SR vs DR

SR and DR are two optical reach classes used in parallel single-lane and multi-lane transceivers. SR stands for Short Reach and uses multimode fiber at 850 nm for distances up to 100 meters. DR stands for Data center Reach and uses single-mode fiber at 1310 nm for distances up to 500 meters. Both are IEEE-defined interfaces for 100G, 200G, 400G, and 800G Ethernet, and both use parallel optics with multiple lanes operating simultaneously.

The distinction between SR and DR is not a matter of one being faster than the other. It is a matter of fiber type, reach, optical engine technology, connector interface, and the application segment each serves. SR is optimized for intra-rack and adjacent-rack connections over multimode fiber, where cost per port and port density are the dominant constraints. DR is optimized for rack-to-rack and row-to-row connections over single-mode fiber, where reach beyond the multimode limit is required and the fiber plant is single-mode.

SR and DR are often deployed together in the same data center fabric. SR connects servers to Top-of-Rack switches within a rack. DR connects leaf switches to spine switches across rows, or GPU nodes to leaf switches in AI clusters where the distance exceeds what multimode fiber can support. The two reach classes are complementary, and the choice between them is determined by the fiber plant, the distance requirement, and the cost structure of each link.

1. What Is SR?

SR (Short Reach) is an optical reach class defined in IEEE 802.3 for multimode fiber transmission. It uses 850 nm VCSELs (vertical-cavity surface-emitting lasers) and PIN photodetectors. The interface is parallel optics: multiple lanes, each with its own transmit and receive fiber, operating simultaneously.

SR is defined for multiple data rates. 100GBASE-SR4 uses four lanes of 25G NRZ over eight multimode fibers at 850 nm, reaching 70 meters over OM3 and 100 meters over OM4. 400GBASE-SR8 uses eight lanes of 50G PAM4 over sixteen multimode fibers, reaching 100 meters over OM4. 800GBASE-SR8 uses eight lanes of 100G PAM4 over sixteen fibers, also reaching 100 meters over OM4. The connector for SR8 is MPO-16 or dual MPO-12.

1.1 SR Characteristics

• Fiber type: Multimode fiber (OM3, OM4, OM5).

• Wavelength: 850 nm.

• Light source: VCSEL.

• Reach: 70–100 meters, depending on fiber grade and data rate.

• Connector: MPO-12 or MPO-16.

• Optical engine: Parallel optics with one VCSEL and one PIN per lane.

• Power consumption: Lower than single-mode equivalents.

• Cost: Lower transceiver cost, higher fiber cost per meter.

2. What Is DR?

DR (Data center Reach) is an optical reach class defined in IEEE 802.3 for single-mode fiber transmission. It uses 1310 nm EMLs (electro-absorption modulated lasers) or DMLs (directly modulated lasers) and PIN photodetectors. Like SR, DR uses parallel optics, but the transmission is over single-mode fiber.

DR is defined for multiple data rates. 100GBASE-DR4 uses four lanes of 25G NRZ over eight single-mode fibers at 1310 nm, reaching 500 meters. 400GBASE-DR4 uses four lanes of 100G PAM4 over eight single-mode fibers, also reaching 500 meters. 800GBASE-DR8 uses eight lanes of 100G PAM4 over sixteen single-mode fibers, reaching 500 meters. The connector for DR4 is MPO-12; for DR8 it is MPO-16.

2.1 DR Characteristics

• Fiber type: Single-mode fiber (OS2).

• Wavelength: 1310 nm.

• Light source: EML or DML.

• Reach: 500 meters.

• Connector: MPO-12 (DR4) or MPO-16 (DR8).

• Optical engine: Parallel optics with one laser and one PIN per lane.

• Power consumption: Moderate; higher than SR but lower than FR/LR.

• Cost: Higher transceiver cost, lower fiber cost per meter.

3. Fiber Type and Reach

The most visible difference between SR and DR is the fiber type and the reach each supports. SR uses multimode fiber and reaches 100 meters. DR uses single-mode fiber and reaches 500 meters. The reach difference is driven by the fiber itself: multimode fiber suffers modal dispersion, which limits the bandwidth-distance product, while single-mode fiber has no modal dispersion and can reach much farther.

The reach difference determines the application segment. SR is limited to connections within a rack or between immediately adjacent racks, where the distance is under 100 meters. DR extends the reach to 500 meters, covering leaf-to-spine connections within a row or between adjacent rows, and GPU-to-leaf connections in AI clusters where the rack-to-rack distance exceeds the multimode limit.

4. Optical Architecture and Signal Modulation

Both SR and DR use parallel optics. Each lane has its own optical transmitter and receiver, and the lanes operate simultaneously. The difference is the light source and the modulation format.

4.1 SR Optical Architecture

SR uses 850 nm VCSELs. VCSELs emit light perpendicular to the chip surface, which allows on-wafer testing and low-cost manufacturing. They operate uncooled over a wide temperature range. Each lane is a direct-modulated VCSEL, and the receiver is a PIN photodiode. SR uses NRZ modulation at 25G per lane and PAM4 modulation at 50G or 100G per lane.

4.2 DR Optical Architecture

DR uses 1310 nm EMLs or DMLs. EMLs integrate a DFB laser with an electro-absorption modulator on the same chip, providing high extinction ratio and low chirp. DMLs are simpler and lower cost but have higher chirp. Each lane is an externally or directly modulated laser, and the receiver is a PIN photodiode. DR uses NRZ modulation at 25G per lane and PAM4 modulation at 100G per lane.

5. Connector and Fiber Count

Both SR and DR use MPO connectors for parallel optics. The fiber count per module depends on the number of lanes and the data rate.

