Decision summary
“400G LR4” is not precise enough for design, purchasing or acceptance. Three commonly encountered names have different scopes: IEEE 400GBASE-FR4 targets at least 2 km over single-mode fiber; IEEE 400GBASE-LR4-6 targets at least 6 km; and the 100G Lambda MSA's 400G-LR4-10 specification targets at least 10 km. Keep the suffix in the BOM, quotation and test report so the two ends are not selected from different specifications.
All three are four-wavelength, duplex single-mode approaches, but reach is not their only engineering difference. A link must meet the exact PMD's transmitter, receiver and channel requirements, and the host must provide the required electrical interface and FEC behavior. An informal label such as “LR4 10 km” is not a substitute for a datasheet that names the applicable specification and revision.
Name the requested PMD exactly
| Designation | Specification family | Nominal planning reach | Procurement note |
|---|---|---|---|
| 400GBASE-FR4 | IEEE 802.3 | At least 2 km SMF | Use the full BASE-FR4 name and check the channel budget |
| 400GBASE-LR4-6 | IEEE 802.3 | At least 6 km SMF | Retain “-6”; do not collapse it into an ambiguous LR4 label |
| 400G-LR4-10 | 100G Lambda MSA | At least 10 km SMF | Confirm the MSA revision and host FEC requirement |
Build the link budget from the real route
Start with measured or designed fiber length, connector pairs, patch panels, splices and any planned optical distribution. Compare the resulting channel loss with the candidate module's specified transmitter and receiver limits, while retaining engineering headroom for connector contamination, repairs, aging and environmental variation. Do not approve a route merely because its length is below 2, 6 or 10 km.
Also record fiber type, duplex connector, operating temperature, module power and the optics at both ends. Mixed vendors may interoperate only when both ends implement the same PMD and the host settings match. FEC must be written as an observed configuration, not assumed from a product nickname.
RFQ and sample checklist
- Write the complete PMD designation on every BOM line and quotation response.
- Provide host model, software, port mode and required form factor at both ends.
- Provide route length plus planned connector, panel and splice losses.
- Request wavelength, optical-power, receiver and FEC evidence tied to the exact model.
- During sample validation, retain Tx/Rx power, temperature, pre-FEC and post-FEC indicators, uncorrectable blocks, traffic conditions and test duration.
What this guide does not prove
It does not claim that LR4-6 and LR4-10 are automatically interoperable, or that a longer-reach optic is always a safer substitute. Excess received power, different compliance points, host coding and FEC behavior can all matter. The approved record must name both endpoint modules and the tested channel.
Primary references
- IEEE 802.3df public PMD comparison material — published reach and lane context for 400G PMDs.
- IEEE 802.3cu close report — the 400GBASE-LR4-6 designation.
- 100G Lambda MSA, 400G-LR4-10 Technical Specification Rev. 1.0 — 10 km optical and FEC requirements.