Transmission Media

As discussed in Chapter 3, computers and other telecommunication devices use signals to represent data. These signals are transmitted from one device to another in the form of electromagnetic energy. Electromagnetic signals can travel through a vacuum, through air, or through other transmission media.

Electromagnetic energy, a combination of electrical and magnetic fields vibrating in relation to each other, includes power, voice, radio waves, infrared light, visible light, ultraviolet light, and X, gamma, and cosmic rays. Each of these constitutes a portion of the electromagnetic spectrum (see Figure 5.0-1). Not all portions of the spectrum are currently usable for telecommunications, however, and usable media are limited to a few types. Voice-band frequencies are generally transmitted as current over metal cables, such as twisted-pair, or coaxial cable. Radio frequencies can travel through air or space, but require specific transmitting and receiving mechanisms. Visible light, the last type of electromagnetic energy currently used for communications, is harnessed using fiber-optic cable.

Power, voice

1 violet lieht

0 3 KHz

300 GHz

Fig. 5.0-1 Electromagnetic spectrum

Fig. 5.0-1 Electromagnetic spectrum

Transmission media can be divided into two broad categories: guided and unguided (see Figure 5.0-2).

Fig. 5.0-2 Classes of transmission media
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