In the wave of evolution towards 400G Ethernet, FR4 and LR4 are two crucial optical module standards. While both are 400G modules with QSFP-DD and OSFP form factors available, they represent technical paths optimized for drastically different application scenarios. Understanding their core differences is essential for building efficient and cost-effective data center networks.
Core Differences: Fundamental Divergence in Distance and Wavelength
The essential difference between the two stems from their design goals: FR4 is designed for interconnection within data centers within a 2km radius, while LR4 targets metropolitan area network access beyond 10km.
This difference in goals directly drives the divergence in their technical implementations:
Transmission Distance: This is the most intuitive distinction. The standard transmission distance for 400G FR4 is 2km, primarily covering interconnections within data center campuses (such as between different buildings) or large data centers. 400G LR4, on the other hand, targets a distance of 10km, aiming to connect campuses further apart or enable access from data centers to metropolitan area networks.
Wavelength and Multiplexing Technology: Both employ wavelength division multiplexing (WDM) technology to transmit four 100G signals in parallel over a single fiber to achieve a total bandwidth of 400G, but they use different wavelength bands.
400G LR4 uses the traditional LAN WDM band with a wavelength spacing of approximately 800GHz, located near the O-band. This is a mature technology that has been validated over a long period by the LR4 family (derived from 100G), but the laser cost is relatively high.
400G FR4, on the other hand, uses the CWDM band with a wider wavelength spacing (approximately 20nm). CWDM lasers have lower temperature control requirements and simpler device fabrication processes, making them key to achieving low cost.
Typical Application Scenarios Differences
When choosing FR4: You are building or upgrading a spine-leaf network backbone within a large data center, or connecting buildings within the same campus within a 2km radius. The core requirement is to achieve the lowest cost per bit and higher port density (as its power consumption is typically slightly lower than LR4) while maintaining performance.
When choosing LR4: You need to perform long-distance interconnects across campuses, data center interconnects (DCI), or connect to the network edge of a telecom operator. In this case, distance is a rigid requirement, cost is secondary, and reliability and long-distance transmission capabilities become the primary considerations.
Selection Decision Guide: More Than Just Distance
When faced with choices, engineers should follow this decision-making factors:
First, check the physical distance: This is the first filter. For distances exceeding 2km, FR4 is ineffective; LR4 is the only standard option.
Conducting Total Cost of Ownership (TCO) Analysis: For links within 2 kilometers, even using single-mode fiber, the price advantage and slightly lower power consumption of FR4 modules often result in a lower overall TCO than LR4. This is a key reason why FR4 has become increasingly popular in data centers in recent years.
Considering Future Evolution: Choosing LR4 means gaining the technological foundation for longer distances (upgradable to ER4, ZR4, etc. in the future). FR4, on the other hand, is the endpoint technology optimized for high-density, short-distance interconnects.
Summary and Outlook
In short, 400G FR4 vs LR4 is not a competition of superiority or inferiority, but rather a complementary partnership. FR4 is the cost-effective “blood vessels” within the data center, while LR4 is the “arteries” connecting to the outside world.
With technological advancements, particularly the maturation of silicon photonics and the penetration of coherent optics into short-range applications, future 800G and higher speed standards may further blur the boundaries between cost and distance. However, for current 400G deployments, a clear understanding of the differences between FR4 and LR4 in terms of distance, cost, and technological origins remains the cornerstone of making informed network investment decisions. A wise choice begins with the most accurate measurement of one’s own needs—whether geographically or financially.
