10km vs 40km Data Center Interconnect

Data center interconnect across a metropolitan area spans two distinct reach tiers. The 10-kilometer tier connects data centers within a city. The 40-kilometer tier connects data centers across a wider metro region or between nearby cities. Both use single-mode fiber and duplex LC connectors. Both are direct-detect PAM4 interfaces. The differences lie in the optical power budget, the wavelength plan, the laser design, and the fiber plant environment each tier is built to serve.

The 10-kilometer tier is represented by LR interfaces. It handles the majority of metro DCI links, where two facilities are separated by a few kilometers to ten kilometers. The 40-kilometer tier is represented by ER interfaces. It covers the longer metro and regional links, where the distance exceeds what LR can reliably span and the fiber path may traverse multiple carrier patch panels.

Both tiers use the same physical connector and the same fiber count per link. The decision between them is not about cabling. It is about link margin, fiber plant uncertainty, and the cost of the module. A 10-kilometer link on a controlled fiber path can use LR with confidence. A 40-kilometer link on a leased metro path requires ER for the additional margin needed to accommodate unknown losses.

1. What 10 km DCI Covers

A 10-kilometer interconnect connects data centers within a metropolitan area. The fiber runs between two facilities that are typically a few kilometers to ten kilometers apart. The path may traverse leased fiber, but the distance is short enough that the link budget can be engineered with confidence using standard LR modules.

The 10-kilometer tier is defined by LR interfaces. A 100GBASE-LR1 module uses a single 1310 nm wavelength carrying 100G PAM4 over a duplex LC pair. A 400GBASE-LR4 module uses four LAN-WDM wavelengths around 1310 nm, each carrying 100G PAM4, multiplexed onto a single fiber pair. An 800GBASE-LR4 module uses four wavelengths of 200G PAM4.

1.1 10 km DCI Characteristics

• Reach: 10 kilometers.

• Wavelength: LAN-WDM at approximately 1295, 1300, 1305, and 1309 nm for LR4; 1310 nm for LR1.

• Optical architecture: LAN-WDM or single wavelength on a duplex LC pair.

• Connector: Duplex LC.

• Fiber count per link: 2 fibers.

• Power budget: 12 to 14 dB.

• Laser cooling: Temperature-controlled.

• Environment: Metro, leased or carrier fiber.

2. What 40 km DCI Covers

A 40-kilometer interconnect connects data centers across a wider metro region or between nearby cities. The fiber runs between two facilities that may be several tens of kilometers apart, traversing streets, carrier hotels, and utility infrastructure. The fiber is often leased, and the operator may not control the patch panels or splice enclosures along the path. The link must be engineered with margin for the additional connector and splice losses that accumulate over the longer distance.

The 40-kilometer tier is defined by ER interfaces. A 100GBASE-ER4 module uses four LAN-WDM wavelengths around 1310 nm, each carrying 25G NRZ, multiplexed onto a single fiber pair. A 400GBASE-ER8 module uses eight wavelengths, each carrying 50G PAM4. An 800GBASE-ER8 module uses eight wavelengths of 100G PAM4. Some ER variants use wavelengths in the 1550 nm window to take advantage of lower fiber attenuation and the possibility of erbium-doped fiber amplification for extended reaches.

2.1 40 km DCI Characteristics

• Reach: 30 to 40 kilometers.

• Wavelength: LAN-WDM at approximately 1295, 1300, 1305, and 1309 nm, or 1550 nm.

• Optical architecture: LAN-WDM or 1550 nm on a duplex LC pair.

• Connector: Duplex LC.

• Fiber count per link: 2 fibers.

• Power budget: 18 to 25 dB.

• Laser cooling: Temperature-controlled.

• Environment: Metro and regional, leased or carrier fiber.

3. Wavelength Plan and Laser Design

LR uses LAN-WDM wavelengths with approximately 4.5 nm spacing around 1310 nm. The tight spacing requires temperature-controlled lasers and a precise multiplexer. The 1310 nm window is chosen because chromatic dispersion is near zero, which simplifies link engineering over the 10-kilometer reach.

ER uses LAN-WDM wavelengths with similar spacing, or 1550 nm wavelengths for extended reach. The 1550 nm window has lower fiber attenuation, approximately 0.2 dB/km compared to 0.35 dB/km at 1310 nm. Over 40 kilometers, that difference is 6 dB, which is a substantial part of the link budget. The trade-off is higher chromatic dispersion at 1550 nm, which must be managed through link engineering or dispersion compensation.

4. Power Budget and Link Engineering

The 10-kilometer link budget is sized for a metro fiber plant with moderate connector and splice losses. Fiber loss at 1310 nm over 10 kilometers is approximately 3.5 dB. Connector and splice losses in a leased metro path can add 2 to 4 dB. The LAN-WDM multiplexer adds 2 to 3 dB. The total path loss is typically 8 to 11 dB, and the module provides 12 to 14 dB of budget.

The 40-kilometer link budget is sized for a longer metro or regional fiber plant with more connector and splice losses. Fiber loss at 1310 nm over 40 kilometers is approximately 14 dB. At 1550 nm, fiber loss over 40 kilometers is approximately 8 dB. Connector and splice losses can add 2 to 4 dB. The LAN-WDM multiplexer adds 2 to 3 dB. The total path loss is typically 12 to 20 dB, and the module provides 18 to 25 dB of budget.

