McGraw-Hill©The McGraw-Hill Companies, Inc. Chapter 7 Transmission Media

McGraw-Hill©The McGraw-Hill Companies, Inc. Understand the difference between guided and unguided media. Be familiar with twisted-pair cable and the rationale for twisting. Be familiar with shielded twisted-pair cable and the rationale for shielding. Be familiar with coaxial cable. After reading this chapter, the reader should be able to: O BJECTIVES Understand how the air (or vacuum) can be a transmission medium for signals.

McGraw-Hill©The McGraw-Hill Companies, Inc. GUIDEDMEDIAGUIDEDMEDIA 7.1

McGraw-Hill©The McGraw-Hill Companies, Inc. Figure 7-1 Categories of guided media

McGraw-Hill©The McGraw-Hill Companies, Inc. Figure 7-2 Twisted-pair cable

McGraw-Hill©The McGraw-Hill Companies, Inc. A twisted pair consists of two conductors each surrounded by an insulating material. Note:

McGraw-Hill©The McGraw-Hill Companies, Inc. Technical Focus: Why Twisted-Pair Cable Reduces Noise Interference from devices such as a motor can create unequal noise over two parallel lines. The line that is closer to the device receives more interference than the one that is farther. If however, the wires are twisted around each other at regular intervals, each wire is closer to the noise source for half of the time and farther for the other half. Both receive the same amount of noise. The noise from two wires cancel each other out at the receiver.

McGraw-Hill©The McGraw-Hill Companies, Inc. Technical Focus: Categories of Twisted-Pair Cables Category 1: The basic cabling used for voice Category 2: Suitable for voice and data up to 4 Mbps Category 3: Suitable for data up to 10 Mbps Category 4: Suitable for data up to 16 Mbps Category 5: Suitable for data up to 100 Mbps

McGraw-Hill©The McGraw-Hill Companies, Inc. Business Focus: Use of UDP in Telephone Network In the early days of the telephone network, the local loop, the two wires that connect a residence to the telephone office, was made of two parallel wires. The telephone companies later changed the local loop to a pair of twisted-pair wires, which is less prone to noise than the previous parallel wires.

McGraw-Hill©The McGraw-Hill Companies, Inc. Figure 7-3 Shielded twisted-pair cable

McGraw-Hill©The McGraw-Hill Companies, Inc. Technical Focus: Shielding and Lightning We can be immune from lightning in a desert if we are sur- rounded by a metallic cover (inside a car, for example). The reason is that electromagnetic energy (lightning) can penetrate insulators, but not metals. Lightning on a metallic shell creates a current on the outside surface, which can be dangerous; the inside is protected. Note that this phenomenon is opposite from what happens with electricity. The electrical current passes through a metal, but it is stopped by an insulator. Electromagnetic energy passes through an insulator, but is stopped by a metal. The same idea is used in shielded twisted- pair cable. The shield is a metal that protects the cable from outside noise, electromagnetic interference.

McGraw-Hill©The McGraw-Hill Companies, Inc. Figure 7-4 Coaxial cable

McGraw-Hill©The McGraw-Hill Companies, Inc. Figure 7-5 Refraction and reflection

McGraw-Hill©The McGraw-Hill Companies, Inc. Figure 7-6 Propagation modes

McGraw-Hill©The McGraw-Hill Companies, Inc. UNGUIDEDMEDIAUNGUIDEDMEDIA 7.2

McGraw-Hill©The McGraw-Hill Companies, Inc. Figure 7-7 Terrestrial microwave

McGraw-Hill©The McGraw-Hill Companies, Inc. Figure 7-8 Satellite communication

McGraw-Hill©The McGraw-Hill Companies, Inc. Figure 7-9 Satellites in geosynchronous orbit