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When combining GPON and XGS-PON, calculating the optical power budget is crucial because XGS-PON operates at higher rates and longer wavelengths, and is generally more sensitive to loss.
Total loss is calculated by adding the insertion loss of the combined components to the ODN (Optical Distribution Network) loss.
External WDM1r (CEx) component loss
In this case, you will use a separate passive multiplexer.
- WDM1r Insertion Loss: Typically 1.0 dB to 1.5 dB per port.
- Total Loss Formula: Loss total= Loss WDM1r + Loss Patch Cords + Loss Splitters + Loss Fiber + Loss Connectors
- Impact: Because you are adding an extra physical device and extra patch cords between the OLT and the ODN, you usually lose about 1.5 – 2.0 dB of your power budget compared to a standard GPON setup.
Combined PON Module Loss (Integrated)
The combined PON solution integrates WDM into the SFP+ transceiver.
- Internal Loss: The internal WDM filter still causes loss, but it is factory-calibrated.
- Effective Loss: Since there are no external patch cords or extra connectors for the WDM, the “extra” loss is effectively 0 dB relative to the OLT port output.
- Class Advantage: Combo PON modules are usually designed to higher power classes (e.g., Class D / N2) to ensure they can drive the same distance as the original GPON.
Typical Power Budget Comparison
To ensure proper network operation, the transmit power (Ptx) minus path loss must be greater than the receive sensitivity (Psens).
| Component | GPON (Class B+) | XGS-PON (Class N1) | XGS-PON (Class N2) |
| Min. Launch Power (OLT) | +1.5 dBm | +2.0 dBm | +4.0 dBm |
| Min. Sensitivity (ONU) | -28.0 dBm | -28.0 dBm | -28.0 dBm |
| Max. Allowable Path Loss | 28.5 dB | 29.0 dB | 31.0 dB |
【Important】
The “Weak Link” Principle: Although XGS-PON has higher transmit power in Category N2 networks, its 1577nm wavelength is more susceptible to macrobends (sharp bends in optical fibers) than the 1490nm GPON wavelength. In poorly maintained optical distribution networks (ODNs), XGS-PON may fail while GPON continues to function normally.
Example Calculation (Worst-Case)
If you are using an external WDM1r on a standard 1:64 ODN:
- Splitter (1:64): ~20.5 dB
- Fiber (20km @ 1577nm): 20km × 0.22 dB/km = 4.4 dB
- Connectors/Splices (est.): 2.0 dB
- External WDM1r: 1.5 dB
- Total ODN Loss: 28.4 dB
Result for GPON (B+): 28.5−28.4=0.1 dB (Too close! Risk of intermittent signal.
Result for XGS-PON (N2): 31.0−28.4=2.6 dB (Safe margin).
Key Recommendations
- Cleanliness: Connector cleanliness is critical at 10G rates (XGS-PON). A dirty connector that results in 0.5 dB of loss on GPON can lead to bit errors on XGS-PON.
- Target Class: If your optical distribution network (ODN) has a high splitting ratio (1:64 or 1:128), be sure to use N2 or E1 class Combo PON modules to compensate for WDM and splitter losses.
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