What goes into a loss budget
Optical loss (measured in decibels, dB) accumulates from three main sources along a link. Add them up and you have the expected loss; compare that against the budget your transceivers allow, and you know whether the link will work — with margin to spare.
- Fiber attenuation: length × the fiber's loss per km (e.g. ~0.35 dB/km at 1310 nm for single-mode).
- Splice loss: number of fusion splices × loss per splice (typically ~0.1 dB each).
- Connector loss: number of connector pairs × loss per pair (typically ~0.5 dB each).
A worked example
Say you have a 10 km single-mode link with 2 splices and 2 connector pairs at 1310 nm. Fiber: 10 × 0.35 = 3.5 dB. Splices: 2 × 0.1 = 0.2 dB. Connectors: 2 × 0.5 = 1.0 dB. Total expected loss ≈ 4.7 dB. If your transceivers support, say, a 10 dB budget, you have healthy margin for ageing, repairs and future splices.
Why the maths isn't enough
A calculated budget is a plan, not a guarantee. A tight bend, a poor splice or a dirty connector can push real losses well above the estimate. That's why every link should be verified after installation with a power meter and an OTDR, and the measured results kept on record. Design to the budget, then prove it — that's the difference between a link that just works and one that mysteriously drops at 2 a.m.