ESTEL ACADEMY · COURSE 2
Building a Link Loss Budget
Plan for weak signals, strong signals and a practical engineering reserve.
LESSON 4 OF 5 · 5 MIN READ
01 · START WITH SPECIFICATIONS
A budget is an allowance for loss
A basic power-budget calculation compares minimum transmitter power with the receiver sensitivity specified for the same optical interface and compatible measurement conditions.
Screening power budget = Tx minimum − Rx sensitivity
Teaching example: −2 dBm − (−12 dBm) = 10 dB.
This arithmetic is only a screening check. Modern interfaces may impose a smaller channel insertion-loss limit, signal penalties and other requirements. Do not treat sensitivity and OMA specifications as interchangeable with average-power readings.
02 · PLAN THE PATH
Use the limiting allowance
| Example requirement | Value |
|---|---|
| Calculated screening budget | 10 dB |
| Specified maximum channel insertion loss | 8 dB |
| Chosen planning reserve within that limit | 2 dB |
| Maximum planned path loss | 8 − 2 = 6 dB |
| Estimated installed path loss | 5 dB |
| Headroom after the chosen reserve | 6 − 5 = 1 dB |
All numbers are invented for this example. The 2 dB reserve is an explicit design choice, not a universal rule. Avoid deducting the same penalties or reserves twice when the interface specification already includes them.
Estimate losses at the operating wavelength: fibre attenuation in dB/km × route length in km, plus connector, splice and passive-component losses. Use actual component data and validate the installed path.
03 · CHECK THE OTHER LIMIT
Too much power can also fail
Use maximum Tx power and minimum expected path loss to check the strongest possible input. Compare it with the receiver’s permitted maximum operating power.
Separate teaching example: Tx maximum +2 dBm, minimum path loss 1 dB, receiver maximum 0 dBm.
Strongest predicted Rx = +2 − 1 = +1 dBm, which exceeds that receiver limit by 1 dB.
An attenuator may be appropriate, but its minimum and maximum attenuation must satisfy both the strong-signal and weak-signal cases. Never choose it from the reach label alone.
04 · CHECK YOUR UNDERSTANDING