Repeaters are amplifiers placed along the channel that restore the signal power. Some attenuation affects are due to absorption. The amount of attentuation is measured in . This introduces changes in shape of the signal. Propagation delay is the term used to describe the time taken for the signal to pass along the channel.
Electromagnetic signals travel at velocities near that of light in vacuo of .
A more typical value is . The importance of propagation delay varies widely with the length of the channel. The effects of noise on a signal Impulse noise is common in low-frequency circuits and arises from electric fields generated by electrical switching. It appears as bursts at the receiver, and when present can have a catastrophic effect due to its large power. When radio links are used, interference from other transmitters can be problematic.