FR vs LR

FR and LR are two optical reach classes for single-mode fiber transceivers. FR stands for Far Reach and supports distances up to 2 kilometers. LR stands for Long Reach and supports distances up to 10 kilometers. Both operate in the 1310 nm window, both use single-mode fiber, and both are defined by IEEE 802.3 for 100G, 200G, 400G, and 800G Ethernet.

The distinction between FR and LR is not the fiber type—both use single-mode fiber—but the wavelength plan, the optical power budget, and the reach each supports. FR uses CWDM wavelengths in the 1271–1331 nm range, multiplexed onto a single fiber pair. LR uses LAN-WDM wavelengths in the 1295–1309 nm range for multi-lane interfaces, or a single 1310 nm wavelength for single-lane interfaces. The tighter wavelength spacing of LAN-WDM requires more precise laser control and a different multiplexer design than CWDM.

FR is optimized for campus and building-to-building connections up to 2 kilometers, where the fiber plant is single-mode and the link distance exceeds what DR can support. LR is optimized for longer campus and metro connections up to 10 kilometers, where the additional power budget is required. Both reach classes are deployed in data center interconnect and campus networks, and the choice between them depends on the link distance and the fiber infrastructure.

1. 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.

1.1 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).

2. What Is LR?

LR (Long Reach) is an optical reach class defined in IEEE 802.3 for single-mode fiber transmission. It uses 1310 nm EMLs and PIN photodetectors. LR is defined in two variants: single-lane LR1, which uses a single 1310 nm wavelength, and multi-lane LR4, which uses four LAN-WDM wavelengths multiplexed onto a single fiber pair.

LR is defined for multiple data rates. 100GBASE-LR1 uses one wavelength of 100G PAM4 over a single fiber pair, reaching 10 kilometers. 100GBASE-LR4 uses four wavelengths of 25G NRZ over a single fiber pair, reaching 10 kilometers. 400GBASE-LR4 uses four wavelengths of 100G PAM4 over a single fiber pair, reaching 10 kilometers. 800GBASE-LR4 uses four wavelengths of 200G PAM4 over a single fiber pair, reaching 10 kilometers. The connector for LR4 is duplex LC.

2.1 LR Characteristics

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

• Wavelength: 1310 nm (LR1) or 1295–1309 nm LAN-WDM (LR4).

• Light source: EML.

• Reach: 10 kilometers.

• Connector: Duplex LC.

• Optical architecture: Single wavelength (LR1) or LAN-WDM (LR4).

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

3. Wavelength Plan and Multiplexing

The most important difference between FR and LR is the wavelength plan. FR uses CWDM wavelengths with 20 nm spacing. LR4 uses LAN-WDM wavelengths with approximately 4.5 nm spacing. LR1 uses a single 1310 nm wavelength with no multiplexing.

3.1 FR Wavelength Plan

FR4 uses four CWDM wavelengths: 1271, 1291, 1311, and 1331 nm. The 20 nm spacing allows the use of uncooled or lightly cooled lasers and a relatively simple CWDM multiplexer. The wide spacing also tolerates wavelength drift over temperature, which reduces the cost and complexity of the module.

3.2 LR Wavelength Plan

LR4 uses four LAN-WDM wavelengths centered around 1310 nm: approximately 1295, 1300, 1305, and 1309 nm. The 4.5 nm spacing requires temperature-controlled lasers and a high-precision LAN-WDM multiplexer. The tighter spacing is necessary to keep the four wavelengths within the 1310 nm window, where chromatic dispersion is near zero, while maintaining enough separation for the multiplexer to separate them.

4. Fiber Count and Connector

Both FR and LR use a single fiber pair and a duplex LC connector. The fiber count per link is the same: two fibers for transmit and receive. The difference is not in the fiber count but in the wavelength plan and the power budget.

FR and LR are both duplex LC interfaces. They can share the same patch panels, patch cords, and fiber trays. The difference is in the module and the reach, not in the physical fiber infrastructure.

5. Optical Architecture and Signal Modulation

Both FR and LR use 1310 nm EMLs and PIN photodetectors. The difference is the number of wavelengths and the per-lane rate.

5.1 FR Optical Architecture

FR4 uses four CWDM wavelengths, each carrying 100G PAM4 or 200G PAM4. The module contains four lasers, a CWDM multiplexer, a CWDM demultiplexer, and four photodiodes. The CWDM multiplexer combines the four wavelengths onto a single fiber, and the demultiplexer separates them at the receiver.

5.2 LR Optical Architecture

LR1 uses a single 1310 nm wavelength carrying 100G PAM4. The module contains one laser, one modulator, and one photodiode. There is no multiplexer. LR4 uses four LAN-WDM wavelengths, each carrying 100G PAM4 or 25G NRZ. The module contains four lasers, a LAN-WDM multiplexer, a LAN-WDM demultiplexer, and four photodiodes. The LAN-WDM multiplexer is more complex and more expensive than the CWDM multiplexer used in FR.

