Calculating loss budgets
This domain tests the arithmetic and assumptions behind a fiber link loss budget. The exam expects you to convert transmitter output and receiver sensitivity into a dB budget, add fiber, connector, splice, and splitter losses, include margin, and avoid mixing dBm with dB.
Budget formula
A simple power budget is minimum transmitter output minus receiver sensitivity, both in dBm; the result is in dB. The link loss estimate is fiber attenuation (dB/km × km) plus connector pairs, splices, splitter excess loss, and any other components. Subtract the estimated loss from the budget; the remainder must cover aging, repairs, temperature, and measurement uncertainty.
Typical planning values are 0.35 dB/km for single-mode at 1310 nm and about 0.25 dB/km at 1550 nm; multimode 850 nm is much higher, often around 3 dB/km. Connector pairs are commonly planned at 0.3–0.5 dB, fusion splices at 0.1–0.3 dB, and an ideal 1×32 splitter at about 15 dB before excess loss.
Worked example
A 12 km single-mode link at 1310 nm uses four connector pairs, five splices, and a 15 dB budget. Fiber: 12 × 0.35 = 4.2 dB. Connectors: 4 × 0.3 = 1.2 dB. Splices: 5 × 0.1 = 0.5 dB. Total estimated loss = 5.9 dB. Add a 3 dB margin: 8.9 dB. Since 8.9 dB is below the 15 dB budget, the design passes with 6.1 dB unused. If the same design used a 1×32 splitter, add about 15 dB and the link would fail; the splitter must be in the budget, not ignored.
Traps
- Counting connector ends instead of mated connector pairs.
- Forgetting patch-panel connectors, splitters, or WDM components.
- Using 1550 nm attenuation for a 1310 nm calculation.
- Treating dBm and dB as interchangeable.
- Omitting margin and then failing after repairs or aging.