Calculating Loss Budgets
This domain covers the arithmetic that decides whether a design works before anything is installed, and whether a measured result is acceptable afterwards. The exam tests the component-loss values, the difference between power budget and loss budget, and the margin.
Power budget versus loss budget
Power budget = minimum transmitter output − receiver sensitivity, in dB. Loss budget = the sum of worst-case losses of every element in the path. The design passes when loss budget plus margin is less than the power budget. A margin of 2–3 dB is normally reserved for aging, repairs, temperature and additional splices.
Standard worst-case values
- Single-mode attenuation: 0.4 dB/km at 1310 nm, 0.3 dB/km at 1550 nm and 1625 nm (typical installed values are lower, but budgets use maximums).
- Multimode attenuation: about 3.5 dB/km at 850 nm, 1.5 dB/km at 1300 nm.
- Connector pair: 0.75 dB maximum per TIA-568 style budgets; 0.5 dB is a common field allowance, 0.3 dB is typical measured.
- Fusion splice: 0.3 dB maximum, 0.1 dB typical.
- Splitters: about 3.5 dB for 1:2, then roughly 3.1–3.5 dB per additional stage - 1:4 ≈ 7.0, 1:8 ≈ 10.5, 1:16 ≈ 14.0, 1:32 ≈ 17.5, 1:64 ≈ 21.0 dB.
- WDM and filters: typically 0.5–1.0 dB each.
Worked example
A 10 km single-mode link at 1310 nm with 4 connectors and 2 fusion splices. Worst case: fiber 10 × 0.4 = 4.0 dB; connectors 4 × 0.5 = 2.0 dB; splices 2 × 0.1 = 0.2 dB. Total loss budget = 6.2 dB. If the transmitter minimum is −3.0 dBm and receiver sensitivity is −23.0 dBm, power budget = 20.0 dB. Available margin = 20.0 − 6.2 = 13.8 dB, comfortably above the 3 dB planning margin, so the design passes and a measured 5.8 dB is well inside the budget.
Why the arithmetic matters after installation
The measured insertion loss is compared with the computed budget, not just with an abstract pass limit. If measurement exceeds the design by more than about 1 dB, something in the count is wrong: an extra splice, a contaminated connector, a tighter bend, or a wrong fiber count.
Traps that cost marks
Using typical values where maximums are required; forgetting the pigtail splices at each end; counting a connector pair as one connector; forgetting that each mated pair contributes twice; omitting splitters entirely in a PON budget; ignoring the aging margin; comparing a channel measurement with a permanent-link budget.
Key points
- Power budget = minimum Tx power − receiver sensitivity.
- Loss budget sums worst-case fiber, connector, splice and splitter losses.
- Reserve at least 2–3 dB margin for aging, repairs and temperature.
- Use maximum attenuation values: 0.4 dB/km at 1310 nm, 0.3 dB/km at 1550 nm.
- A 1:32 splitter costs about 17.5 dB and must be in every PON budget.