ChapterTransmittingMedia.docVIP

PAGE  PAGE 9 Chapter 10 Transmitting Media In the previous Chapter, we have an overview of some of the most important access network technologies in the Internet. As we described these technologies, we also indicate the physical transmitting media used. Understanding how these media function will help you sort out the various rates for them and determine which is the most appropriate in a given situation. In this subsection we provide a brief overview of these and other transmission media that are commonly employed in the Internet. Transmitting media fall into two categories: guided media and unguided media. With guided media, the waves are guided along a solid medium, such as a fiber-optic cable, a twisted-pair copper wire, or a coaxial cable. With unguided media, the waves propagate in the atmosphere and in outer space, such as Satellite Radio Channels. 10.1 Some popular guided transmitting media 10.1.1.twisted pair The least-expensive and most commonly used transmission medium is twisted-pair wire. For over one-hundred years it has been used by telephone networks. In fact, more than 99 percent of the wired connections from the telephone handset to the local telephone switch use twisted-pair wire. Most of us have seen twisted-pair in our homes and work environments. Twisted pair consists of two insulated copper wires, each about 1mm thick, arranged in a regular spiral pattern. The wires are twisted together to reduce the electrical interference from similar pairs close by. 10.1.2. coaxial cable Coaxial cable consists of two copper conductors, but the two conductors are concentric rather than parallel. （see Fig.10.1）With this construction and a special insulation and shielding, coaxial cable can have higher bit rates than twisted pair. Coaxial cable comes in two varieties: baseband coaxial cable and broadband coaxial cable. Fig.10.1 the Structure of coaxial cable 10.1.3.fiber optics An optical fiber is a thin, flexible medium that conducts pul