attenuation

Shout across a field and a friend nearby hears you clearly, but the same shout reaches someone far away as a faint whisper. The signal got weaker as it travelled — that fading-with-distance is attenuation. Every medium saps a signal's strength as it moves through it: copper turns some signal energy into heat, fiber absorbs and scatters a little light, and radio waves spread out and thin as they fan into space.

Concretely, attenuation is the loss of signal power between sender and receiver, usually measured in decibels (dB): a positive dB loss means the received power is a fraction of what was sent, with each 3 dB roughly halving the power. It generally grows with distance and often with frequency too — higher-frequency signals tend to fade faster, which is why high-speed copper runs are kept short. As the signal weakens but the noise stays put, the signal-to-noise ratio falls, and once SNR drops too low the receiver can no longer tell symbols apart and errors climb. That is why links have a maximum reach: Ethernet over copper is limited to about 100 metres, not because the bits 'run out', but because beyond that the signal is too faint relative to noise.

The fix for distance is not to shout louder forever (more power has limits and adds its own problems) but to refresh the signal along the way. An amplifier boosts an analog signal — but it boosts the noise riding along with it too. A repeater or regenerator does better for digital: it reads the still-recognisable bits, then transmits a clean, full-strength copy, leaving the accumulated noise behind. This is one of digital's great advantages, and it is why long fiber runs and undersea cables are dotted with regenerators.