FiberLinkA GPON link does not pass or fail on loss alone — it passes or fails on whether the power arriving at the ONU lands inside that receiver's working window. Which window applies depends on the optical class the equipment is built to.
The two classes in everyday use are Class B+ and Class C+, whose ONU receiver limits come from ITU-T G.984.2 and its amendments:
| Class | Overload (max) | Sensitivity (min) |
|---|---|---|
| B+ | −8 dBm | −27 dBm |
| C+ | −12 dBm | −32 dBm |
Read those as a window, not a floor. Class B+ works between −8 and −27 dBm. Class C+ works between −12 and −32 dBm. Outside either end, the link does not work.
Why C+ is not simply "better"
C+ buys 5 dB of extra reach at the bottom — −32 dBm instead of −27 dBm — which is roughly one more split stage, or a good stretch of extra route. That is the reason it exists and the reason people reach for it on long rural feeders and deep splits.
But look at the top of the window. C+ overloads at −12 dBm, which is 4 dB less headroom than B+. The window shifts down; it does not simply widen. Swapping a B+ ONU for a C+ ONU on a short drop off a shallow split can take a link that worked and saturate it.
Overload is a real failure, not a theoretical one
Too much light is the failure mode that gets forgotten, because the instinct in optical work is that more power is safer. It is not. Past the overload figure the receiver saturates and the error rate climbs — often presenting as an intermittent or "sometimes it syncs" fault rather than a clean hard failure, which makes it expensive to chase.
It shows up in predictable places: a 1:4 or 1:8 split with a very short drop, a subscriber close to the cabinet on a network designed for the far end of the estate, or a link where someone substituted a higher-output transmitter without recalculating the near-end nodes.
Check both ends of the window at every endpoint.
A design that closes at the furthest drop can still overload the nearest one. The two are different calculations on the same network.
Which class should you design to?
Design to the class of the ONU that will actually be installed — not the best class available. If the deployed fleet is B+, a plan validated against a C+ window is a plan that will pass on paper and fail on the street at −29 dBm.
Where a network is mixed, or the fleet may change over the life of the build, the safe move is to design against the intersection of the windows you must support rather than the union: keep every endpoint between −12 and −27 dBm, and the link works on either class.
| Situation | Design window |
|---|---|
| Known B+ fleet | −8 to −27 dBm |
| Known C+ fleet | −12 to −32 dBm |
| Mixed or uncertain fleet | −12 to −27 dBm |
Spec limits are not field targets
Every number above is the point of failure, not the point of comfort. Designing a drop to land at −26.8 dBm on a B+ network is technically inside spec and practically reckless — it leaves nothing for ageing connectors, a future repair splice, a bend that appears when someone else works in the same chamber, or ordinary measurement uncertainty.
Crews generally hold a few dB back from both ends of the window and treat that tighter band as the real target. The spec window is what the equipment tolerates; the field target is what you hand over.
Putting it on the node
None of this is hard arithmetic — it is bookkeeping, at every endpoint, against the right window. That is exactly the part that erodes on a long build.
FiberLink carries the window with the design. Pick GPON Class B+, Class C+, a tighter Field Target, or your own custom window and tolerance, and every reading you log is judged against it automatically — flagged against the design prediction and against the receiver limits as two separate verdicts. The class you used is printed in the exported PDF, so the report says not just what measured, but what it was measured against.
Presets for B+, C+ and your own window.
Set the criteria once and every logged reading is judged the same way, all the way down the tree. Get FiberLink free on Google Play →
The short version
B+ runs −8 to −27 dBm; C+ runs −12 to −32 dBm. C+ reaches further but overloads sooner. Design to the fleet you will actually install, check the top of the window as carefully as the bottom, and hold a margin back from both.