Link budget engineering

A link budget is the one calculation every fibre link is designed by and every failing link is judged against: does the light that leaves the transmitter, minus everything the plant takes, still exceed what the receiver needs — with room to spare for ageing, repairs and a dirty connector? This page gives the equation and its terms, the standard allowances for each loss, the penalties that are not losses but behave like them, a margin policy, four worked examples, and the opposite problem: too much light.

The equation

Power budget      = Tx power (min)  −  Rx sensitivity (worst)                        [dB]
Plant loss        = fibre (km × dB/km) + connector pairs + splices + passive devices   [dB]
Penalties         = dispersion + reflections + crosstalk + modal noise (spec-defined)  [dB]
Ageing / repair   = 1–3 dB reserved                                                   [dB]
Margin remaining  = Power budget − Plant loss − Penalties − Ageing  (target ≥ 3 dB)

Where the numbers come from:

Allowances by standard

Use the stricter of the customer's standard and the practical numbers; document which.

Penalties that are not losses

IEEE PMD budgets already contain the penalties for the standard reach on standard fibre; add your own only for non-standard combinations.

Margin policy

Worked examples

1. 10GBASE-LR over 10 km SMF, 4 connector pairs, 4 splices

2. 10GBASE-ER over 35 km SMF at 1550 nm, 6 pairs, 20 splices

3. 100GBASE-SR4 over 90 m OM4, 2 MPO pairs

4. GPON class B+ over 18 km, 1:32 split, 5 pairs, 8 splices

Too much light: overload and attenuators

Receivers have a maximum input (overload, −1 … +3 dBm for PIN, lower for APD). A 40 km ER module on a 2 km link delivers −5 dBm to a receiver rated to +1 — fine; a ZR module with +4 dBm Tx on a patch cord delivers +3.5 dBm to an APD rated to −7 — errors or damage.

Both ends must be checked: budgets are per direction and the two ends' modules may differ (Rx power & budget).

From design to acceptance

1. Design with allowances → margin ≥ 3 dB.
2. Install; measure insertion loss with LSPM at the working wavelengths (Testing).
3. Compare measured loss with design loss; investigate any connector > 0.5 dB.
4. Light the link; record Tx and Rx DDM at both ends — the link passport (Monitoring).
5. Margin = Rx power − sensitivity; alert if it drops by 2 dB from the passport.

In CodingBox

Step 4 starts on the bench: read each module's Tx power and thresholds before install, and after install read the Rx values on the switch — the pair of numbers is the measured budget. CodingBox stores the module's values with its serial so the passport survives module swaps (DDM in the app, Code database).

The numbers on one page — attenuation, allowances, reflectance, geometry, dispersion, budgets of common interfaces: Typical values; the formulas with worked examples: Formulas & calculations; the module side of the budget — launch power, sensitivity, overload, FEC: Transceivers in the link; when the budget runs out — amplifiers, OSNR and regeneration: Amplifiers & regeneration.