|  | 4.3 | Basics of Signals and Noise in Communcations Systems
| 4.3.1 | Comparing analog and digital signals
Signal refers to a desired electrical voltage, light pattern, or modulated electromagnetic wave. All of these can carry networking data. One type of signal is analog. An analog signal has the following characteristics: is wavy has a continuously varying voltage-versus-time graph is typical of things in nature has been widely used in telecommunications for over 100 years The main graphic shows a pure sine wave. The two important characteristics of a sine wave are its amplitude (A) - its height and depth - and its period (T) - length of time to complete 1 cycle. You can calculate the frequency (f) - wiggley-ness - of the wave with the formula f = 1/T. Another type of signal is digital. A digital signal has the following characteristics: has discrete, or jumpy, voltage-versus-time graphs is typical of technology, rather than nature The graphic shows a digital networking signal. Digital signals have a fixed amplitude, even though their pulsewidth, T, and frequency, can be changed. Digital signals from modern sources can be approximated by a square wave, which has seemingly instantaneous transitions from low to high voltage states, with no wiggles. While this is an approximation, it is a reasonable one, and will be used in all future diagrams.