5. Fiber Plant and Environment

LR and ER both use duplex LC connectors and two fibers per link. The physical connector type is identical. The difference lies in the fiber plant environment and the engineering discipline each tier requires.

A 10-kilometer metro link is engineered with a fiber path that is known with reasonable confidence. The operator may have a fiber route diagram showing the patch panels and splice enclosures along the path. The link budget calculation is straightforward, and the module provides enough margin for the expected losses.

A 40-kilometer regional link is engineered with less certainty. The fiber may be leased, and the operator may not know the exact number of patch panels or splice enclosures along the path. The link budget must include margin for unexpected losses. The temperature-controlled lasers and higher-power transmitters in ER modules provide that margin.

6. Application Boundaries

The 10-kilometer tier covers the majority of metro DCI links. It connects two data centers within a city, a data center to a carrier hotel, or a campus to a nearby metro facility. The distance is short enough that no amplification is required, and the fiber path is known well enough that the link budget can be calculated with confidence.

The 40-kilometer tier covers longer metro and regional DCI links. It connects data centers across a wider metro area or between nearby cities. The distance is long enough that the link budget must be engineered with margin, but still short enough that no amplification is required for the 1310 nm variants. The 1550 nm variants can be amplified if the reach needs to extend beyond 40 kilometers.

7. Module Cost and Power

LR modules cost less than ER modules. The 10-kilometer power budget requires less transmitter output power and a less sensitive receiver, which reduces the module cost. ER modules cost more because of the higher launch power, the more sensitive receiver, and the tighter wavelength control required for the longer reach. The premium for ER over LR is typically 30 to 50 percent depending on the data rate and the vendor.

Power consumption is higher for ER modules. A 400GBASE-LR4 module draws approximately 12 to 14 watts. A 400GBASE-ER8 module draws approximately 14 to 16 watts. An 800GBASE-LR4 module draws approximately 16 to 18 watts. An 800GBASE-ER8 module draws approximately 20 to 24 watts. The additional power in ER comes from the higher laser drive current and the more sensitive receiver.

8. Standards and Ecosystem

LR is defined by IEEE 802.3ba for 100GBASE-LR4, by IEEE 802.3cu for 100GBASE-LR1, by IEEE 802.3bs for 400GBASE-LR4, and by IEEE 802.3df for 800GBASE-LR4. ER is defined by IEEE 802.3ba for 100GBASE-ER4, by IEEE 802.3bs for 400GBASE-ER8, and by IEEE 802.3df for 800GBASE-ER8.

9. Comparison Summary

10. Selection Framework

The choice between the 10-kilometer and 40-kilometer tiers is determined by the physical distance and the fiber plant environment.

• Distance under 10 kilometers on a known fiber path: Use LR. The module is cheaper, the power budget is sufficient, and the link budget is straightforward.

• Distance between 10 and 40 kilometers: Use ER. LR cannot reach, and ER provides the additional power budget and tighter wavelength control needed for the longer path.

• Distance beyond 40 kilometers: Use ZR or coherent optics. ER is rated for 30 to 40 kilometers and does not have the budget for longer reaches without amplification or coherent detection.

The fiber plant environment matters as much as the distance. A 10-kilometer link on a fiber path with unknown patch panel losses may require ER for the additional margin. A 10-kilometer link on a controlled fiber path can use LR with confidence. The choice is not just about distance; it is about how much uncertainty the fiber path introduces.

11. Common Misconceptions

• "10 km and 40 km DCI use different connectors." False. Both use duplex LC connectors and two fibers per link. The physical fiber plant is identical.

• "40 km DCI requires amplification." False. The 40-kilometer tier is direct-detect PAM4 and does not require amplification for the 1310 nm variants. The 1550 nm variants can be amplified if the reach needs to extend beyond 40 kilometers, but amplification is not required at the rated distance.

• "LR modules can be used for 40 km links." False. LR is rated for 10 kilometers. For 40-kilometer links, ER is required.

• "ER modules are always more expensive per gigabit." Not necessarily. ER modules cost more per port, but both tiers use two fibers per link. The cost difference is in the module, not the fiber. The cost per gigabit depends on the data rate and the module vendor.

• "10 km DCI is obsolete." False. Metro DCI links under 10 kilometers are a permanent part of data center infrastructure. LR remains the standard interface for these links because it matches the distance and the controlled fiber environment.

12. Summary

The 10-kilometer and 40-kilometer DCI tiers serve different physical environments. The 10-kilometer tier, represented by LR interfaces, uses LAN-WDM wavelengths with 4.5 nm spacing or a single 1310 nm wavelength. It connects data centers within a metropolitan area over a fiber path that is known with reasonable confidence. The 40-kilometer tier, represented by ER interfaces, uses LAN-WDM wavelengths with 4.5 nm spacing or 1550 nm wavelengths for extended reach. It connects data centers across a wider metro region or between nearby cities over leased or carrier fiber.

Both tiers use duplex LC connectors and two fibers per link. Both are direct-detect PAM4 interfaces. The differences lie in the power budget, the laser design, and the fiber plant environment. LR is cheaper and simpler for metro links under 10 kilometers. ER provides additional margin for metro and regional links up to 40 kilometers.

The choice between them is determined by the physical distance and the fiber plant environment, not by a performance comparison. A metro area uses LR for short links and ER for longer links. The two tiers coexist in the same network, connected by ZR or coherent optics where the distance exceeds 40 kilometers.

13. Q&A