As data centers and transport networks evolve to adopt 400G Ethernet, many architects face the challenge of breaking out 400G QSFP-DD ports into multiple 100G links, especially when connecting to existing 100G NRZ-based infrastructure. This guide walks through all validated breakout options, explains key modulation differences, and proposes the right architecture depending on distance, compatibility, and budget.
PAM4 vs NRZ: The Modulation Wall
At the heart of breakout interoperability is the modulation mismatch between modern and legacy optics. Understanding this distinction is critical when planning your 400G to 100G breakout strategy.
| Modulation | Used In | Lane Speed | Compatibility |
|---|---|---|---|
| NRZ | Most 100G legacy optics (SR4, LR4, CWDM4, ER4L) | 25G/lane | Incompatible with PAM4 |
| PAM4 | All 400G QSFP-DD optics (DR4, FR4, SR8) and 100G DR1/FR1 | 50G or 100G/lane | PAM4 family only |
Important: You cannot passively connect PAM4 to NRZ. You need either modulation conversion or optics with matching modulation on both sides.
Breakout Options Overview
Below is a comprehensive overview of all validated breakout options available for 400G QSFP-DD to 100G connectivity.
| # | 400G Module | Breakout To | Cabling | Max Reach | Status |
|---|---|---|---|---|---|
| 1 | 400G-SRBD (BiDi) | 4x 100G-SRBD/SR1.2 | MPO to 4x LC | 100m | Correct |
| 2 | 400G-SR8 | 2x 100G-SR4 | MPO-16 to 2x MPO-8 | 100m | BW loss |
| 3 | 400G-DR4 | 4x 100G-DR1 | MPO to 4x LC | 500m | Standard |
| 4 | 400G-DR4+ / xDR4 | 4x 100G-FR1 | MPO to 4x LC | 2 km | Longer reach |
| 5 | 400G-2FR4 | 2x 100G-CWDM4 | Duplex LC | 2 km | BW loss |
| 6 | 400G-PLR4 | 4x 100G-LR1 | MPO to 4x LC | 10 km | Long-reach |
| 7 | 4x SN DR4/FR4/PLR4 | 4x DR1/FR1/LR1 | SN to LC (x4) | 500m - 10 km | Compact |
| 8 | 400G AOC (PAM4) | 4x 100G QSFP28 NRZ | Fixed AOC | 3 - 10 m | No gearbox |
| 9 | 400G AOC w/ Gearbox | 4x 100G NRZ | Fixed AOC | 1 - 20 m | Works |
| 10 | Passive DAC | 2x 100G PAM4 | Fixed Cable | 1 - 3 m | PAM4 only |
| 11 | Active Breakout Module | 4x 100G CWDM4/LR4 | SN or MPO32 | Up to 10 km | PAM4-NRZ |
| 12 | Muxponder Breakout | 4x 100G-ER4L | Any | 30 - 40 km | Best reach |
Key Use Case Scenarios
Selecting the right breakout option depends on your distance requirements, existing infrastructure, and whether you need to support legacy NRZ equipment.
Short Reach
Up to 10 km - Modern PAM4 Networks- Direct DR4 to DR1 or PLR4 to LR1 breakout
- All optics are PAM4 single lambda
- No active conversion needed
Medium Reach
10 - 15 km - Legacy Compatibility- Active breakout modules (PAM4 to NRZ)
- Support CWDM4/LR4 optics
- Max ~12 km power budget
Long Reach
20 - 40 km - Legacy NRZ (ER4L)- Muxponder converts 400G PAM4 to 4x 100G NRZ
- ER4L optics rated for 30 km+
- Clean electrical separation
Modulation Compatibility Summary
Use this quick reference to understand which combinations require active conversion and which work with passive breakout cables.
| 400G Side | 100G Side | Passive Breakout | Conversion? | Works? |
|---|---|---|---|---|
| DR4 (PAM4) | DR1/FR1/LR1 (PAM4) | Yes | No | Yes |
| DR4 | LR4/CWDM4/ER4L (NRZ) | No | Active needed | Via muxponder |
| SR8 | SR4 (NRZ) | 200G only | N/A | BW loss |
| Any | AOC/DAC (NRZ) | No | Yes | Active cable |
Match the Option to Your Application
Use this summary table to quickly identify the best solution for your target distance and optics requirements.
Quick Reference Guide
| Target Distance | 100G Optics | Recommended Solution |
|---|---|---|
| < 500m | DR1 / SR1.2 | Passive DR4 or SRBD breakout |
| 1 - 2 km | FR1 / CWDM4 | Passive PAM4 or active conversion |
| 10 km | LR1 / LR4 | PLR4 passive or AOE |
| 25 - 40 km | ER4L | Muxponder with NRZ output |
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