Digital and Analog Signaling

Digital and Analog Signaling

Signaling amounts to communicating information. The information being communicated can take one of two forms—analog or digital:

Analog information changes continuously and can take on many different values. An analog clock’s hands move constantly, displaying time on a continuous scale.
Digital information is characterized by discrete states. A light bulb, for example, is on or off. A digital clock represents the time in one-minute intervals and doesn’t change its numbers again until the next minute. A digital clock can represent exact minutes but not the seconds that pass in between.

Frequently, information existing as one form must be converted to the other. This conversion often involves the use of some encoding scheme that enables the original information to be recovered from a signal after the signal has been received.

When an analog or a digital signal is altered so that it contains information, the process is called modulation or encoding. AM radio, for example, transmits information by modulating the radio signal, which increases or decreases the amplitude (signal strength) depending on the information content. Many similar schemes are used to communicate information through different types of signals.

Figure 7.1 illustrates the difference between analog and digital signals. The analog signal constantly changes and takes on values throughout the range of possible values. The digital signal takes on only two (or a few) specific states.

A modem is the most common computer connectivity device that transmits an analog signal. (Refer to Chapter 6, “Connectivity Devices,” for more on modems.) Modems transmit digital computer signals over telephone lines by converting them to analog form. Modems are wonderfully handy for PC-to-PC communications or for accessing a LAN from a remote location, but modems generally are too slow and too unreliable for the high-tech task of linking busy LAN segments into a WAN. Because computer data is inherently digital, most WANs use some form of digital signaling.

Further Information

Analog Waveforms