How the limit is calculated
Loss limit = attenuation coefficient × length + connector pairs × loss per pair + splices × loss per splice + other components, separately at each test wavelength: 1310 and 1550 nm for single-mode, 850 and 1300 nm for multimode.
| Allowance | TIA-568.3-D | Design allowances |
|---|---|---|
| Connector pair | 0.75 dB | 0.5 dB |
| Splice | 0.3 dB | 0.1 dB |
| Single-mode fibre, 1310 / 1550 nm | 0.5 / 0.5 dB/km | 0.35 / 0.22 dB/km |
| Multimode fibre, 850 / 1300 nm | 3.0 / 1.5 dB/km | 3.0 / 1.0 dB/km |
A standard gives the most a compliant link may lose; a well-built link measures well below it. ISO/IEC 14763-3 grades connections and customer or carrier specifications are often stricter — enter their allowances as custom values, use the stricter set and record which one you used.
Measuring the link
Measure insertion loss with a light source and a power meter at both working wavelengths. With a one-jumper reference the result includes the connections at both ends of the link — count them as connector pairs; the two- and three-jumper methods leave one or both of them out. Investigate any connection above 0.5 dB, even when the link as a whole passes.
More in the documentation: Testing and measurement, Link budget engineering, Typical values.