80km vs 120km Coherent Optics

Eighty kilometers and one hundred twenty kilometers are the two standard reach tiers for coherent pluggable optics in metro and regional data center interconnect. The 80-kilometer tier is the baseline reach of the OIF 400ZR specification. The 120-kilometer tier is the extended reach of the OpenZR+ MSA and the upper bound of the 400ZR implementation agreement. Both use the same coherent detection architecture, the same tunable laser in the C-band, and the same QSFP-DD or OSFP form factors. The differences lie in the optical signal-to-noise ratio budget, the amplifier spacing, the forward error correction strength, and the line system design required to close the link.

The distance gap is fifty percent, but the engineering gap is shaped by the nonlinear nature of optical amplification and the OSNR penalty that accumulates with each span. An 80-kilometer link and a 120-kilometer link may look similar on a fiber map, but the 120-kilometer link requires either a more powerful FEC, a shorter amplifier spacing, or a higher-performance coherent DSP to maintain the same error-free operation.

1. What the 80 km Tier Represents

The 80-kilometer tier is the baseline reach for 400ZR and 800ZR coherent pluggables. It corresponds to a link with three to five spans of 15 to 25 kilometers each, amplified by erbium-doped fiber amplifiers at each span endpoint. The line system is designed so that the received OSNR at the coherent receiver remains above the threshold required by the FEC in the module.

The 80-kilometer tier is the most common deployment scenario for metro and regional DCI. It covers the distance between two data centers in the same metropolitan area, or between a data center and a carrier point of presence in a nearby city. The fiber plant is typically leased or owned, and the amplifier spacing is determined by the existing line system or by the splice enclosure locations along the route.

1.1 80 km Coherent Optics Characteristics

• Reach: 80 kilometers.

• Typical span count: 3 to 5 spans.

• Typical span length: 15 to 25 kilometers.

• Amplification: EDFA at each span endpoint.

• OSNR budget: Moderate.

• FEC: Standard or moderate-strength.

• Standard: OIF 400ZR baseline.

• Application: Metro and regional DCI.

2. What the 120 km Tier Represents

The 120-kilometer tier is the extended reach for 400ZR+ and OpenZR+ coherent pluggables. It corresponds to a link with four to eight spans of 15 to 25 kilometers each, or fewer spans of longer length. The OSNR budget is tighter, and the FEC must provide more coding gain to compensate for the additional span loss and the accumulated noise from each amplifier.

The 120-kilometer tier covers longer regional DCI links, where the distance between data centers exceeds the 80-kilometer baseline. It is also used in AI scale-across deployments, where compute clusters in separate campuses or cities must be connected as a single fabric. The line system may include ROADMs for wavelength routing, and the amplifier spacing may be shorter to maintain the OSNR budget.

2.1 120 km Coherent Optics Characteristics

• Reach: 120 kilometers.

• Typical span count: 4 to 8 spans.

• Typical span length: 15 to 25 kilometers.

• Amplification: EDFA at each span endpoint, possibly with Raman assistance.

• OSNR budget: Tighter than 80 km.

• FEC: Stronger coding gain required.

• Standard: OpenZR+ MSA and 400ZR upper bound.

• Application: Extended regional DCI and AI scale-across.

3. OSNR Budget and Amplifier Spacing

The optical signal-to-noise ratio is the primary limiter of coherent reach. Each EDFA adds amplified spontaneous emission noise to the signal. The OSNR at the receiver degrades with each span, and the rate of degradation depends on the span loss and the amplifier noise figure.

An 80-kilometer link with 20-kilometer spans has four spans. A 120-kilometer link with 20-kilometer spans has six spans. The additional two spans add approximately 3 dB of OSNR penalty. To compensate, the 120-kilometer link requires either a stronger FEC, a lower-noise amplifier, or a shorter span length to reduce the loss per span.

4. Forward Error Correction and Coding Gain

Forward error correction is the primary tool for closing the OSNR gap between 80 kilometers and 120 kilometers. The baseline 400ZR specification uses a moderate-strength FEC with a coding gain sufficient for 80 kilometers. The OpenZR+ MSA adds a stronger FEC option that provides additional coding gain for 120 kilometers and beyond.

The stronger FEC comes at a cost. It adds latency, because the codeword length is longer. It also adds power consumption, because the DSP must perform more computation. For latency-sensitive applications, the additional FEC latency may be a concern. For regional DCI and AI scale-across, where the propagation delay already dominates the latency budget, the additional FEC latency is acceptable.

5. Line System Design and Amplifier Placement

The 80-kilometer tier is designed for line systems with standard EDFA spacing. The amplifiers are placed at splice enclosures or carrier hotels along the route, typically every 15 to 25 kilometers. The line system may include fixed or reconfigurable optical add/drop multiplexers, depending on whether the wavelengths need to be routed to different destinations.

