Page 14 - Optical Communications Essentials

Basic Concepts of Communication Systems

4 Chapter One

Figure 1.1. Block diagram of a typical communication link connecting
separate LANs.

1.2. Analog Signal Formats
The signals emitted by information sources and the signals sent over a transmis-
sion channel can be classified into two distinct categories according to their phys-
ical characteristics. These two categories encompass analog and digital signals.
An analog signal conveys information through a continuous and smooth vari-
ation in time of a physical quantity such as optical, electrical, or acoustical inten-
sities and frequencies. Well-known analog signals include audio (sound) and video
messages. As examples,

■ An optical signal can vary in color (which is given in terms of its wavelength
or its frequency, as described in Chap. 3), and its intensity may change from
dim to bright.
■ An electric signal can vary in frequency (such as the kHz, MHz, GHz desig-
nations in radio communications), and its intensity can range from low to
high voltages.
■ The intensity of an acoustical signal can range from soft to loud, and its tone
can vary from a low rumble to a very high pitch.

The most fundamental analog signal is the periodic sine wave, shown in
Fig. 1.2. Its three main characteristics are its amplitude, period or frequency,
and phase. The amplitude is the size or magnitude of the waveform. This is
generally designated by the symbol A and is measured in volts, amperes, or
watts, depending on the signal type. The frequency (designated by f ) is the

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