SR vs FR

SR and FR are two optical reach classes for Ethernet transceivers. SR stands for Short Reach and supports distances up to 100 meters. FR stands for Far Reach and supports distances up to 2 kilometers. SR uses multimode fiber at 850 nm with VCSELs. FR uses single-mode fiber at 1310 nm with EMLs. Both are defined by IEEE 802.3 for 100G, 200G, 400G, and 800G Ethernet.

The distinction between SR and FR is the fiber type, the wavelength, the light source, the connector, and the reach. SR is optimized for intra-rack and adjacent-rack connections over multimode fiber, where cost per port and port density are the dominant constraints. FR is optimized for campus and building-to-building connections over single-mode fiber, where the reach exceeds what multimode fiber can support and the fiber plant is single-mode.

SR and FR are often deployed in the same network. SR connects servers to Top-of-Rack switches within a rack. FR connects leaf switches to spine switches across a campus or between buildings. The two reach classes are complementary, and the choice between them is determined by the distance and the fiber infrastructure.

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 SR4 is MPO-12; for SR8 it 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 architecture: 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 FR?

FR (Far Reach) is an optical reach class defined in IEEE 802.3 for single-mode fiber transmission using coarse wavelength division multiplexing (CWDM). It uses 1310 nm EMLs and PIN photodetectors. Multiple wavelengths are multiplexed onto a single fiber pair, and each wavelength carries one lane.

FR is defined for multiple data rates. 100GBASE-FR1 uses one wavelength of 100G PAM4 over a single fiber pair, reaching 2 kilometers. 400GBASE-FR4 uses four wavelengths of 100G PAM4 over a single fiber pair, reaching 2 kilometers. 800GBASE-FR4 uses eight wavelengths of 100G PAM4 over a single fiber pair, or four wavelengths of 200G PAM4, reaching 2 kilometers. The connector for FR4 is duplex LC.

2.1 FR Characteristics

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

• Wavelength: 1310 nm CWDM (1271, 1291, 1311, 1331 nm for FR4).

• Light source: EML.

• Reach: 2 kilometers.

• Connector: Duplex LC.

• Optical architecture: Wavelength division multiplexing, one wavelength per lane.

• Fiber count per 400G link: 2 fibers (1 Tx + 1 Rx).

• Fiber count per 800G link: 2 fibers (1 Tx + 1 Rx).

3. Fiber Type and Reach

The most visible difference between SR and FR is the fiber type and the reach each supports. SR uses multimode fiber and reaches 100 meters. FR uses single-mode fiber and reaches 2 kilometers. 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. FR extends the reach to 2 kilometers, covering building-to-building connections within a campus and leaf-to-spine connections where the distance exceeds the multimode limit.

4. Wavelength and Light Source

SR uses 850 nm VCSELs. FR uses 1310 nm EMLs. The wavelength and light source differ because of the fiber type and the reach requirement.

4.1 SR Wavelength and Light Source

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 FR Wavelength and Light Source

FR uses 1310 nm EMLs. EMLs integrate a DFB laser with an electro-absorption modulator on the same chip, providing high extinction ratio and low chirp. Each lane is an externally modulated laser, and the receiver is a PIN photodiode. FR uses CWDM wavelengths: 1271, 1291, 1311, and 1331 nm for FR4. Each wavelength carries 100G PAM4, and the four wavelengths are multiplexed onto a single fiber pair using a CWDM multiplexer inside the module.

5. Connector and Fiber Count

SR uses parallel fibers and an MPO connector. FR uses a single fiber pair and a duplex LC connector. The fiber count per link differs significantly.

FR is significantly more fiber-efficient than SR. A 400G FR4 link uses two fibers, while a 400G SR8 link uses sixteen fibers. An 800G FR4 link uses two fibers, while an 800G SR8 link uses sixteen fibers. In fiber-constrained environments, FR allows more links to be deployed on the same fiber cable.

6. Optical Architecture and Signal Modulation

SR uses parallel optics with one VCSEL per lane. FR uses wavelength division multiplexing with one wavelength per lane. The per-lane rate and the modulation format differ between the two.

6.1 SR Optical Architecture

SR uses parallel optics. Each lane has its own VCSEL and photodiode. 400GBASE-SR8 uses eight lanes of 50G PAM4, each on a separate fiber. 800GBASE-SR8 uses eight lanes of 100G PAM4, each on a separate fiber. The module contains eight laser drivers, VCSELs, photodiodes, and transimpedance amplifiers.

6.2 FR Optical Architecture

FR uses wavelength division multiplexing. 400GBASE-FR4 uses four wavelengths, each carrying 100G PAM4, multiplexed onto a single fiber pair. The module contains four EMLs, a CWDM multiplexer, a CWDM demultiplexer, and four photodiodes. 800GBASE-FR4 uses eight wavelengths of 100G PAM4 or four wavelengths of 200G PAM4, multiplexed onto a single fiber pair.

