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A link budget answers one question: does the transmitter have enough power to reach the receiver with something left over? Enter the span, the connectors and splices in the channel, and the transmitter and receiver figures from the transceiver datasheet. Everything recalculates as you type.
What the calculation does. Power budget is simply the minimum transmitter output minus the receiver sensitivity — the total number of decibels you have to spend. Channel loss is what the fibre, the connectors, the splices and any passive components take out of it. Subtract the channel loss and your safety margin from the power budget and what is left is the remaining margin. Positive means the link works; negative means it does not.
Getting the inputs right. Two mistakes account for most failed link budgets. The first is using the map distance instead of the installed route length — cable follows ducts, not straight lines, and 20% extra is normal. The second is forgetting the mated pairs inside patch panels: a link that looks like two connectors usually has four or six once you count the panels at each end. Count every mated pair in the channel.
Why keep a safety margin. Fibre and connectors age, temperature moves the numbers around, and sooner or later somebody puts a digger through the duct and you gain a repair splice. Three decibels of margin is the normal planning figure for a link you expect to run for years. On a link you can re-engineer easily, you can run tighter.
Around 3 dB is the common planning figure — it covers ageing, temperature drift and a repair splice or two. Some operators use 2 dB indoors where the environment is controlled and nothing gets dug up, and 5 dB or more on long external routes that are likely to be repaired during their life.
For single-mode, about 0.35 dB/km at 1310 nm and 0.22 dB/km at 1550 nm. For multimode at 850 nm, about 3.0 dB/km — but multimode links are almost never attenuation-limited, they are limited by modal bandwidth, so check the standard's reach limit as well as the loss.
Budget 0.3 dB per mated pair for single-mode and 0.5 dB for multimode as a planning figure. Good factory-terminated connectors measure well under that — our assemblies are specified at 0.35 dB maximum and typically measure 0.25 dB — but plan with the worst case, not the typical.
Three usual causes. Receiver overload — on a short link with a long-reach optic, there can be too much light, and you need a fixed attenuator. Dirty connectors — the single most common cause, and invisible without an inspection scope. Or dispersion rather than loss, which the dispersion calculator will tell you about.
Pulse spread from Δt = D · L · Δλ, and the distance at which dispersion — not loss — ends the link
Open tool →Convert optical power between logarithmic and linear units, in both directions
Open tool →Turn insertion loss in decibels into the percentage of light that actually gets through
Open tool →Transceivers, MPO assemblies, WDM and patch cords — built and tested to the figures you just calculated.