How a fibre link budget works
The power budget of a link is the difference between the transmitter's minimum launch power and the receiver's sensitivity, both from the transceiver datasheet. Everything between them uses up that budget: fibre attenuation (dB per km times length), each mated connector pair and each splice. What is left must cover a safety margin for ageing, temperature, repair splices and dirty connectors — 3 dB is common practice.
Connector and splice loss can be taken at the maximum allowed by TIA-568.3-D (0.75 dB per mated pair, 0.3 dB per splice), which is the safe design basis, or at typical installed values for factory-polished connectors and fusion splices (around 0.5 dB and 0.1 dB). If a link passes only on typical values, measure the installed loss before commissioning.
On multimode fibre the distance is often limited by modal bandwidth, not by loss. 1000BASE-SX reaches 275 m on OM1 and 550 m on OM2 according to IEEE 802.3, even though the power budget would allow more. A link longer than the standard's reach can fail with power to spare, so the calculator checks both limits and reports the smaller one as the maximum usable length.
Short single-mode links with high-power optics can overload the receiver. The calculator compares the strongest transmitter with a low estimate of the link loss against the receiver overload level; if exceeded, an in-line attenuator is needed.
Formulas
- Power budget:
- PB = Ptx,min − Prx,sens (dB)
- Link loss:
- L = α·ℓ + Nc·Lc + Ns·Ls (fibre attenuation α in dB/km, length ℓ, connector pairs Nc, splices Ns)
- Margin left:
- M = PB − L − Msafety ≥ 0 for the link to pass
- Maximum length by loss:
- ℓmax = (PB − Msafety − Nc·Lc − Ns·Ls) / α
IEEE 802.3 reach used by the calculator
| Standard | Wavelength | Fibre | Reach |
|---|---|---|---|
| 100BASE-FX | 1300 nm | OM1 / OM2 multimode | 2 km |
| 100BASE-LX10 | 1310 nm | OS2 single-mode | 10 km |
| 1000BASE-SX | 850 nm | OM1 / OM2 (OM3, OM4) | 275 m / 550 m |
| 1000BASE-LX | 1310 nm | Single-mode / OM1–OM2 with mode-conditioning cord | 5 km / 550 m |
| 1000BASE-LX10 | 1310 nm | OS2 single-mode | 10 km |
1000BASE-ZX is a manufacturer specification, not an IEEE 802.3 standard. OM3 and OM4 are taken at the OM2 reach for 1000BASE-SX, which is conservative.
Frequently asked questions
How many connectors should I count?
Count every mated pair: the patch cord into the panel at each end and any intermediate patch panels. A simple panel-to-panel link with patch cords at both ends usually has 2 mated pairs on the permanent link, plus the equipment connections if the budget includes the patch cords.
Is 3 dB margin enough?
It is the usual design margin for substation and industrial links. Use more for long outdoor routes where repair splices are likely, and keep a record of the measured loss at commissioning to compare later.
Why does my 1000BASE-SX link fail at 400 m on OM1 with power to spare?
The IEEE 802.3 reach for 1000BASE-SX on OM1 (62.5 µm, 200 MHz·km) is 275 m. Beyond that the pulses spread (modal dispersion) and bit errors rise regardless of power. Use OM3/OM4 or single-mode with LX optics.
Where do the transceiver values come from?
The presets are representative datasheet values. Real modules differ, so enter the minimum transmit power and receiver sensitivity from the datasheet of the module you use.