|  | 4.3 | Basics of Signals and Noise in Communications 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) - wiggleyness - 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 but their pulsewidth and frequency can be changed. Digital Signals have a fixed amplitude but their pulsewidth and frequency can be changed. While this is an approximation, it is a reasonable one, and will be used in all future diagrams.