How the calculator works
Margin = power budget − plant loss − penalties − reserve, where the power budget is the minimum transmit power minus the worst-case receiver sensitivity. All terms are in dB; the usual design target is a margin of at least 3 dB.
| Term | Formula | Typical design value |
|---|---|---|
| Power budget | Tx minimum − Rx sensitivity | 10GBASE-LR: −8.2 − (−14.4) = 6.2 dB |
| Fibre | length × attenuation coefficient | 0.35 dB/km at 1310 nm, 0.22 dB/km at 1550 nm, 3.0 dB/km at 850 nm (multimode) |
| Connectors | mated pairs × loss per pair | 0.5 dB per pair (0.75 dB by TIA-568.3-D) |
| Splices | splices × loss per splice | 0.1 dB per fusion splice |
| Other components | sum of their insertion loss | CWDM mux 1–2.5 dB, 1:2 splitter 3.5 dB |
| Penalties | dispersion, reflections, crosstalk | 0 for a standard interface at its rated reach; 0.5–1.5 dB otherwise |
| Reserve | laser ageing and future repairs | 1–3 dB |
The presets carry typical average-power figures from the IEEE and MSA specifications; for multi-lane modules they are per lane. A real module's data sheet — or its own memory — always wins.
Margin and verdict
| Margin | Verdict |
|---|---|
| at or above the target (3 dB by default) | the design passes |
| 1 dB up to the target | passes with conditions: connectors verified clean, monitoring alerts on |
| 0–1 dB | works on day one, fails on the first dirty connector or hot day — redesign |
| below 0 | the link does not close: a longer-reach module, fewer connections or an amplifier |
Overload and attenuators
A long-reach module on a short link can deliver more light than the receiver accepts. The best case at the receiver is Tx maximum − plant loss — penalties and reserve do not reduce the power. If it is above the overload level, the link needs a fixed attenuator at the receiving end. The calculator picks a standard value that brings the best case below overload while the worst case keeps the target margin. Budgets are per direction, so check both ends.
Maximum length
L max = (power budget − connectors − splices − other − penalties − reserve − target margin) / attenuation coefficient — the longest fibre that still leaves the target margin with the same connections.
Worked example
10GBASE-LR over 10 km at 1310 nm with 4 connector pairs and 4 splices: 6.2 − (3.5 + 2.0 + 0.4) = 0.3 dB of margin with design allowances — marginal. With typical connectors at 0.2 dB the margin is 1.5 dB. Measure the installed loss, or use a 10GBASE-ER module with an attenuator.
More in the documentation: Link budget engineering, Rx power and the link budget, Typical values.