This property is one of the principal reasons digital communication became increasingly important in the last years of the 20th century. Noise is the backdrop of the universe. Atoms and molecules in motion create random electromagnetic signals that prevent any communication channel from being perfectly clear. Of course, all sorts of events, from elevator motors and electric mixers to lightning and solar flares, also contribute noise to our communications environment. Some encoding techniques are less susceptible to particular kinds of noise than others.
Crosstalk is a special form of noise that is induced by other signals on a common medium.
Digital signal transmission is used in LANs, where cable lengths are relatively short and thus not subject to severe attenuation. This code is the archetype of what a digital signal looks like, although there are innumerable variations in how best to transmit a digital signal across various media.