Access Networks and Physical Media in Java

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communities, providing access rates of about 128 Kbps over twisted pair We also saw that ADSL (Asymmetric Digital Subscriber Loop) technology has enabled residential users to access the Web at rates in excess of 6 Mbps over twisted pair Coaxial-Cable Like twisted pair, coaxial cable consists of two copper conductors, but the two conductors are concentric rather than parallel 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 Baseband coaxial cable, also called 50-ohm cable, is about a centimeter thick, lightweight, and easy to bend It is commonly used in LANs; in fact, the computer you use at work or at school is probably connected to a LAN with either baseband coaxial cable or with UTP Take a look at the the connection to your computer's interface card If you see a telephone-like jack and some wire that resembles telephone wire, you are using UTP; if you see a T-connector and a cable running out of both sides of the T-connector, you are using baseband coaxial cable The terminology "baseband" comes from the fact that the stream of bits is dumped directly into the cable, without shifting the signal to a different frequency band 10 Mbps Ethernets can use either UTP or baseband coaxial cable As we will discuss in the 5, it is a little more expensive to use UTP for 10 Mbps Ethernet, as UTP requires an additional networking device, called a hub Broadband coaxial cable, also called 75-ohm cable, is quite a bit thicker, heavier, and stiffer than the baseband variety It was once commonly used in LANs and can still be found in some older installations For LANs, baseband cable is now preferable, since it is less expensive, easier to physically handle, and does not require attachment cables Broadband cable, however, is quite common in cable television systems As we saw in Section 151, cable television systems have been recently been coupled with cable modems to provide residential users with Web access at rates of 10 Mbps or higher With broadband coaxial cable, the transmitter shifts the digital signal to a specific frequency band, and the resulting analog signal is sent from the transmitter to one or more receivers Both baseband and broadband coaxial cable can be used as a guided shared medium Specifically, a number of end systems can be connected directly to the cable, and all the end systems receive whatever any one of the computers transmits We will look at this issue in more detail in 5 Fiber Optics An optical fiber is a thin, flexible medium that conducts pulses of light, with each pulse representing a bit A single optical fiber can support tremendous bit rates, up to tens or even hundreds of gigabits per second They are immune to electromagnetic interference, have very low signal attenuation up to 100 kilometers, and are very hard to tap These characteristics have made fiber optics the preferred long-haul guided transmission media, particularly for overseas links Many of the long-distance telephone networks in the United States and elsewhere now use fiber optics exclusively Fiber optics is also prevalent in the backbone of the Internet However, the high cost of optical devices -- such as transmitters, receivers, and switches -- has hindered their deployment for short-haul transport, such as in a LAN or into the home in a residential access network AT&T Labs provides an excellent site on fiber optics, including several nice animations Terrestrial and Satellite Radio Channels Radio channels carry signals in the electromagnetic spectrum They are an attractive media because require no physical "wire" to be installed, can penetrate walls, provide connectivity to a mobile user, and can potentially carry a signal for long distances The characteristics a radio channel depend significantly on the propagation environment and the distance over which a signal is to be carried Environmental considerations determine path loss and shadow fading (which decrease in signal strength as it travels over a distance and around/through obstructing objects), multipath fading (due to signal reflection off of interfering objects), and interference (due to other radio channels or electromagnetic signals) Terrestrial radio channels can be broadly classified into two groups: those that operate as local area networks (typically spanning 10's to a few hundred meters) and wide-area radio channels that are used for mobile data services (typically operating within a metropolitan region) A number of wireless LAN products are on the market, operating in the 1 to 10's of Mbps range
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