IEEE 802.3cu Task Force
Public 400GBASE-FR4 work defines the 2 km, four-wavelength single-mode optical path.
400G OPTICS · DR4 VS FR4
400G DR4 vs FR4: Fiber, Connector and Reach Tradeoffs
Both are 400G single-mode optical paths, but they use different physical architectures. DR4 is a parallel-fiber design; FR4 wavelength-multiplexes four optical lanes onto a duplex fiber pair.
DR4Parallel SMF · 500 m class
FR44-wavelength WDM · 2 km class
FiberMulti-fiber vs duplex pair
DecisionReach + fiber plant + breakout + platform
Architecture Comparison
The same 400G rate can require a completely different fiber plant
400G DR4
- Four parallel 100G-class optical lanes over single-mode fiber.
- IEEE 400GBASE-DR4 reach is 500 m class.
- Current Cisco QSFP112 implementations use MPO-12 APC.
- Parallel architecture can support documented 4×100G DR1 breakout use cases.
- Fiber count and polarity become part of the deployment design.
400G FR4
- Four optical wavelengths are multiplexed onto a duplex single-mode fiber pair.
- IEEE 400GBASE-FR4 reach is 2 km class.
- Current Cisco QSFP112 implementations use duplex LC UPC.
- Lower fiber count can simplify structured cabling for longer links.
- Do not assume the same breakout behavior as a parallel DR4 design.
Connector boundary: MPO-12 for DR4 and duplex LC for FR4 are current mainstream implementations, but procurement must still follow the exact module PN and platform compatibility matrix.
Decision Matrix
Choose from the installed fiber plant and topology
| Question | DR4 tends to fit when… | FR4 tends to fit when… |
|---|---|---|
| Reach | The link is inside the 500 m class | The design needs up to the 2 km class |
| Fiber plant | Parallel SMF is available or planned | Duplex SMF is preferred or already deployed |
| Breakout | Parallel-lane breakout is a documented requirement | Native 400G duplex transport is the priority |
| Operations | MPO polarity and multi-fiber handling are controlled | Duplex connector operations are preferred |
| Selection | Confirm host platform, exact PN, connector, FEC, firmware and interoperability before volume purchase. | |
Evidence Sources
Standards baseline plus current product implementation
Cisco 400G QSFP Transceiver Data
Current data lists DR4 as MPO-12 parallel SMF / 500 m and FR4 as duplex LC SMF / 2 km.
Need to choose DR4 or FR4 for a live BOM?
Send the switch / NIC, reach, existing fiber, connector preference, breakout requirement, quantity and target date.