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Types of Guided and Unguided Transmission Media

This chapter provides an overview of twisted-pair cable, shielded twisted-pair cable, coaxial cable, and various types of fiber-optic cable. It also explains the concept of guided and unguided media, and their applications in communication networks.

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Types of Guided and Unguided Transmission Media

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  1. Chapter 7 Transmission Media

  2. OBJECTIVES 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. Understand how the air (or vacuum) can be a transmission medium for signals. Understand the difference between guided and unguided media. After reading this chapter, the reader should be able to:

  3. 7.1 GUIDED MEDIA

  4. Figure 7-1 Categories of guided media • Guided media, which are those that provide a physically limited path for the signal, include twisted-pair cable, and fiber-optic cable.

  5. Figure 7-2 Twisted-pair cable • A twisted pair is made of two insulated copper wires twisted together to reduce the effect of electrical interference of other electrical devices and other nearby cables. • Twisted-pair cable comes in two forms: unshielded and shielded.

  6. Note: A twisted pair consists of two conductors each surrounded by an insulating material.

  7. 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.

  8. 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 Technical Focus:Categories of Twisted-Pair Cables

  9. 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.

  10. Figure 7-3 Shielded twisted-pair cable • Shielded twisted-pair (STP) cable has a metal foil or braided-mesh covering that encases each pair of insulated conductors..

  11. 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.

  12. Figure 7-4 Coaxial cable • Coaxial cable carries signals of higher-frequency ranges than twisted-pair cable.

  13. Figure 7-5 Refraction and reflection • If the angle of incidence is less than the critical angle, the light is refracted. • If the angle of incidence is greater than the critical angle, the light is reflection.

  14. Figure 7-6 Propagation modes • Three types of fiber-optic cables:

  15. 7.2 UNGUIDED MEDIA

  16. Figure 7-7 Terrestrial microwave • To increase the distance served by terrestrial microwave, a system of repeaters can be installed with each antenna. • A signal received by one antenna is relayed the next antenna.

  17. Figure 7-8 Satellite communication • Satellite communicationis much like line-of-sight microwave transmission with one of the stations, a satellite, orbiting the earth. • The principle is the same as terrestrial microwave, with a satellite acting as a supertall antenna and repeater.

  18. Figure 7-9 Satellites in geosynchronous orbit • Figure shows three satellites, each 120 degrees from another in geosynchronous orbit around the equator. The view is from the North Pole.

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