Optical budget maths for a first GPON rollout

Splitter ratio, distance and connector count all spend from the same budget. Work out what you have before you promise a subscriber count.

A Class B+ GPON link gives you 28 dB to spend between the OLT port and the subscriber ONU. Every split, every kilometre and every connector takes a bite. Plan the spend before you plan the subscriber count, because the ratio you choose at the cabinet decides how far you can reach.

What each element costs you

A 1:8 splitter costs roughly 10.5 dB. 1:16 costs about 13.7 dB, and 1:32 about 17.5 dB. Single-mode fibre at 1490 nm loses around 0.25 dB per kilometre. Each fusion splice costs about 0.1 dB, and each mated connector pair about 0.5 dB. Those connector losses add up faster than operators expect — an ODF at the cabinet, a patch at the enclosure and the ONU itself is already 1.5 dB.

Run the sum before you run the fibre

Take a 1:32 design at six kilometres. The splitter is 17.5 dB, the fibre is 1.5 dB, four connector pairs are 2 dB, and eight splices are 0.8 dB. Total spend: 21.8 dB against a 28 dB budget. You have 6.2 dB of margin, which is healthy.

Now stretch the same design to twelve kilometres and add a second splitter stage. The fibre doubles to 3 dB and the extra 1:2 stage costs another 3.5 dB. You are at 28.3 dB — over budget, on paper, before anything has been installed badly.

Keep margin for the repair you have not had yet

Hold at least 3 dB back. Fibre gets cut, and every repair adds two splices and a slack loop. An operator with no margin on day one is an operator who cannot repair a break without a truck roll to re-engineer the split.

Choose the ratio for the density, not the spreadsheet

1:32 looks efficient per OLT port, and in a dense urban rollout it is. Spread across a thinly populated area, the same ratio strands capacity in splitters that will never fill while eating the budget you needed for distance. A 1:8 in low density and a 1:32 in high density is usually the right answer even though it means stocking both.