7. Power Budget and Reach

SR supports 100 meters, and FR supports 2 kilometers. The reach difference is driven by the fiber type and the optical power budget. Multimode fiber has higher attenuation and modal dispersion, which limits the reach. Single-mode fiber has lower attenuation and no modal dispersion, which allows longer reaches.

FR has a higher power budget than SR because the link must span 2 kilometers instead of 100 meters. The additional 5 to 6 dB of budget is provided by higher transmitter output power and better receiver sensitivity. The CWDM multiplexer in FR4 also has insertion loss, which is accounted for in the module specifications.

8. Power Consumption and Thermal

Power consumption differs between SR and FR because of the laser technology and the number of lanes. SR uses uncooled VCSELs, which are low-power. FR uses EMLs at 1310 nm, which require more drive current and more complex bias control.

FR modules consume slightly more power than SR modules of the same data rate. The higher power consumption is driven by the EML lasers and the CWDM multiplexer. In high-density switches, the additional power and thermal load must be included in the thermal design.

9. Application Scenarios

SR and FR serve different segments of the data center and campus network. The choice between them is determined by the link distance and the fiber infrastructure.

9.1 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.

9.2 FR Application Scenarios

• Campus building-to-building: Distances up to 2 kilometers.

• Data center interconnect: Between buildings on a campus.

• Metro DCI: Short metro links up to 2 kilometers.

• Fiber-constrained environments: Where the fiber count per link must be minimized.

• Longer leaf-to-spine runs: Where the distance exceeds 100 meters but is under 2 kilometers.

10. Cost Structure

The cost structure of SR and FR differs in module cost, fiber cost, and connector cost. SR modules are less expensive because VCSELs are low-cost and uncooled. FR modules are more expensive because 1310 nm EMLs and CWDM multiplexers cost more. However, FR uses fewer fibers per link, which reduces the fiber and connector cost in fiber-constrained environments.

The cost comparison depends on the deployment scale and the fiber plant. In a data center with abundant multimode fiber, SR is the lower-cost option because the modules are cheaper and the fiber is already installed. In a campus or metro environment where fiber is scarce or expensive to install, FR is the lower-cost option because it uses only two fibers per link, reducing the fiber and connector cost.

11. Standard and Ecosystem

SR and FR 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). FR is defined in 802.3cu (100GBASE-FR1, 400GBASE-FR4) and 802.3df (800GBASE-FR4).

12. Comparison Summary

13. Selection Framework

14. Common Misconceptions

• "SR and FR can be mixed on the same link." False. SR requires multimode fiber, and FR requires single-mode fiber. The two fiber types and wavelengths are incompatible. A link must use either SR or FR, not both.

• "FR is always more expensive than SR." Not entirely true. FR modules cost more than SR modules, but FR uses only two fibers per link while SR uses eight or sixteen. In fiber-scarce or high-installation-cost environments, the total cost of ownership for FR can be lower than for SR.

• "SR can reach 2 km." False. SR is rated for 100 meters. For links beyond 100 meters, FR or DR or LR modules are required.

• "FR uses four fibers for 400G." False. 400GBASE-FR4 uses a single fiber pair (two fibers), with four wavelengths multiplexed onto the same fiber. One fiber carries the transmit direction, and the other carries the receive direction.

• "FR replaces SR." False. SR and FR serve different distance segments. SR is used for intra-rack and adjacent-rack connections under 100 meters, while FR is used for campus and building-to-building connections between 100 meters and 2 kilometers. The two coexist in data center and campus networks.

15. Summary

SR and FR are two IEEE-defined optical reach classes for Ethernet transceivers. SR uses multimode fiber at 850 nm with VCSELs, reaching 100 meters. FR uses single-mode fiber at 1310 nm with EMLs, reaching 2 kilometers. Both support 100G, 200G, 400G, and 800G Ethernet.

The choice between SR and FR is determined by the link distance and the fiber plant. SR is the lower-cost option for intra-rack and adjacent-rack connections under 100 meters, where the module cost dominates and the fiber plant is multimode. FR is the appropriate option for campus and building-to-building connections between 100 meters and 2 kilometers, where the fiber count per link must be minimized and the duplex LC infrastructure is available.

SR and FR are not competing technologies. They are complementary reach classes that serve different segments of the data center and campus network. The most efficient network design uses SR where the distance is under 100 meters and the fiber plant supports MPO connections, and FR where the distance exceeds 100 meters and the fiber plant is built around duplex LC connections.

16. Q&A