As data demands grow and backbone networks transition to 100G, many organizations look to upgrade their infrastructure without replacing existing fiber. The 100GBASE-ER4 optical module, offering up to 40km of reach over single-mode fiber (SMF), is an attractive option. However, deploying 100G ER4 over legacy fiber links comes with critical considerations. Without proper planning, the advantages of ER4 may be offset by signal degradation, mismatch, or costly troubleshooting.
Understanding the 100G ER4 Module
The 100GBASE-ER4 is a QSFP28 optical transceiver that uses four wavelengths (LAN-WDM) and EML (Electro-Absorption Modulated Lasers) to transmit data at 25Gbps per channel. Designed for long-range connections, it operates at a center wavelength around 1300nm and supports distances up to 40km over standard G.652 single-mode fiber. This makes it ideal for metropolitan networks, inter-data center links, and carrier backbones.
The Legacy Fiber Challenge
Many enterprise and telecom operators have fiber plants deployed a decade or more ago, based on earlier specifications such as ITU-T G.652.A or G.652.B. These legacy fibers often have higher attenuation, dispersion, or splicing losses than modern G.652.D fiber. When upgrading to 100G ER4, several risks arise:
Optical Signal Loss (Attenuation):
ER4 modules operate within a strict power budget. If legacy fibers introduce excessive attenuation (e.g., >6 dB over 30km), the signal at the receiver may drop below the required threshold, causing link instability or failure.
Connector Compatibility:
Modern ER4 modules typically use LC UPC duplex connectors. Older fiber plants might still use SC connectors or feature mixed UPC/APC interfaces. Incompatible connector types can cause reflection or poor insertion loss, affecting performance.
Chromatic Dispersion Impact:
Though ER4 modules include dispersion tolerance mechanisms, old fiber may not meet the dispersion specifications for long-reach 100G transmission. Over longer distances, this can increase bit error rates (BER).
Best Practices for a Smooth ER4 Deployment
To maximize performance and avoid costly downtime, IT and network engineers should take the following steps:
Perform an End-to-End Link Budget Analysis:
Calculate total loss across the fiber span, including splice points, connectors, and patch cords. Ensure it stays within the ER4 module’s receiver sensitivity range (typically around -4.5 dBm).
Test Legacy Fiber Before Deployment:
Use an OTDR (Optical Time Domain Reflectometer) to measure fiber quality, identify high-loss segments, and confirm link length.
Upgrade to Compatible Patch Cords:
Always use low-loss LC UPC duplex OS2 patch cables with ≤0.2dB insertion loss. Avoid mixing UPC and APC connectors unless you’re using appropriate mating adapters.
Consider Using Optical Amplifiers or DCMs:
For borderline cases, EDFA (Erbium-Doped Fiber Amplifiers) or dispersion compensation modules can help extend reach or correct fiber impairments.
Monitor Post-Deployment Performance:
Use digital diagnostic monitoring (DDM) to track real-time optical power levels and pre-FEC BER, allowing early detection of degradation.
Conclusion
While 100G ER4 provides a cost-effective solution for upgrading long-distance links, deploying it over legacy fiber infrastructure requires careful planning. By understanding the limitations of older fiber and applying best practices in link design and compatibility, network operators can successfully bridge performance demands with their existing infrastructure, without compromising reliability.