Class budget
The most loss the optics are rated to cross: 28 dB for B+ and 32 dB for C+. The link passes when the total loss fits inside it and the ONT still gets at least its sensitivity.
GPON splitter calculator
Check that a PON reaches every ONT: add the fibre, splitters, connectors and splices, and see the power left at the ONT against a B+ or C+ budget. Nothing you type is sent anywhere.
- Fibre loss
- 3.5 dB
- Splitter loss
- 17.8 dB
- Connector loss
- 2.0 dB
- Splice loss
- 0.6 dB
- Total loss
- 23.9 dB
- Power at the ONT
- -20.9 dBm
- Margin to class budget
- 4.1 dB
- Margin over ONT sensitivity
- 6.1 dB
Pass: 1:32 split, 4.1 dB inside the budget.
How the budget is worked out
Every loss is in dB and adds up. Power at the ONT is the launch power less that total.
Splitter loss
Each halving of the light costs a little over 3 dB. Typical PLC splitters lose 3.7 dB at 1:2 up to 20.5 dB at 1:64. Cascaded stages add, so a 1:8 then a 1:4 costs about what one 1:32 does. Use your splitter's datasheet figure if you have it.
Fibre, connectors and splices
Single-mode fibre loses about 0.35 dB per km at 1310 nm, the upstream wavelength and the worse of the two. Allow around 0.5 dB a connector and 0.1 dB a fusion splice.
Margin
Leave 3 dB or so for ageing, repairs and dirty connectors. A link that passes with nothing to spare is the one that drops subscribers after its first fibre cut.