# Duplex, parallel, or breakout?

Editorial revision 2026-11-01 — engineering guidance, not a released model specification.

Choose the optical path before choosing the assembly.

## Duplex describes the path

A duplex connection uses two fibers, commonly one for each direction. Wavelength multiplexing can carry multiple optical channels on a fiber; optical channel count is not the same as physical strand count.

## Parallel describes multiple optical lanes

Parallel interfaces distribute traffic across separate optical lanes. Confirm the populated connector positions and the endpoint's transmit/receive allocation. An MPO shell can contain positions the chosen application does not use.

## Breakout depends on the endpoints

A harness presents fibers to separate connectors. It does not itself make a host or optical module support breakout. Check the supported lane grouping, optical interface and destination requirements before assigning each leg. Preserve the exact connector-position map alongside the physical drawing.

In practice / Worked example

## Four wavelengths. Two physical fibers.

Count optical channels separately from installed strands. In the described 400G-FR4 architecture, multiplexing combines four wavelengths in each direction.

| Direction | Optical channels | Physical path |
| Endpoint A → Z | Four wavelengths | Fiber 1 |
| Endpoint Z → A | Four wavelengths | Fiber 2 |
The duplex connection uses two physical fibers. Four wavelengths do not imply four connector positions or four independently usable links. This establishes topology only. Model limits, host electrical application and FEC remain separate checks.

Source: 100G Lambda MSA · 400G-FR4 ↗ Rev 2.0 · 18 September 2018 · §§1.1–1.3

## Before you specify

- Physical strands and populated positions

- A/Z optical lane allocation

- Supported host breakout and leg destinations

## Sources & scope

General engineering guidance. External examples describe their own products and do not establish WBN availability or qualification.

- 400G-FR4 technical specification ↗ 100G Lambda MSA · Rev 2.0 · 18 Sep 2018

- LinkX 100G-PAM4 product line overview ↗ NVIDIA · Online reference · accessed 1 Nov 2026

Source: 100G Lambda MSA · 400G-FR4 ↗ — https://100glambda.com/specifications/send/2-specifications/7-400g-fr4-technical-spec-d2p0
Source: 400G-FR4 technical specification ↗ — https://100glambda.com/specifications/send/2-specifications/7-400g-fr4-technical-spec-d2p0
Source: LinkX 100G-PAM4 product line overview ↗ — https://docs.nvidia.com/networking/display/400g100gpam4ovdev/LinkX-100G-PAM4-Product-Line-Overview