4.3 Basics of Signals and Noise in Communications Systems 4.3.7 Noise Noise is unwanted additions to voltage, optical, or electromagnetic signals. In other words, each bit receives additional unwanted signals from various sources. Too much noise can corrupt a bit turning a binary 1 into a binary 0, or a 0 into a 1, destroying the message.
Figure shows five sources of noise that can affect a bit on a wire.
When electrical noise on the cable originates from signals on other wires in the cable, this is known as crosstalk . NEXT stands for near-end crosstalk . When two wires are near each other and untwisted, energy from one wire can wind up in an adjacent wire and vice versa. This can cause noise at both ends of a terminated cable. There are actually many forms of cross-talk which must be considered when building networks.