This document discusses transmission impairments in communication systems. It describes three main causes of signal impairment: attenuation, distortion, and noise. Attenuation is the loss of energy as the signal strength decreases with distance. Distortion occurs when different frequency components of a composite signal travel at different speeds, causing them to arrive at different times and be out of phase. Noise refers to unwanted random signals that mix with the original signal, such as induced noise from devices, thermal noise from electron movement, crosstalk between wires, and impulse noise from sources like lightning.