What Is the Difference Between 400GBASE-DR4 and 400GBASE-FR4?

1. What Is 400GBASE-DR4?

400GBASE-DR4 is a 400 Gigabit Ethernet physical-layer interface designed for parallel transmission over single-mode optical fiber. It uses four optical lanes in each transmission direction, with each lane carrying approximately 100Gbps of data using PAM4 signaling.

The four transmit lanes operate in parallel rather than being combined through wavelength-division multiplexing. At the receiving end, four corresponding optical lanes deliver the signals to the receiving circuitry.

Typical characteristics of 400GBASE-DR4 include:

• Aggregate bandwidth: 400Gbps Ethernet connectivity.

• Optical architecture: Four parallel transmit lanes and four parallel receive lanes.

• Nominal wavelength region: Around 1310nm.

• Fiber type: Single-mode fiber.

• Maximum standard reach: Typically 500 meters for compliant implementations.

• Connector: Commonly an MPO-12 interface with eight active fiber positions for transmit and receive.

DR4 is suitable for high-bandwidth data center links where single-mode fiber is available and the required distance fits within its reach specification. Its parallel-fiber design also provides opportunities for supported breakout deployments, such as four 100G links, when the transceiver, host port, cabling, and Ethernet configuration explicitly support the intended arrangement.

For module options, see the C-LIGHT 400G QSFP-DD Optical Transceiver product range, which includes 400GBASE-DR4 configurations.

2. What Is 400GBASE-FR4?

400GBASE-FR4 is a 400 Gigabit Ethernet optical interface designed for single-mode fiber links up to 2 kilometers under the applicable standard. Instead of using four separate spatial optical lanes, FR4 transmits four optical wavelengths over a duplex fiber connection.

Each wavelength carries approximately 100Gbps of data using PAM4 signaling. A wavelength multiplexer combines the four optical channels into one outgoing fiber path, while a demultiplexer separates the channels at the receiving end. The opposite fiber carries traffic in the reverse direction.

Typical characteristics of 400GBASE-FR4 include:

• Aggregate bandwidth: 400Gbps Ethernet connectivity.

• Optical architecture: Four wavelength channels multiplexed in each direction.

• Wavelength plan: Four CWDM wavelengths nominally centered at 1271nm, 1291nm, 1311nm, and 1331nm.

• Fiber type: Single-mode fiber.

• Maximum standard reach: Typically 2 kilometers for compliant implementations.

• Connector: Commonly duplex LC, using one fiber for transmit and one for receive.

FR4 is useful when a network requires more reach than a typical DR4 link can provide, while retaining a duplex single-mode fiber connection. Wavelength multiplexing allows four optical channels to share each directional fiber, reducing the number of fiber strands required between network devices.

See the C-LIGHT 400G QSFP-DD FR4 2km transceiver for a product example with a duplex LC interface.

3. Key Differences Between 400GBASE-DR4 and 400GBASE-FR4

The critical distinction is the optical lane arrangement. DR4 uses multiple fiber paths to carry the parallel signals, while FR4 uses four different wavelengths on the same fiber in each direction. Both deliver the same nominal Ethernet bandwidth over single-mode fiber.

4. Parallel Optics vs. Wavelength Multiplexing

DR4 and FR4 achieve 400Gbps using different optical arrangements. The difference influences transceiver components, fiber connections, and how the network can use existing cabling.

With DR4, each optical lane is carried on its own fiber strand. The transmitter and receiver operate multiple channels simultaneously, and the MPO connector provides the required fiber positions in a compact interface.

With FR4, the four wavelength channels are combined through wavelength-division multiplexing. All four channels travel along the same transmit fiber and are separated at the remote receiver. A second fiber carries the return direction.

This difference also affects fiber infrastructure planning. DR4 requires more active fiber strands between the endpoints, while FR4 can provide the same aggregate bandwidth over a duplex fiber pair. The trade-off is that FR4 requires optical multiplexing and demultiplexing components within the transceiver.

5. Wavelengths and PAM4 Signal Transmission

Both DR4 and FR4 use PAM4 signaling in their standard 400G optical implementations. PAM4 represents two bits per symbol through four signal levels, allowing each optical lane to carry approximately 100Gbps of data without requiring twice the symbol rate of a same-rate NRZ implementation.

Although the two interfaces use the same general modulation family, they are not automatically interoperable at the optical level. A DR4 receiver expects the optical lanes and wavelength characteristics defined by DR4, while an FR4 receiver expects the four-channel wavelength plan and multiplexed optical signal defined by FR4.

Matching the nominal data rate and fiber type is insufficient. The optical standard, wavelength arrangement, connector, link budget, and host configuration must all match the intended application.