6. Reach and Power Budget

FR supports 2 kilometers, and LR supports 10 kilometers. The reach difference is driven by the optical power budget and the receiver sensitivity. Both use single-mode fiber, which has no modal dispersion, and both operate near 1310 nm, where chromatic dispersion is near zero. The reach difference is primarily a function of the transmitter output power, the receiver sensitivity, and the link budget allocated by the standard.

LR has a higher power budget than FR because the link must span 10 kilometers instead of 2 kilometers. The additional 5 to 6 dB of budget is provided by higher transmitter output power and better receiver sensitivity. The LAN-WDM multiplexer in LR4 also has slightly higher insertion loss than the CWDM multiplexer in FR4, which is accounted for in the module specifications.

7. Power Consumption and Thermal

Power consumption differs between FR and LR because of the laser cooling requirements and the multiplexer design. LR4 modules use temperature-controlled LAN-WDM lasers, which consume more power than the uncooled or lightly cooled CWDM lasers in FR4 modules.

LR modules consume 2 to 4 watts more than FR modules of the same data rate. The higher power consumption is driven by the temperature-controlled lasers, the higher transmitter output power, and the more complex LAN-WDM multiplexer. In high-density switches, the additional power and thermal load must be included in the thermal design.

8. Application Scenarios

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

8.1 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.

• Leaf-to-spine across a large campus: Where the distance exceeds 500 meters but is under 2 kilometers.

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

8.2 LR Application Scenarios

• Metro DCI: Distances up to 10 kilometers.

• Regional DCI: Between data centers in the same metropolitan area.

• Campus backbone: Long building-to-building connections.

• Carrier access: Metro access links up to 10 kilometers.

• Longer leaf-to-spine runs: Where the distance exceeds 2 kilometers.

9. Cost Structure

The cost structure of FR and LR differs in module cost and laser cost. FR modules use CWDM multiplexers and uncooled or lightly cooled lasers, which are less expensive. LR modules use LAN-WDM multiplexers and temperature-controlled lasers, which are more expensive. The fiber cost is the same for both, because both use a single fiber pair and a duplex LC connector.

The cost comparison depends on the deployment scale and the link distance. In a campus environment with links under 2 kilometers, FR is the lower-cost option because the modules are cheaper and the reach is sufficient. In a metro environment with links between 2 and 10 kilometers, LR is the only viable option, and its higher cost is justified by the additional reach.

10. Standard and Ecosystem

FR and LR are both IEEE 802.3 standards. FR is defined in 802.3cu (100GBASE-FR1, 400GBASE-FR4) and 802.3df (800GBASE-FR4). LR is defined in 802.3ba (100GBASE-LR4), 802.3cu (100GBASE-LR1), 802.3bs (400GBASE-LR4), and 802.3df (800GBASE-LR4).

11. Comparison Summary

12. Selection Framework

13. Common Misconceptions

• "FR and LR use different fiber types." False. Both use single-mode fiber (OS2). The difference is the wavelength plan and the reach, not the fiber type.

• "FR and LR use different connectors." False. Both use duplex LC connectors. The physical fiber infrastructure—patch panels, patch cords, and fiber trays—is the same for both.

• "LR is always more expensive." LR modules cost more than FR modules because of the LAN-WDM multiplexer and temperature-controlled lasers. However, LR is the only option for links beyond 2 kilometers, so the cost comparison is only relevant when both can reach the required distance.

• "FR can reach 10 km." False. FR is rated for 2 kilometers. For 10 km links, LR is required.

• "LR replaces FR." False. FR and LR serve different distance segments. FR is used for campus and building-to-building connections up to 2 km, while LR is used for metro and regional connections up to 10 km. The two coexist in campus and metro networks.

14. Summary

FR and LR are two IEEE-defined optical reach classes for single-mode fiber transceivers. FR uses CWDM wavelengths at 1310 nm with a duplex LC connector, reaching 2 kilometers. LR uses LAN-WDM wavelengths or a single 1310 nm wavelength with a duplex LC connector, reaching 10 kilometers. Both support 100G, 200G, 400G, and 800G Ethernet.

The choice between FR and LR is determined by the link distance. FR is the lower-cost option for campus and building-to-building connections up to 2 kilometers, where the CWDM multiplexer and uncooled lasers reduce module cost and power consumption. LR is the required option for metro and regional connections between 2 and 10 kilometers, where the additional power budget and LAN-WDM wavelength plan are necessary to span the distance.

FR and LR are not competing technologies. They are complementary reach classes that serve different segments of the campus and metro network. The most efficient network design uses FR where the distance is under 2 kilometers, and LR where the distance exceeds 2 kilometers and up to 10 kilometers.

15. Q&A