# Choose the optical interface.

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

Start with the path between the ports. Then resolve the module and host requirements.

## Start at both ends

Record the intended protocol, port mode and available fiber at A and Z. An aggregate data rate alone does not specify the electrical lanes, optical arrangement or supported host configuration.

## Read the optical standard separately

For example, 400G FR4 combines four optical wavelengths on each direction of a duplex single-mode path. Its optical specification does not assign the package, power envelope or host qualification of a particular WBN model.

## Resolve the actual model

Check the released model against the full channel: fiber type, connector, distance, loss and operating conditions. Confirm the electrical interface and FEC at both hosts. An unknown requirement stays open; a familiar suffix is not evidence that two implementations are interchangeable.

In practice / Worked example

## 400G over an existing duplex path

Assume two usable single-mode fibers over a 900 m route. A 400G-FR4 interface is a candidate: its four wavelengths share one transmit fiber and one receive fiber.

| Requirement | Initial finding | Still required |
| Two-fiber path | Duplex arrangement fits | Connector and polish at both ends |
| 900 m route | Within the MSA’s 2 m–2 km range | Channel loss and dispersion |
| 400GbE ports | Target aggregate rate matches | Electrical application, FEC and host support |
This narrows the optical family; it does not qualify a module. Record the exact model and document revision at A and Z. Distance and available fibers here are illustrative inputs, not a WBN reach guarantee.

Source: 100G Lambda MSA · 400G-FR4 ↗ Rev 2.0 · 18 September 2018 · §§1.1–1.8, Table 2-2

## Before you specify

- A/Z host and port mode

- Optical interface and fiber path

- Released model limits and document revision

## 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