QSFP56

QSFP56 doubles QSFP28 to 200 Gb/s (4 × 50G) by moving the four lanes to PAM4 signalling — the same step SFP56 takes for a single lane. Mechanically it is a QSFP28; electrically and thermally it is a new generation.

Key facts

Specifications

• Mechanical: the SFF-8665 QSFP28 outline and the 28 Gb/s connector and cage family (SFF-8662/8672, SFF-8663), qualified for PAM4 signalling.
• Electrical: 200GAUI-4 (IEEE 802.3cd/802.3bs) — four PAM4 lanes.
• Management: SFF-8636 with PAM4-era code extensions, or CMIS on newer modules and hosts.

Management interface

QSFP56 sits at the boundary between the two management generations:

• Many QSFP56 modules use the paged SFF-8636 model with extended compliance codes for 50G PAM4 lanes.
• Newer modules — and all InfiniBand HDR optics on recent platforms — use CMIS, gaining the module state machine and banked pages.

A host must recognise which one a module speaks; reading a CMIS module with an SFF-8636 decoder yields nonsense past the identifier byte.

Compatibility notes

• Accepts QSFP28 and QSFP+ modules at their rates.
• Breakout to 2×100G or 4×50G (SFP56) links.
• Fits a QSFP-DD cage on four of the eight lanes.

In CodingBox

CodingBox detects whether a QSFP56 module follows SFF-8636 or CMIS and applies the matching decoder, so identity, per-lane DDM and configuration read correctly in either case.