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.
FIGURE 4.3 One cycle of a sine curve.