Hong Life CalculatorsUnderstanding the fibre optic link loss budget calculator — concept and principle
A loss budget tells you whether light from an optical transmitter will still be strong enough at the far end for the receiver. Installers use it before buying optics and when checking a link after splicing. The calculator sums the losses along the fibre and compares them with the transceiver pair's power budget.
How it is calculated
- Total loss (dB) = length (km) × attenuation (dB/km) + connectors × 0.5 dB + splices × 0.1 dB.
- Power budget (dB) = transmitter output (dBm) − receiver sensitivity (dBm).
- Margin (dB) = power budget − total loss.
The 0.5 dB per connector and 0.1 dB per splice are typical values for good workmanship. The TIA-568 cabling standard allows up to 0.75 dB per mated connector pair and 0.3 dB per splice, so a worst-case budget would use those figures instead.
The principle
Decibels turn multiplication into addition. Each piece of fibre, connector and splice passes only a fraction of the light, and the fractions multiply. On a logarithmic scale (dB = 10 × log₁₀ of the power ratio) those factors simply add, so every loss can be summed. dBm is power relative to 1 mW, so subtracting two dBm values gives a ratio in dB. Fibre attenuation comes mainly from scattering in the glass and depends strongly on wavelength: standard single-mode fibre loses roughly 0.3–0.4 dB/km at 1310 nm and less at 1550 nm, while multimode fibre loses several dB/km at 850 nm.
Worked example
- Inputs: Tx −3 dBm, Rx sensitivity −20 dBm, 20 km at 0.35 dB/km, 4 connectors, 6 splices.
- Fibre loss: 20 × 0.35 = 7.0 dB.
- Connector loss: 4 × 0.5 = 2.0 dB; splice loss: 6 × 0.1 = 0.6 dB.
- Total loss: 7.0 + 2.0 + 0.6 = 9.6 dB.
- Power budget: −3 − (−20) = 17 dB.
- Margin: 17 − 9.6 = 7.4 dB.
| Fibre length (km) | Total loss (dB) |
|---|---|
| 5 | 4.35 |
| 10 | 6.1 |
| 15 | 7.85 |
| 20 | 9.6 |
| 25 | 11.35 |
| 30 | 13.1 |
| 40 | 16.6 |
How to read the result
A positive margin means the link should work; a negative one means the receiver will see too little light. Designers usually keep at least about 3 dB of margin for ageing, temperature changes, repairs that add splices and dirty connectors. Expected received power here is −3 − 9.6 = −12.6 dBm. On short links with powerful optics, also check the receiver's overload level; an attenuator may be needed.
Common mistakes
- Using the transceiver's typical output instead of the minimum guaranteed output.
- Forgetting the connectors at patch panels, not just at the two ends.
- Treating dBm and dB as interchangeable; dBm is absolute power, dB is a ratio.
Frequently asked questions
What is a good loss margin for fibre?
Many designers aim for 3 dB or more after all known losses. Very long or critical links often get more.
Does dispersion appear in the loss budget?
No. Chromatic and modal dispersion limit bandwidth and reach separately, so a link can pass the loss budget and still fail at high data rates.
Sources: ANSI/TIA-568.3-D, Optical Fiber Cabling and Components Standard · ITU-T Recommendation G.652, Characteristics of a single-mode optical fibre and cable