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In the previous subsection we gave an overview of some of the most important access network technologies in the Internet While describing these technologies, we also indicated the physical media used For example, we said that HFC uses a combination of fiber cable and coaxial cable We said that ordinary modems, ISDN, and ADSL use twisted-pair copper wire And we said that mobile access network use the radio spectrum In this subsection we provide a brief overview of these and other transmission media that are commonly employed in the Internet In order to define what is meant by a "physical medium,", let us reflect on the brief life of a bit Consider a bit traveling from one end system, through a series of links and routers, to another end system This poor bit gets transmitted many, many times! The source end-system first transmits the bit and shortly thereafter the first router in the series receives the bit; the first router then transmits the bit and shortly afterwards the second router receives the bit, etc Thus our bit, when traveling from source to destination, passes through a series of transmitter-receiver pairs For each transmitter-receiver pair, the bit is sent by propagating electromagnetic waves across a physical medium The physical medium can take many shapes and forms, and does not have to be of the same type for each transmitter-receiver pair along the path Examples of physical media include
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twisted-pair copper wire, coaxial cable, multimode fiber optic cable, terrestrial radio spectrum and satellite radio spectrum Physical 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 cooper wire or a coaxial cable With unguided media, the waves propagate in the atmosphere and in outer space, such as in a digital satellite channel or in a CDPD system
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Suppose you want to wire a building to allow computers to access the Internet or an intranet -- should you use twisted-pair copper wire, coaxial cable, or fiber optics Which of these media gives the highest bit rates over the longest distances We shall address these questions below But before we get into the characteristics of the various guided medium types, let us say a few words about their costs The actual cost of the physical link (copper wire, fiber optic cable, etc) is often relatively minor compared with the other networking costs In particular, the labor cost associated with the installation of the physical link can be orders of magnitude higher than the cost of the material For this reason, many builders install twisted pair, optical fiber, and coaxial cable to every room in a building Even if only one medium is initially used, there is a good chance that another medium could be used in the near future, and so money is saved but not having to lay additional wires Twisted-Pair Copper Wire The least-expensive and most commonly-used transmission medium is twisted-pair copper wire For over one-hundred years it has been used by telephone networks In fact, more than 99% of the wired connections from the telephone handset to the local telephone switch use twisted-pair copper wire Most of us have seen twisted pair in our homes and work environments Twisted pair consists of two insulated copper wires, each about 1 mm thick, arranged in a regular spiral pattern; see Figure 15-3 The wires are twisted together to reduce the electrical interference from similar pairs close by Typically, a number of pairs are bundled together in a cable by wrapping the pairs in a protective shield A wire pair constitutes a single communication link
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Figure 15-3: Twisted Pair Unshielded twisted pair (UTP) is commonly used for computer networks within a building, that is, for local area networks (LANs) Data rates for LANs using twisted pair today range from 10 Mbps to 100 Mbps The data rates that can be achieved depend on the thickness of the wire and the distance between transmitter and receiver Two types of UTP are common in LANs: category 3 and category 5 Category 3 corresponds to voice-grade twisted pair, commonly found in office buildings Office buildings are often prewired with two or more parallel pairs of category 3 twisted pair; one pair is used for telephone communication, and the additional pairs can be used for additional telephone lines or for LAN networking 10 Mbps Ethernet, one of the most prevalent LAN types, can use category 3 UTP Category 5, with its more twists per centimeter and Teflon insulation, can handle higher bit rates 100 Mbps Ethernet running on category 5 UTP has become very popular in recent years In recent years, category 5 UTP has become common for preinstallation in new office buildings When fiber-optic technology emerged in the 1980s, many people disparaged twisted-pair because of its relatively low bit rates Some people even felt that fiber optic technology would completely replace twisted pair But twisted pair did not give up so easily Modern twisted-pair technology, such as category 5 UTP, can achieve data rates of 100 Mbps for distances up to a few hundred meters Even higher rates are possible over shorter distances In the end, twisted-pair has emerged as the dominant solution for high-speed LAN networking As discussed in Section 151, twisted-pair is also commonly used for residential Internet access We saw that dial-up modem technology enables access at rates of up to 56 Kbps over twisted pair We also saw that ISDN is available in many
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