Selection Guide for Data Center 400G Optical Modules

Driven by the rapid development of AI training clusters, hyperscale cloud data centers, and 5G core networks, 400G Ethernet interfaces have become the standard for next-generation networks. When selecting modules, the choice between form factors (QSFP-DD vs. OSFP) and optical interface types (such as SR4, DR4, or FR4) significantly impacts network performance, power consumption, cost, and future scalability. This article outlines the selection logic for 400G optical modules based on application scenarios and provides specific deployment recommendations.

1. Two Mainstream Form Factors: QSFP-DD and OSFP

Currently, there are two mainstream physical form factor standards for 400G optical modules, each with its own established ecosystem. QSFP-DD (Quad Small Form-factor Pluggable Double Density) shares the same physical dimensions as QSFP28/QSFP+ modules; it integrates eight high-speed electrical lanes into a single module to deliver a total bandwidth of 400G. It is backward compatible with QSFP28/QSFP+ cages—meaning 100G modules can be installed in 400G ports—thereby protecting customer investment. This standard is primarily driven by data center switch vendors (such as Arista, Cisco, Juniper, H3C, and Huawei) and holds the largest market share.

OSFP (Octal Small Form-factor Pluggable) is slightly larger than QSFP-DD. While it also features eight internal lanes and a larger footprint, this design offers a greater surface area for heat dissipation; it was used as a transitional solution in early 400G switches. It is not directly compatible with QSFP28 modules and requires an OSFP-to-QSFP28 adapter. Initially promoted by vendors like Arista, OSFP interfaces are still utilized in some AI cluster network interface cards (NICs) and switches. Recommendation: When building new data center switch interconnects, opt for QSFP-DD switches, as they offer the best ecosystem support and the widest range of available modules. When server network interface cards (NICs) or certain GPU clusters utilize OSFP interfaces (such as early NVIDIA 400G NICs), OSFP modules can be selected; these can subsequently be integrated into QSFP-DD networks using OSFP-to-QSFP-DD adapter modules.

2. Detailed Overview of QSFP-DD Form Factor Optical Modules

Based on transmission distance, fiber type, and cost, there are four primary types of QSFP-DD 400G optical modules:

2.1 SR4: Short-range multimode; most cost-effective

SR4 (Short Range 4) is based on VCSEL (Vertical-Cavity Surface-Emitting Laser) arrays and employs four pairs of multimode fibers for parallel transmission, with each pair operating at 53.125 Gbps PAM4. Wavelength: 850 nm. Fiber: OM3/OM4/OM5 multimode fiber; MPO-12/APC or MPO-16 connectors (QSFP-DD SR4 typically uses MPO-12). Transmission distance: up to 70 m (OM3), 100 m (OM4), and 150 m (OM5). Power consumption: approximately 8–10 W.

Typical use cases:

Short-range interconnects between racks or between ToR (Top-of-Rack) and Leaf switches within the same data center hall. Suitable for budget-sensitive scenarios where distance requirements are met; offers the lowest overall link cost.

2.2 DR4: Medium-range single-mode; the preferred choice for high density

DR4 (Datacenter Reach 4) utilizes four parallel single-mode fiber lanes, with each channel operating at 100G. It features a center wavelength of 1310 nm and uses MPO-12/APC connectors. It supports a transmission distance of 500 meters over standard single-mode fiber, with a power consumption of approximately 9–10 W.

Typical use cases:

Inter-row and inter-hall connections within the data center; the 500-meter range is sufficient to cover the vast majority of medium-to-large data centers. It also supports breakout configurations into four 100G-DR single-mode interfaces, thereby protecting investments in existing 100G single-mode networks.

2.3 FR4: Single-mode, long-reach solution ideal for inter-building connectivity

FR4 (Future Reach 4) utilizes Coarse Wavelength Division Multiplexing (CWDM4) technology to multiplex four 100G optical signals at different wavelengths onto a single pair of single-mode fibers for transmission. It operates at wavelengths of 1271nm, 1291nm, 1311nm, and 1331nm (LAN-WDM), employs a single-mode duplex LC interface, supports a transmission distance of 2km over standard single-mode fiber, and has a power consumption of approximately 10–12W.

Typical use cases:

Data Center Interconnect (DCI) between different buildings within a campus, or connections between data centers and metro edge nodes.

3. Detailed overview of OSFP-packaged optical modules

Like the QSFP-DD form factor, SR4 and DR4 variants are also available in the OSFP form factor. Their key technical specifications match those of the QSFP-DD versions; the only differences lie in physical dimensions and electrical interface layout.

OSFP SR4: Uses an MPO-12 multimode interface and supports transmission distances of up to 100 meters. Commonly used with NVIDIA Spectrum switches or ConnectX-6/7 network adapters.

OSFP DR4: Uses an MPO-12 single-mode interface and supports transmission distances of up to 500 meters. It also supports breakout configurations into four 100G OSFP or QSFP28 modules.

Note that inserting an OSFP module into a QSFP-DD cage requires a mechanical adapter; however, the electrical connection also requires the switch to support this mode. Generally, it is recommended to match the module form factor with the switch port form factor to avoid signal integrity issues introduced by adapters.

4. Selection decision matrix for different models

To help engineers quickly identify the appropriate 400G optical module, this article outlines a decision-making path based on the following three dimensions:

Power Consumption and Thermal Considerations:

SR4 and DR4 modules have comparable power consumption, both remaining under 10W; FR4 modules consume slightly more power due to the inclusion of WDM (Wavelength Division Multiplexing) components. In high-density QSFP-DD switches, a single board with 32 fully loaded ports can exceed 300W in total power consumption, necessitating adequate cabinet cooling capabilities. For the "All-to-All" communication patterns common in AI clusters, the thermal stability of optical modules also impacts bit error rates and training efficiency.

Compatibility Note:

When selecting optical modules, it is essential to verify firmware compatibility with the target switches or network interface cards (NICs). Leading third-party module vendors (such as YXFiber) adapt their firmware for equipment from manufacturers like Arista, Cisco, and H3C to ensure plug-and-play functionality and support for CMIS 4.0 digital diagnostic monitoring, thereby facilitating efficient operations and maintenance.

Conclusion

Selecting 400G optical modules essentially involves balancing distance, fiber type, and cost. The SR4, DR4, and FR4 solutions address connectivity needs ranging from tens of meters to two kilometers—covering both intra-data center and campus interconnect scenarios—while the QSFP-DD and OSFP form factors cater to different equipment ecosystems. Understanding the technical characteristics and connectivity schemes of each module type enables the selection of the most cost-effective solution for real-world projects, laying a solid foundation for high-speed interconnectivity in the AI and cloud era.