How a fiber loss budget works
Every part of a link costs some light. The loss budget adds those costs up and compares the total with what the optics can tolerate.
Total loss = length × attenuation + connector pairs × loss per pair + splices × loss per splice + margin
The power budget is the other side of the ledger: the optic's minimum transmit power minus the receiver's sensitivity. If the power budget is bigger than the total loss, the link should come up with headroom to spare.
Power budget = Tx power − Rx sensitivity · Headroom = power budget − total loss
Worked example
A 12.5 km single-mode run at 1310 nm, two mated connector pairs (one patch panel at each end), six fusion splices and a 3 dB margin, lit with an optic rated −3 dBm transmit and −24 dBm sensitivity.
- Fiber: 12.5 km × 0.35 dB/km = 4.38 dB
- Connectors: 2 × 0.75 dB = 1.50 dB
- Splices: 6 × 0.3 dB = 1.80 dB
- Margin: 3.00 dB
Total loss is 10.68 dB against a 21 dB power budget, leaving 10.3 dB of headroom. The receiver should see about −10.7 dBm today (the margin is a reserve, not a loss that exists yet).
Design limits or typical values?
Use the TIA-568 maximums when you are writing a spec or setting acceptance limits: 0.75 dB per mated connector pair, 0.3 dB per splice, and the cable maximums in the dropdown. A link built from parts that each meet their maximum is guaranteed to meet the budget.
Use typical values when you want to predict what your light source and power meter will actually read. A good fusion splice reads well under 0.1 dB and a clean, modern connector pair around 0.3 dB, so a real link usually tests well below its calculated maximum. If it tests above, something is wrong: a dirty connector, a tight bend, or a bad splice.
What the margin is for
The margin covers what will happen to the link after you leave: repair splices after a cable cut, connectors that get dirtier with every mating, and aging of the optics. Three dB is a common choice for outside plant. Short premises links are often designed with less. Do not count the margin when you compare against a measured result; compare the meter reading with the loss without margin.
Common mistakes
- Counting connectors instead of mated pairs. Loss happens where two connectors meet. A link with a patch panel at each end usually has two mated pairs, not four connectors' worth of loss.
- Using the typical Tx power instead of the minimum. Optics vary. The datasheet minimum is what you can count on.
- Too much light. A short single-mode link with a long-reach optic can overload the receiver. If the expected Rx level is above the optic's maximum input, add an attenuator.
- Ignoring wavelength. Loss at 1310 nm is higher than at 1550 nm per km, but bends hurt 1550 nm (and 1625 nm) more. Test at the wavelengths the link will carry.