Cabling · Networking
Attenuation
The loss of signal strength as it travels through a cable or fiber. Too much attenuation and the signal is too weak to be reliably decoded at the far end.
All transmission media attenuate signals — copper cable through resistance and capacitance, fiber through light scattering and absorption in the glass. The question is whether the attenuation over a given distance falls within the budget for that link.
Attenuation is measured in decibels (dB). A 3dB loss cuts the signal power in half; 10dB cuts it to one-tenth. Fiber links have a loss budget — the maximum total attenuation the transceiver pair can accommodate — and every connector, splice, and meter of cable consumes part of that budget.
For copper Ethernet, attenuation limits are built into the category specification. The 100-meter max run for Cat6 is partly an attenuation limit — beyond that, signal loss is too high for reliable 1Gbps operation. PoE is even more constraining, since power also degrades over resistance.
For fiber, attenuation varies by:
- Fiber type — single-mode glass has lower attenuation per kilometer than multimode, which is why single-mode is used for long-distance runs.
- Wavelength — 1310nm and 1550nm windows in single-mode fiber are where glass is most transparent.
- Connectors and splices — each connection point adds insertion loss. A clean, well-polished connector adds ~0.3dB; a dirty or damaged one can add several dB.
- Bends — tight bends cause microbending loss in fiber, which is why bend radius specifications exist.