By Madan Kc, ISP/GPON engineer at DG Link Network · Updated 13 November 2026
PON optical power budget
How it is calculated
Total loss = fiber length x attenuation + splitter loss + connector loss + splice loss. The expected ONU Rx power is the OLT Tx power minus that loss (downstream, 1490 nm). The upstream direction uses the ONU Tx power and the OLT receiver, with the higher 1310 nm fiber attenuation. The maximum reach is how far the fiber could run before the remaining margin reaches zero, with the other losses unchanged, for the weaker of the two directions.
| Item | Typical value | Note |
|---|---|---|
| GPON Class B+ budget | 28 dB | OLT Tx +1.5 to +5 dBm, ONU sensitivity -28 dBm, overload -8 dBm (ITU-T G.984.2) |
| GPON Class C+ budget | 32 dB | OLT Tx +3 to +7 dBm, ONU sensitivity -32 dBm, overload -12 dBm |
| Fiber, 1490 nm | 0.25 dB/km | G.652 fiber; use your cable’s data sheet |
| Fiber, 1310 nm | 0.35 dB/km | Higher than 1490 nm |
| PLC splitter | 1:2 about 4 dB, 1:8 about 10.5 dB, 1:32 about 17.4 dB | Check the splitter data sheet |
| Connector | 0.3 to 0.5 dB | 0.5 dB is a safe planning value |
| Fusion splice | 0.05 to 0.1 dB | Per splice |
Compare the result with real readings: a healthy ONU usually reads somewhere between -8 and -27 dBm, as explained in ONU Rx power (dBm): good range, bad range and fixes. If the ONU is weaker than expected, see ONT red LOS light.
FAQ
What is the maximum distance for GPON?
The logical reach is up to 20 km and the optical budget decides the real reach. Class B+ gives about 28 dB, so a 1:32 splitter (about 17.4 dB) leaves roughly 10 dB for fiber, connectors, splices and margin.
What ONU Rx power is too high?
Above the ONU’s overload level, typically -8 dBm for Class B+. Short links with a low split ratio can be too strong, and an attenuator solves it.
Why does the tool check upstream as well?
Both directions must work. The ONU transmitter is weaker than the OLT’s, and 1310 nm loses more per km, so on long links upstream can limit reach even when downstream looks fine.
Does this replace a field measurement?
No. It is a planning estimate. Always measure with an optical power meter at the ONU and compare it with the calculated value.
Related
Sources and testing: Class B+ and C+ figures follow ITU-T G.984.2 as commonly published; loss values per component are typical planning numbers, so use your own data sheets. I checked the formulas by calculation but did not test the tool against live OLT hardware.