ODN classes are the contract
GPON optics come in classes: B+ = 28 dB, C+ = 32 dB of allowed ODN loss between OLT and ONT. XGS-PON defines N1/N2/E1/E2 similarly. The class printed in the design is the ceiling every drop must fit under — at the *worst-case* wavelength and the *farthest* customer.
Where the decibels go
A 1:32 PLC splitter consumes ~17.5 dB; 1:64 ~21 dB. Add fiber (0.35 dB/km at 1310 upstream — the worse wavelength), ~0.3–0.5 dB per connector pair (APC), splice allowances, and 1–3 dB of lifetime margin for repairs and ageing. On a B+ budget with a 1:32, only ~8–9 dB remain for everything else — roughly 20 km of reach *if* the plant is clean.
Differential loss
The nearest ONT must not overload the OLT receiver while the farthest still reaches sensitivity. Check both extremes in design; in the field, an ONT reporting Rx far outside its siblings flags a drop problem, not an OLT problem.
Acceptance testing a PON
At turn-up: power at the FDH output per branch, ONT received power vs prediction, and a filtered-OTDR trace of feeder + one representative distribution leg archived as baseline. A PON accepted on "the light is green" is a PON you will re-visit at scale.