ITU-T PON ODN loss classes
Every ODN class is a window with a minimum as well as a maximum — and the minimum is the one that catches people out.
Loss classes
| Class | Minimum | Maximum | Window | Standard |
|---|---|---|---|---|
| GPON Class A | 5.0 dB | 20.0 dB | 15.0 dB | ITU-T G.984.2 |
| GPON Class B | 10.0 dB | 25.0 dB | 15.0 dB | ITU-T G.984.2 |
| GPON Class B+ | 13.0 dB | 28.0 dB | 15.0 dB | ITU-T G.984.2 |
| GPON Class C | 15.0 dB | 30.0 dB | 15.0 dB | ITU-T G.984.2 |
| GPON Class C+ | 17.0 dB | 32.0 dB | 15.0 dB | ITU-T G.984.2 |
| XGS-PON Class N1 | 14.0 dB | 29.0 dB | 15.0 dB | ITU-T G.9807.1 |
| XGS-PON Class N2 | 16.0 dB | 31.0 dB | 15.0 dB | ITU-T G.9807.1 |
| XGS-PON Class E1 | 18.0 dB | 33.0 dB | 15.0 dB | ITU-T G.9807.1 |
| XGS-PON Class E2 | 20.0 dB | 35.0 dB | 15.0 dB | ITU-T G.9807.1 |
The minimum matters as much as the maximum
Drop below the class minimum and the receiver overloads: too much light saturates the photodiode and the bit error rate collapses. It is a common failure on short drops behind a small splitter, and in the field it does not present as an optical budget problem at all — the power meter reads healthy, so people go hunting for a fault that is not there. The fix is an attenuator, not more light.
Operating wavelengths and fibre attenuation
Upstream and downstream run at different wavelengths and the fibre does not attenuate them equally, so the binding direction is not always the one you assume. Values below are design figures for G.652.D single-mode fibre.
| System | Direction | Wavelength | dB / km |
|---|---|---|---|
| GPON | downstream | 1490 nm | 0.28 |
| GPON | upstream | 1310 nm | 0.35 |
| XGS-PON | downstream | 1577 nm | 0.25 |
| XGS-PON | upstream | 1270 nm | 0.40 |
Splitter insertion loss
Insertion loss is the theoretical split plus the device's excess loss, which is why cascading costs more than splitting once. A 1:4 followed by a 1:8 serves the same 32 subscribers as a single 1:32 — the theoretical loss is identical, since 10·log10(4) + 10·log10(8) = 10·log10(32) — but you pay a second helping of excess loss, around 0.8 dB.
| Split ratio | Theoretical | Excess | Typical max IL |
|---|---|---|---|
| 1:2 | 3.01 dB | 0.6 dB | 3.61 dB |
| 1:4 | 6.02 dB | 1.3 dB | 7.32 dB |
| 1:8 | 9.03 dB | 1.5 dB | 10.53 dB |
| 1:16 | 12.04 dB | 1.6 dB | 13.64 dB |
| 1:32 | 15.05 dB | 2.0 dB | 17.05 dB |
| 1:64 | 18.06 dB | 2.5 dB | 20.56 dB |
| 1:128 | 21.07 dB | 2.9 dB | 23.97 dB |
Excess loss figures are typical for PLC splitters. Use your device's datasheet when you have it.
Related tool: PON optical power budget, which checks a link against both ends of the class window.