Transmission Media

Chapter 7TransmissionMediaUnderstand 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 forshielding.Be familiar with coaxial cable.After reading this chapter, the reader should be able to:OBJECTIVESUnderstand how the air (or vacuum) can be a transmissionmedium for signals.GUIDEDMEDIA7.1Figure 7-1Categories of guided mediaFigure 7-2Twisted-pair cableA twisted pair consists of two conductors each surrounded by an insulating material.Note:Technical Focus: Why Twisted-Pair Cable Reduces NoiseInterference from devices such as a motor can create unequal noise over two parallel lines. The line that is closer to the devicereceives 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 thetime and farther for the other half. Both receive the same amount of noise. The noise from two wires cancel each otherout at the receiver.Technical Focus: Categories of Twisted-Pair CablesCategory 1: The basic cabling used for voiceCategory 2: Suitable for voice and data up to 4 MbpsCategory 3: Suitable for data up to 10 MbpsCategory 4: Suitable for data up to 16 MbpsCategory 5: Suitable for data up to 100 MbpsBusiness Focus: Use of UDP in Telephone NetworkIn 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. Figure 7-3Shielded twisted-pair cableTechnical Focus: Shielding and LightningWe 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) canpenetrate insulators, but not metals. Lightning on a metallicshell creates a current on the outside surface, which can bedangerous; the inside is protected. Note that this phenomenonis 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 isstopped 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.Figure 7-4Coaxial cableFigure 7-5Refraction and reflectionFigure 7-6Propagation modesUNGUIDEDMEDIA7.2Figure 7-7Terrestrial microwaveFigure 7-8Satellite communicationFigure 7-9Satellites in geosynchronous orbit