The fiber count per link is determined by the number of lanes. 400GBASE-DR4 uses four lanes and consumes eight fibers; 400GBASE-SR8 uses eight lanes and consumes sixteen fibers. This difference in fiber count affects the fiber plant design: DR4 is more fiber-efficient than SR8 for the same aggregate data rate, because it uses higher per-lane modulation (100G PAM4 vs 50G PAM4) and fewer lanes.

6. Power Consumption and Thermal

Power consumption differs between SR and DR because of the laser technology and the modulation format. VCSELs are uncooled and low-power; EMLs and DMLs at 1310 nm are also uncooled in DR modules but require more drive current and more complex bias control.

800G DR8 modules are available in both QSFP-DD and OSFP form factors. QSFP-DD provides higher port density and backward compatibility with QSFP28. OSFP is larger, has an integrated heat sink, and is used in platforms where higher power and thermal dissipation are required. SR8 modules are available in both form factors as well, with QSFP-DD being the natural choice for SR8 because its power draw stays within QSFP-DD thermal limits.

7. Application Scenarios

SR and DR serve different segments of the data center network. The choice between them is determined by the distance and the fiber plant.

7.1 SR Application Scenarios

• Server-to-ToR: Within-rack connections under 100 meters.

• GPU-to-leaf in AI clusters: Short-reach connections within a rack or adjacent rack.

• Storage area networks: Short-reach SAN connectivity over multimode fiber.

• InfiniBand NDR: 400G SR4 for HDR/NDR InfiniBand within a rack.

• Existing multimode fiber plant: Where OM3/OM4/OM5 fiber is already installed.

7.2 DR Application Scenarios

• Leaf-to-spine: Rack-to-rack or row-to-row connections up to 500 meters.

• AI cluster GPU-to-leaf: Where the distance exceeds the multimode limit.

• Spine-to-super-spine: Within a single data hall over single-mode fiber.

• Existing single-mode fiber plant: Where OS2 fiber is already installed.

• Campus building-to-building: Short-reach inter-building links under 500 meters.

8. Cost Structure

The cost structure of SR and DR differs in module cost, fiber cost, and connector cost. SR modules are less expensive because VCSELs are low-cost and uncooled. DR modules are more expensive because 1310 nm EMLs and DMLs cost more than VCSELs. However, the fiber cost per meter is lower for single-mode fiber than multimode fiber, and the fiber count per link may be lower for DR4 than SR8 at 400G, which affects the total cost of ownership.

The cost comparison depends on the deployment scale and the existing fiber plant. In a data center already built with multimode fiber, SR is the lower-cost option because the fiber is already installed. In a data center built with single-mode fiber, DR is the natural choice because it uses the existing fiber. In greenfield deployments, single-mode fiber is increasingly the default choice because it supports DR, FR, and LR modules and has a longer upgrade path.

9. Standard and Ecosystem

SR and DR are both IEEE 802.3 standards. SR is defined in 802.3ae (10GBASE-SR), 802.3bm (40GBASE-SR4, 100GBASE-SR4), 802.3cd (50GBASE-SR, 100GBASE-SR2, 200GBASE-SR4), and 802.3bs (400GBASE-SR16, later 400GBASE-SR8). DR is defined in 802.3bs (200GBASE-DR4, 400GBASE-DR4) and 802.3cd (100GBASE-DR).

10. Comparison Summary

11. Selection Framework

12. 常见误区

• “DR is faster than SR.” 错误。DR 和 SR 是传输距离等级，而非速率等级。100G SR4 和 100G DR4 都提供 100 Gbps，400G SR8 和 400G DR4 都提供 400 Gbps。区别在于光纤类型和传输距离。

• “SR can reach 500 meters.” 错误。SR 基于多模光纤，最长距离为 100 米（OM4）。超过 100 米需要 DR、FR 或 LR 等单模光纤模块。

• “DR is always more expensive.” 不完全正确。DR 模块比 SR 模块贵，但 DR 使用的单模光纤每米成本更低，且 DR4 比 SR8 使用更少的光纤（8 根 vs 16 根）。在大规模部署中，总拥有成本可能因光纤和连接器节省而接近或低于 SR。

• “SR and DR can be mixed on the same link.” 错误。SR 模块需要多模光纤，DR 模块需要单模光纤。两者不能在同一链路中混用，因为光纤类型和波长都不兼容。

• “DR replaces SR.” 错误。SR 和 DR 服务于不同的距离段。SR 仍然是机架内连接的首选，因为其模块成本更低。DR 用于超过 100 米的连接。两者在数据中心中互补共存。

13. Summary

SR and DR are two IEEE-defined optical reach classes for parallel single-lane and multi-lane transceivers. SR uses multimode fiber at 850 nm with VCSELs, reaching 100 meters. DR uses single-mode fiber at 1310 nm with EMLs or DMLs, reaching 500 meters. Both use parallel optics with MPO connectors, and both support 100G, 200G, 400G, and 800G Ethernet.

The choice between SR and DR is determined by the distance and the fiber plant. SR is the lower-cost option for short-reach intra-rack and adjacent-rack links over multimode fiber. DR is the appropriate option for rack-to-rack and row-to-row links up to 500 meters over single-mode fiber. In AI clusters, SR handles GPU-to-leaf connections within a rack, while DR handles the cross-rack connections that exceed the multimode reach limit.

SR and DR are not competing technologies. They are complementary reach classes that serve different segments of the data center fabric. The most efficient network design uses SR where multimode fiber is available and the distance is under 100 meters, and DR where single-mode fiber is available and the distance is between 100 and 500 meters.

14. Q&A