Technologism Essay Research Paper The Internet is — страница 3
- Просмотров 444
- Скачиваний 5
- Размер файла 27 Кб
head end is installed to receive satellite and traditional over-the-air broadcast television signals. These signals are then carried to subscriber’s homes over coaxial cable that runs from the head end throughout the community Figure 3.1: Coaxial cable tree-and-branch topology To achieve geographical coverage of the community, the cables emanating from the head end are split (or “branched”) into multiple cables. When the cable is physically split, a portion of the signal power is split off to send down the branch. The signal content, however, is not split: the same set of TV channels reach every subscriber in the community. The network thus follows a logical bus architecture. With this architecture, all channels reach every subscriber all the time, whether or not the subscriber’s TV is on. Just as an ordinary television includes a tuner to select the over-the-air channel the viewer wishes to watch, the subscriber’s cable equipment includes a tuner to select among all the channels received over the cable. 3.1.1. Technological evolution The development of fiber-optic transmission technology has led cable network developers to shift from the purely coaxial tree-and-branch architecture to an approach referred to as Hybrid Fiber and Coax(HFC) networks. Transmission over fiber-optic cable has two main advantages over coaxial cable: ? A wider range of frequencies can be sent over the fiber, increasing the bandwidth available for transmission; ? Signals can be transmitted greater 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.