Networking Explained, 2nd Edition

142 Networking Explained, Second Edition

9. Tell me more about analog communication.

The term analog refers to any physical device or signal that can continuously vary in

strength or quantity, for example, voltage in a circuit. The term analog communication

refers to any method of communication based on analog principles. Typically, this term is

associated with voice transmission instead of data transmission because voice transmis-

sion facilities, such as the telephone, were initially analog-based. The “other” type of

physical communication is digital communication, which we’ll talk about later.

10. What does harmonic motion have to do with analog communication?

To answer this question, consider an object attached to a spring that is suspended from

a ceiling (Figure 4.2). If you pull on the attached weight and release it, the spring begins

oscillating up and down. In a frictionless environment, this up and down motion would

continue forever. This idealized motion is called simple harmonic motion, which is the

basic model for vibratory or oscillatory motion and can occur in many different types of

wave motion. Examples include mechanical oscillators such as mass-spring systems simi-

lar to that shown in Figure 4.2, and pendulums; periodic motion found in the earth sci-

ences such as water waves, tides, and climatic cycles; and electromagnetic waves such as

alternating electric currents, sound waves, light waves, radio waves, and television waves.

11. Okay, but what does this have to do with analog communication?

In any computer communications system, data are transmitted across a medium from

sender to receiver in the form of electrical signals. In analog communications, signals flow

across a wire in the form of electromagnetic waves. When viewed by an oscilloscope,

these signals appear as continuous waves called sinusoidal waves, which resemble a sine

curve (Figure 4.3). Sinusoidal waves are characteristic of anything that oscillates, and they

have the following three attributes: amplitude, which is the level of voltage on a wire (or

the intensity of a light beam when dealing with fiber-optic cable); frequency, which is the

number of oscillations, or cycles, of a wave in a specified length of time; and phase, which

is the point a wave has advanced within its cycle. A frequency rate of one cycle per second

is defined as 1 hertz (abbreviated Hz) in honor of Heinrich Rudolph Hertz (1857–1894), a

FIGURE 4.2 An object is attached to a spring that is suspended from a ceiling. When pulled and

released, the spring oscillates up and down. This oscillation is called simple harmonic.

Chapter 4: Physical Layer Concepts 143

German physicist who in the late 1880s was the first to produce radio waves artificially.

Thus, hertz is a measure of frequency in cycles per second. The reciprocal of frequency is

called period, which is the amount of time it take to complete a single cycle, that is, sec-

onds per cycle.

12. Can you bring this down to earth for me?

Sure. Consider the act of speaking over the telephone. If we speak softly or whisper,

the amplitude (volume) decreases. If we speak loudly or scream, the amplitude increases.

If we speak in a high-pitched voice, the frequency changes to more cycles per second than

if we speak in a low-pitched voice, which requires fewer cycles per second (see Figure

4.4). AM/FM radio, television speakers, public address systems, and most important of all,

traditional telephones, are all examples of analog devices (although there is a fast-growing

trend toward a full-digital telephone system both in business and wireless networks).

13. How does this relate to data communications?

In data communications, data are represented in analog form by varying the voltage of

the wave (called amplitude modulation, abbreviated AM), by varying the frequency

(called frequency modulation, abbreviated FM), or by varying the phase (called phase

modulation or phase shifting) of a wave. Phase modulation is often used for modems,

which is discussed in Chapter 15.

14. How does the term “wavelength” relate to this discussion?

Wavelength, as the name implies, is a measure of the length of a wave. It is the dis-

tance an electrical or light signal travels in one complete cycle. Radio signals are often

described and classified according to their wavelength. For example, “the 40-meter ham

band” is an explicit reference to radio waves that are approximately 40 m long. In the RF

spectrum (discussed later in this chapter), “short-wave radio” and “microwave radar” are

relative references to different wavelengths.

1/4 cycle

Amplitude

t

Period

(One cycle)

FIGURE 4.3 One cycle of a sine curve.