DR vs LR

DR and LR are two optical reach classes for single-mode fiber transceivers. DR stands for Data center Reach and supports distances up to 500 meters. 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 DR and LR is not the fiber type—both use single-mode fiber—but the way the optical lanes are organized and the reach each supports. DR uses parallel single-mode lanes: multiple fibers, each carrying one lane, with an MPO connector. LR uses wavelength division multiplexing: multiple wavelengths multiplexed onto a single fiber pair, with a duplex LC connector. LR also includes single-lane variants that use a single 1310 nm wavelength over a duplex LC pair.

DR is optimized for rack-to-rack and row-to-row connections within a data hall, where the distance is between 100 and 500 meters and the fiber plant is single-mode. LR is optimized for longer campus and metro connections up to 10 kilometers, where the additional power budget and wavelength plan are required. Both reach classes are deployed in AI clusters and cloud data centers, and the choice between them depends on the link distance and the fiber infrastructure.

1. What Is DR?

DR (Data center Reach) is an optical reach class defined in IEEE 802.3 for parallel single-mode fiber transmission. It uses 1310 nm EMLs or DMLs and PIN photodetectors. Each lane has its own transmit and receive fiber, and the lanes operate simultaneously.

DR is defined for multiple data rates. 100GBASE-DR uses one lane of 25G NRZ over two single-mode fibers at 1310 nm, reaching 500 meters. 400GBASE-DR4 uses four lanes of 100G PAM4 over eight single-mode fibers, 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.

1.1 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 architecture: Parallel optics, one laser per lane.

• Fiber count per 400G link: 8 fibers (4 Tx + 4 Rx).

• Fiber count per 800G link: 16 fibers (8 Tx + 8 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. Fiber Count and Connector

The most visible difference between DR and LR is the fiber count and the connector type. DR uses parallel fibers and an MPO connector. LR uses a single fiber pair and a duplex LC connector.

LR is significantly more fiber-efficient than DR. A 400G LR4 link uses two fibers, while a 400G DR4 link uses eight fibers. An 800G LR4 link uses two fibers, while an 800G DR8 link uses sixteen fibers. In fiber-constrained environments, LR allows more links to be deployed on the same fiber cable.

4. Wavelength Plan and Multiplexing

DR uses a single wavelength—1310 nm—on each parallel fiber. LR uses multiple wavelengths in the LAN-WDM grid for multi-lane interfaces, or a single 1310 nm wavelength for single-lane interfaces.

4.1 DR Wavelength Plan

DR uses 1310 nm on every lane. There is no wavelength multiplexing. Each lane has its own fiber, and the same wavelength is used on all lanes. The optical engine is a parallel array of lasers and photodiodes, one per lane.

4.2 LR Wavelength Plan

LR4 uses four LAN-WDM wavelengths centered around 1310 nm: approximately 1295, 1300, 1305, and 1309 nm. Each wavelength carries 100G PAM4 or 200G PAM4, and the four wavelengths are multiplexed onto a single fiber pair using a LAN-WDM multiplexer inside the module. The receiver demultiplexes the four wavelengths and routes each to a photodiode. LR1 uses a single 1310 nm wavelength with no multiplexing.

5. Optical Architecture and Signal Modulation

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

5.1 DR Optical Architecture

DR uses parallel optics. Each lane has its own laser and photodiode. 400GBASE-DR4 uses four lanes of 100G PAM4, each on a separate fiber. 800GBASE-DR8 uses eight lanes of 100G PAM4, each on a separate fiber. The module contains four or eight laser drivers, lasers, photodiodes, and transimpedance amplifiers.

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. LR4 uses four LAN-WDM wavelengths, each carrying 100G PAM4 or 200G PAM4. The module contains four lasers, a LAN-WDM multiplexer, a LAN-WDM demultiplexer, and four photodiodes.

6. Reach and Power Budget

DR supports 500 meters, 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 DR because the link must span 10 kilometers instead of 500 meters. The additional 6 to 8 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 parallel optics in DR, which is accounted for in the module specifications.

7. Power Consumption and Thermal

Power consumption differs between DR 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 lasers in DR modules.

LR modules consume 2 to 4 watts more than DR 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

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

8.1 DR Application Scenarios

• Leaf-to-spine within a row: Distances up to 500 meters.

• Adjacent-row connections: Where the distance exceeds 100 meters but is under 500 meters.

• AI cluster GPU-to-leaf: Cross-rack connections under 500 meters.

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

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

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 500 meters.

9. Cost Structure

The cost structure of DR and LR differs in module cost and laser cost. DR modules use parallel optics with MPO connectors, which are less expensive than the LAN-WDM multiplexers used in LR modules. LR modules cost more because of the internal WDM and the tighter wavelength control required. However, LR 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 single-mode fiber, DR 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, LR is the lower-cost option because it uses only two fibers per link, reducing the fiber and connector cost.

10. Standard and Ecosystem

DR and LR are both IEEE 802.3 standards. DR is defined in 802.3bs (200GBASE-DR4, 400GBASE-DR4) and 802.3cd (100GBASE-DR). 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

• "DR and LR both use single-mode fiber, so they are interchangeable." False. DR uses parallel single-mode fibers with an MPO connector. LR uses LAN-WDM multiplexing with a duplex LC connector. The fiber count, connector type, and internal optical architecture are completely different, and the two cannot be used interchangeably.

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

• "DR can reach 10 km." False. DR is rated for 500 meters. For links beyond 500 meters, LR or FR modules are required.

• "LR uses four fibers for 400G." False. 400GBASE-LR4 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.

• "DR replaces LR." False. DR and LR serve different distance segments. DR is used for rack-to-rack and row-to-row connections under 500 meters, while LR is used for metro and regional connections between 500 meters and 10 kilometers. The two coexist in data center and metro networks.

14. Summary

DR and LR are two IEEE-defined optical reach classes for single-mode fiber transceivers. DR uses parallel single-mode lanes at 1310 nm with an MPO connector, reaching 500 meters. 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 DR and LR is determined by the link distance and the fiber plant. DR is the lower-cost option for rack-to-rack and row-to-row connections under 500 meters, where the module cost dominates and the fiber plant can accommodate the higher fiber count. LR is the appropriate option for metro and regional connections between 500 meters and 10 kilometers, where the fiber count per link must be minimized and the duplex LC infrastructure is available.

DR and LR are not competing technologies. They are complementary reach classes that serve different segments of the data center and metro network. The most efficient network design uses DR where the distance is under 500 meters and the fiber plant supports MPO connections, and LR where the distance exceeds 500 meters and the fiber plant is built around duplex LC connections.

15. Q&A