Fibre Optic Link Budget Calculator
Will this fibre run work with these optics? Add up the loss from the fibre, connectors, splices and patch panels, then compare it with what the transceiver can handle. You'll see the margin, the light level at the far end, whether it needs an attenuator and the longest run that would still work.
- Single-mode and multimode
- 1G to 100G optics
- IEEE limits and TIA losses
Result
Loss breakdown
This transceiver
Loss figures we use
| Part | Typical | Worst case (TIA-568.3-D maximum) |
|---|---|---|
| Mated connector pair (LC, SC, MPO) | 0.3 dB | 0.75 dB |
| Fusion splice | 0.15 dB | 0.3 dB |
| Mechanical splice | 0.3 dB | 0.3 dB |
| OS2 single-mode at 1310 nm | 0.35 dB/km | 1.0 dB/km inside buildings, 0.5 dB/km outside |
| OS2 single-mode at 1550 nm | 0.25 dB/km | 1.0 dB/km inside buildings, 0.5 dB/km outside |
| OM3, OM4, OM5 at 850 nm | 3.0 dB/km | 3.0 dB/km |
| OM1, OM2 at 850 nm | 3.0 dB/km | 3.5 dB/km (the older limit, to be safe with legacy cable) |
Transceiver figures are average optical power from IEEE 802.3 (as published in Cisco's datasheets and IEEE documents). ZX, ZR and 20 km BiDi optics aren't IEEE standards, so we use typical vendor datasheet figures: always check your own module's datasheet. For 40G and 100G optics, figures are per lane.
How the calculation works
- Total loss is the fibre's length times its loss per kilometre, plus every connector pair and splice.
- For IEEE optics we compare it with the standard's maximum channel insertion loss. That's the most loss the standard allows between the two transceivers, after allowing for the optics' own penalties. For 1G optics it's well below the transmitter minus the receiver figure.
- For other optics the power budget is the weakest transmitter minus the receiver sensitivity, and we keep a 3 dB safety margin for ageing, dirt and future repairs.
- Multimode distance is also limited by bandwidth: past the standard's reach for your fibre grade, the signal smears even if there's enough light.
- Overload: long-reach optics can blind a receiver on a short run. If the strongest transmitter minus the least loss is above the receiver's maximum, fit an attenuator.
Testing a fibre link
- Light source and power meter (OLTS): the test the standards are based on. It measures the total loss end to end at each wavelength. Set a reference with good test leads first, and compare the result with this calculator's figure.
- OTDR: sends pulses down the fibre and plots the reflections, so you can see the distance to each connector, splice, bend or break and its loss. Use launch and receive leads so the end connectors are measured too.
- On a live link, most switches show the optic's transmit and receive power (DOM or DDM), for example show interfaces transceiver on Cisco. Compare the received power with the range above.
- Inspect and clean every end face before connecting. A single speck of dust on a single-mode core can add more loss than a kilometre of fibre.