Data communications deals with two types of information: analog and digital. An analog signal is characterized by a continuous mathematical function — when the input changes from one value to the next, it does so by moving through all possible intermediate values. In contrast, a digital signal has a fixed set of valid levels, and each change consists of an instantaneous move from one valid level to another. Figure 6.1 illustrates the concept by showing examples of how the signals from an analog source and a digital source vary over time. In the figure, the analog signal might result if one measured the output of a microphone, and the digital signal might result if one measured the output of a computer keyboard.

          ANALOG SIGNAL; 

An analog signal is a continuous wave denoted by a sine wave (pictured below) and may vary in signal strength (amplitude) or frequency (waves per unit time). The sine wave's amplitude value can be seen as the higher and lower points of the wave, while the frequency value is measured in the sine wave's physical length from left to right.


There are many examples of analog signals around us. The sound from a human voice is analog, because sound waves are continuous, as is our own vision, because we see various shapes and colors in a continuous manner due to light waves. Even a typical kitchen clock having its hands moving continuously can be represented as an analog signal.

                                                                              Analog Signal - DIAGRAM

Analog signal represented as a sine wave

Analog Signal - Sine Wave

Digital Signal

A digital signal - a must for computer processing - is described as using binary (0s and 1s), and therefore, cannot take on any fractional values. As illustrated in the graphic below, digital signals retain a uniform structure, providing a constant and consistent signal. Because of the inherent reliability of the digital signal, technology using it is rapidly replacing a large percentage of analog applications and devices. For example, the wristwatch, showing the time of day, with its minute, hour, and sweeping second hands, is being replaced by the digital watch, which offers the time of day and other information using a numerical display. A typical digital signal is represented below. Note the equally dispersed 1s and 0s.

                                                                                 Digital signal with binary