04Chapter04 - CHAPTER 4 Transmission Media Computers and...

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79 CHAPTER 4 Transmission Media 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. Figure 4.1 shows the electromagnetic spectrum. Transmission media for LANs can be divided into two broad categories: guided and unguided (see Figure 4.2). Figure 4.1 Electromagnetic spectrum Figure 4.2 Classes of transmission media Power, voice Infrared light Ultra- violet light 0 3 kHz 300 GHz Visible light, 430 Ð 750 THz Radio communication Radio, microwave, satellite X, gamma, cosmic rays Transmission media Guided Unguided
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80 CHAPTER 4 TRANSMISSION MEDIA 4.1 GUIDED MEDIA Guided media, which are those that provide a conduit from one device to another, include twisted-pair cable, coaxial cable, and fiber-optic cable (see Figure 4.3). A sig- nal traveling along any of these media is directed and contained by the physical limits of the medium. Twisted-pair and coaxial cable use metallic (copper) conductors that accept and transport signals in the form of electrical current. Optical fiber is a glass or plastic cable that accepts and transports signals in the form of light. Twisted-Pair Cable Twisted-pair cable comes in two forms: unshielded and shielded. Unshielded Twisted-Pair (UTP) Cable Unshielded twisted-pair (UTP) cable is the most common type of telecommunication medium in use today. A twisted pair consists of two conductors (usually copper), each with its own colored plastic insulation. The plastic insulation is color-banded for identi- fication (see Figure 4.4). Colors are used both to identify the specific conductors in a cable and to indicate which wires belong in pairs and how they relate to other pairs in a larger bundle. Figure 4.3 Categories of guided media Figure 4.4 Twisted-pair cable Guided media Twisted-pair cable Fiber-optic cable Coaxial cable Solid copper conductors Outer insulator or PVC
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SECTION 4.1 GUIDED MEDIA 81 In the past, two parallel flat wires were used for communication. However, electro- magnetic interference from devices such as a motor can create noise over those wires. If the two wires are parallel, the wire closest to the source of the noise gets more interfer- ence and ends up with a higher voltage level than the wire farther away, which results in an uneven load and a damaged signal. If, however, the two wires are twisted around each other at regular intervals (between 2 and 12 twists per foot), each wire is closer to the noise source for half the time and farther away for the other half. With twisting, therefore, the cumulative effect of the interference is equal on both wires. Twisting does not always eliminate the impact of noise, but does significantly reduce it.
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04Chapter04 - CHAPTER 4 Transmission Media Computers and...

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