The 120-kilometer tier may require a more carefully engineered line system. The amplifier spacing may be shorter to reduce the loss per span and maintain the OSNR budget. Low-noise EDFAs or Raman-assisted amplification may be used to improve the OSNR. The line system may include dispersion compensation modules if the accumulated dispersion at 1550 nm exceeds what the coherent DSP can handle without penalty.

6. Module Design and Power Consumption

The 80-kilometer and 120-kilometer tiers often use the same coherent DSP and the same tunable laser. The difference is in the FEC mode and the DSP configuration. The 120-kilometer mode uses a stronger FEC and may require more DSP power to perform the additional error correction computation.

Power consumption varies by module type. A 400ZR module for 80 kilometers draws approximately 15 to 20 watts. A 400ZR+ module for 120 kilometers draws approximately 20 to 25 watts. The difference is primarily due to the stronger FEC and the higher-performance DSP required for the extended reach.

7. Standards and Ecosystem

The 80-kilometer tier is defined by the OIF 400ZR Implementation Agreement. The 120-kilometer tier is defined by the OpenZR+ MSA, which builds on the OIF standards and adds stronger FEC and multi-rate operation. The 800ZR and 800ZR+ variants follow the same pattern, with the baseline supporting 80 kilometers and the extended variants supporting 120 kilometers.

8. Application Boundaries

The 80-kilometer tier covers metro and regional DCI links where the distance is under 80 kilometers. It is the most common deployment scenario for 400ZR and 800ZR modules. The 120-kilometer tier covers regional DCI links where the distance exceeds 80 kilometers but remains under 120 kilometers. It is also used in AI scale-across deployments where compute clusters are separated by 80 to 120 kilometers.

9. Cost Profile

The 120-kilometer tier costs more than the 80-kilometer tier because of the stronger FEC and the higher-performance DSP. The module premium is typically 20 to 40 percent depending on the vendor and the data rate. The line system cost may also be higher if low-noise amplifiers or shorter amplifier spacing are required.

10. Comparison Summary

11. Selection Framework

The choice between 80 kilometers and 120 kilometers is determined by the physical distance and the OSNR budget of the line system.

• Distance under 80 kilometers on a standard line system: Use the baseline 400ZR or 800ZR module. The standard FEC is sufficient, and the module cost and power consumption are lower.

• Distance between 80 and 120 kilometers on a standard line system: Use the 400ZR+ or 800ZR+ module with stronger FEC. The additional coding gain closes the OSNR gap.

• Distance beyond 120 kilometers: Use 400ZR+ or 800ZR+ with a carefully engineered line system. The reach can extend to 500 kilometers or more with adequate amplification and OSNR management.

The line system design matters as much as the distance. A 120-kilometer link with low-noise amplifiers and short spans may use the baseline FEC. An 80-kilometer link with high-loss spans and noisy amplifiers may require the stronger FEC. The choice between the two tiers is not just about distance; it is about the OSNR budget of the complete link.

12. Common Misconceptions

• "80 km and 120 km coherent optics use different lasers." False. Both use the same tunable laser in the C-band. The difference is in the FEC and the DSP configuration, not the laser.

• "120 km always requires a different module." Not always. Some 400ZR modules support both 80 km and 120 km modes, with the FEC mode selected in software. The module is the same; the configuration is different.

• "The 120 km tier requires Raman amplification." Not necessarily. Raman amplification can improve the OSNR, but it is not required for 120 kilometers if the span loss is low and the EDFA noise figure is good.

• "80 km modules cannot be used for 120 km links." False in some cases. A module configured for the 80 km FEC mode may not reach 120 kilometers, but if the line system has enough margin, it may. The FEC mode determines the OSNR threshold, and the line system determines the available OSNR.

• "The 120 km tier is always more expensive." The module costs more, but the line system cost may be similar if the existing amplifiers and span lengths already support the reach. The cost difference is primarily in the module.

13. Summary

Eighty kilometers and one hundred twenty kilometers are the two standard reach tiers for coherent pluggable optics. The 80-kilometer tier is the baseline reach of the OIF 400ZR specification, supported by standard FEC and standard EDFA line systems. The 120-kilometer tier is the extended reach of the OpenZR+ MSA, supported by stronger FEC and, in some cases, low-noise amplifiers or shorter span lengths.

Both tiers use the same coherent detection architecture, the same tunable laser, and the same form factors. The differences lie in the OSNR budget, the FEC coding gain, the amplifier design, and the line system complexity. The 120-kilometer tier requires approximately 3 dB more OSNR than the 80-kilometer tier, which is achieved through stronger FEC or improved line system design.

The choice between them is determined by the physical distance and the OSNR budget of the line system, not by a fundamental technology difference. A metro link under 80 kilometers uses the baseline module. A regional link between 80 and 120 kilometers uses the extended-reach module with stronger FEC. The two tiers coexist in the same network, and the same module may support both through software configuration.

14. Q&A