Technologism Essay Research Paper The Internet is — страница 7

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distances without amplification. The main disadvantage of fiber is that the optical components required to send and receive data over it are expensive. Because lasers are still too expensive to deploy to each subscriber, network developers have adopted an intermediate Fiber to the Neighborhood (FTTN)approach. Figure 3.3: Fiber to the Neighborhood (FTTN) architecture Various locations along the existing cable are selected as sites for neighborhood nodes. One or more fiber-optic cables are then run from the head end to each neighborhood node. At the head end, the signal is converted from electrical to optical form and transmitted via laser over the fiber. At the neighborhood node, the signal is received via laser, converted back from optical to electronic form, and transmitted to

the subscriber over the neighborhood’s coaxial tree and branch network. FTTN has proved to be an appealing architecture for telephone companies as well as cable operators. Not only Continental Cablevision and Time Warner, but also Pacific Bell and Southern New England Telephone have announced plans to build FTTN networks. Fiber to the neighborhood is one stage in a longer-range evolution of the cable plant. These longer-term changes are not necessary to provide Internet service today, but they might affect aspects of how Internet service is provided in the future. 3.2 ISDN Technology Unlike cable TV networks, which were built to provide only local redistribution of television programming, telephone networks provide switched, global connectivity: any telephone subscriber can

call any other telephone subscriber anywhere else in the world. A call placed from a home travels first to the closest telephone company Central Office (CO) switch. The CO switch routes the call to the destination subscriber, who may be served by the same CO switch, another CO switch in the same local area, or a CO switch reached through a long- distance network. Figure 4.1: The telephone network The portion of the telephone network that connects the subscriber to the closest CO switch is referred to as the local loop. Since all calls enter and exit the network via the local loop, the nature of the local connection directly affects the type of service a user gets from the global telephone network. With a separate pair of wires to serve each subscriber, the local telephone network

follows a logical star architecture. Since a Central Office typically serves thousands of subscribers, it would be unwieldy to string wires individually to each home. Instead, the wire pairs are aggregated into groups, the largest of which are feeder cables. At intervals along the feeder portion of the loop, junction boxes are placed. In a junction box, wire pairs from feeder cables are spliced to wire pairs in distribution cables that run into neighborhoods. At each subscriber location, a drop wire pair (or pairs, if the subscriber has more than one line) is spliced into the distribution cable. Since distribution cables are either buried or aerial, they are disruptive and expensive to change. Consequently, a distribution cable usually contains as many wire pairs as a

neighborhood might ever need, in advance of actual demand. Implementation of ISDN is hampered by the irregularity of the local loop plant. Referring back to Figure 4.3, it is apparent that loops are of different lengths, depending on the subscriber’s distance from the Central Office. ISDN cannot be provided over loops with loading coils or loops longer than 18,000 feet (5.5 km). 4.0 Internet Access This section will outline the contrasts of access via the cable plant with respect to access via the local telephon network. 4.1 Internet Access Via Cable The key question in providing residential Internet access is what kind of network technology to use to connect the customer to the Internet For residential Internet delivered over the cable plant, the answer is broadband LAN

technology. This technology allows transmission of digital data over one or more of the 6 MHz channels of a CATV cable. Since video and audio signals can also be transmitted over other channels of the same cable, broadband LAN technology can co-exist with currently existing services. Bandwidth The speed of a cable LAN is described by the bit rate of the modems used to send data over it. As this technology improves, cable LAN speeds may change, but at the time of this writing, cable modems range in speed from 500 Kbps to 10 Mbps, or roughly 17 to 340 times the bit rate of the familiar 28.8 Kbps telephone modem. This speed represents the peak rate at which a subscriber can send and receive data, during the periods of time when the medium is allocated to that subscriber. It does not