In practice, we don't come close to approaching this limit, because Nyquist's rule applies only to noiseless channels. In practice, every channel has background noise. It cannot be eliminated, and depends on the temperature, bandwidth, and Boltzman's constant K . Is uniformly distributed across the frequency spectrum and thus called white noise. Crosstalk noise results from unwanted coupling between signal paths.
Impulse noise consists of sharp, shortlived disturbances, from such sources such as lightning.
Stallings Fig 2.15. Unlike Nyquist's limit, Shannon's limit is valid regardless of the encoding method. Let's consider a phone line again. Thus, the maximum data rate bps. Read the fine print.