6. Transmission Distance and Optical Power Budget

The specified reach is one of the most important reasons to choose between DR4 and FR4. DR4 is designed for links up to 500 meters, while FR4 supports up to 2 kilometers under the applicable Ethernet specifications.

Actual link performance depends on fiber attenuation, connector losses, patch panels, splices, transmitter output power, receiver sensitivity, and the module's allowable optical loss. A module's nominal reach should not be treated as a guarantee when the installed fiber path exceeds the permitted optical budget.

For a link near the maximum supported distance, calculate the complete optical loss budget using the selected module's datasheet and the cable plant specifications. Do not assume that a transceiver will operate reliably just because the fiber length falls below its nominal reach.

7. Fiber Connectivity, MPO and LC Interfaces

DR4 and FR4 typically use different connectors because their optical architectures are different.

400GBASE-DR4: The common interface is MPO-12, with four transmit fibers and four receive fibers. Only eight fiber positions carry the four-lane bidirectional data arrangement. Fiber polarity and lane mapping must match the transceiver and cabling specification.

400GBASE-FR4: The common interface is duplex LC. One fiber carries the four multiplexed optical wavelengths in the transmit direction, and the other carries the receive wavelengths. This reduces the number of active fiber strands required between endpoints.

For DR4 deployments, inspect the MPO/MTP trunk and breakout configuration carefully. For FR4, verify the duplex LC patching path and the single-mode fiber quality. In both cases, connector cleanliness and insertion loss are important at 400GbE speeds.

C-LIGHT provides a range of MPO/MTP fiber connectivity products for parallel-optics network designs.

8. Compatibility, Interoperability and Breakout

Both interfaces can be used in 400G Ethernet networks, but their optical architectures differ. A DR4-to-DR4 link is the natural parallel-optics configuration, while an FR4-to-FR4 link is the natural wavelength-multiplexed configuration. The two module types should not be assumed to communicate directly with one another.

Before deployment, confirm the following:

• Host support: Verify the switch, router, or network adapter supports the exact transceiver type and operating mode.

• Optical standard: Match DR4 with a compatible DR4 link or FR4 with a compatible FR4 link, unless a documented architecture provides an alternative.

• Fiber and connector: Confirm single-mode fiber, the required MPO or LC interface, and correct polarity.

• Link budget: Calculate the expected attenuation through the installed fiber and all connection points.

• FEC and configuration: Follow the applicable Ethernet specification and host configuration requirements.

• Breakout mode: Confirm that the specific module, switch port, and software support the intended split configuration.

DR4's parallel optical architecture can support specific breakout configurations, such as four 100GBASE-DR1 links, when permitted by the module and host platform. FR4's four wavelength channels are multiplexed onto a duplex optical connection, so a similar physical breakout arrangement cannot be assumed without a product-specific design.

For C-LIGHT product configurations, review the 400G QSFP-DD product range and the 400G FR4 2km product information.

9. Cost, Fiber Utilization and Network Design

Choosing DR4 or FR4 involves more than comparing the transceiver price. The total deployment cost includes optics, fiber assemblies, patch panels, installation labor, maintenance, and the possible reuse of the existing cabling plant.

DR4 may be attractive when the fiber plant already supports parallel single-mode connectivity and the reach is within 500 meters. Its architecture can also simplify defined breakout deployments. However, the greater number of active fiber strands may require more complex cabling when existing infrastructure is duplex-based.

FR4 can be attractive for longer links and environments where duplex single-mode fiber is already installed. Four wavelengths share each directional fiber, reducing the fiber count between endpoints. That efficiency relies on a more complex wavelength-multiplexed optical design.

The most economical choice depends on the existing fiber plant and the complete link requirement. A longer-reach module is not automatically the better choice if the link is short and parallel-fiber cabling is already installed. Conversely, FR4 may reduce infrastructure changes when the network already uses duplex single-mode fiber and requires up to 2km reach.

10. Conclusion

400GBASE-DR4 and 400GBASE-FR4 provide 400GbE connectivity over single-mode fiber, but they use different optical transmission architectures. DR4 uses four parallel optical lanes and typically supports up to 500 meters through an MPO-family interface. FR4 uses four CWDM wavelengths combined on a duplex single-mode fiber link and typically supports up to 2 kilometers through duplex LC.

DR4 is suited to shorter links using parallel-fiber infrastructure and may support defined 4 × 100G breakout configurations. FR4 is suited to longer links where two fibers are preferred over multiple parallel strands. Both require compatible optics at each end, the correct cabling, and a verified optical power budget.

For product selection, confirm reach, fiber type, connector, optical standard, and host compatibility before choosing either interface. C-LIGHT offers relevant options in its 400G QSFP-DD transceiver range and its 400G FR4 2km product line.

11. Frequently Asked Questions