What is an important factor that limits the transmission of digital signal over long distances?

Diffraction describes the bending of waves around obstacles or through apertures, which affects signal directionality but is not the primary cause of signal loss over long transmission distances.

Coherence refers to the consistent phase relationship between wave cycles and is relevant in optical or laser systems, but it is not the primary limiting factor for general digital signal transmission over distance.

Attenuation refers to the reduction in signal amplitude and power as the signal propagates through a medium over distance. As a digital signal weakens, it becomes increasingly difficult to distinguish binary values, leading to bit errors and eventual communication failure. Repeaters or amplifiers must be deployed at intervals to compensate for attenuation and extend the usable transmission range.

Wave velocity affects propagation delay but remains relatively constant within a given medium and does not cause the signal power degradation that restricts long-distance digital transmission.