A fibre link works only if enough light reaches the receiver. A loss budget adds up everything that removes light along the route and compares the total with what the equipment can tolerate. Doing the sum before laying the cable avoids a link that tests as unusable after installation.
The formula
Total link loss (dB) equals fibre loss plus connector loss plus splice loss plus design margin.
| Term | How to calculate it | Where the value comes from |
|---|---|---|
| Fibre loss | Route length in km multiplied by attenuation in dB per km | The fibre datasheet, at the wavelength in use |
| Connector loss | Number of connector pairs multiplied by loss per pair | The connector supplier |
| Splice loss | Number of splices multiplied by loss per splice | The splicing method and the splice machine records |
| Design margin | A reserve for repairs, ageing and temperature | The network designer, commonly a few decibels |
Compare with the equipment budget
The equipment maker gives a power budget, which is the difference between transmitter output and receiver sensitivity. The total link loss must stay below that figure. A link that is close to the limit on paper will fail as soon as a repair splice is added.
Worked example with assumed values
The figures below are assumed for the arithmetic and are not APAR ratings. Use the datasheet values for the fibre actually supplied.
| Item | Assumed value | Quantity | Loss (dB) |
|---|---|---|---|
| Fibre, single-mode | 0.4 dB per km | 20 km | 8.0 |
| Connector pairs | 0.5 dB each | 2 | 1.0 |
| Splices | 0.1 dB each | 8 | 0.8 |
| Design margin | 3.0 dB | 1 | 3.0 |
| Total link loss | 12.8 | ||
In this example the equipment power budget must be at least 12.8 dB. If it is lower, shorten the route, reduce the number of splices and connectors, or choose equipment with a larger budget.
Ways to keep the loss down
Choose cable lengths that reduce the number of joints, since each splice adds loss. Keep to the bend radius stated on the cable datasheet, because tight bends add loss. Single-mode fibre suits long routes because of its lower attenuation, while multi-mode suits shorter campus and in-building runs. APAR’s single-mode and multi-mode fibre page covers both types. For choosing a cable construction for the route, see the fibre optic cable selection guide.