G depending on the module PMD.
  • The competing eight-lane form factor is OSFP —

larger, with an integrated heat sink; the two are not mechanically interchangeable.

In CodingBox

CodingBox supports QSFP-DD through CMIS 4.0/5.0: module state, identity, applications, per-lane DDM and configuration pages are decoded and editable, with per-lane values on the DDM screen revealing a single failed lane out of eight.