Technologism Essay Research Paper The Internet is — страница 4

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 imply that every subscriber can transfer data at that rate

simultaneously. The effective average bandwidth seen by each subscriber depends on how busy the LAN is. Therefore, a cable LAN will appear to provide a variable bandwidth connection to the Internet Full-time connections Cable LAN bandwidth is allocated dynamically to a subscriber only when he has traffic to send. When he is not transferring traffic, he does not consume transmission resources. Consequently, he can always be connected to the Internet Point of Presence without requiring an expensive dedication of transmission resources. 4.2 Internet Access Via Telephone Company In contrast to the shared-bus architecture of a cable LAN, the telephone network requires the residential Internet provider to maintain multiple connection ports in order to serve multiple customers

simultaneously. Thus, the residential Internet provider faces problems of multiplexing and concentration of individual subscriber lines very similar to those faced in telephone Central Offices. The point-to-point telephone network gives the residential Internet provider an architecture to work with that is fundamentally different from the cable plant. Instead of multiplexing the use of LAN transmission bandwidth as it is needed, subscribers multiplex the use of dedicated connections to the Internet provider over much longer time intervals. As with ordinary phone calls, subscribers are allocated fixed amounts of bandwidth for the duration of the connection. Each subscriber that succeeds in becoming active (i.e. getting connected to the residential Internet provider instead of

getting a busy signal) is guaranteed a particular level of bandwidth until hanging up the call. Bandwidth Although the predictability of this connection-oriented approach is appealing, its major disadvantage is the limited level of bandwidth that can be economically dedicated to each customer. At most, an ISDN line can deliver 144 Kbps to a subscriber, roughly four times the bandwidth available with POTS. This rate is both the average and the peak data rate. A subscriber needing to burst data quickly, for example to transfer a large file or engage in a video conference, may prefer a shared-bandwidth architecture, such as a cable LAN, that allows a higher peak data rate for each individual subscriber. A subscriber who needs a full-time connection requires a dedicated port on a

terminal server. This is an expensive waste of resources when the subscriber is connected but not transferring data. The Internet is a network of networks that interconnects computers around the world, supporting both business and residential users. In 1994, a multimedia Internet application known as the World Wide Web became popular. The higher bandwidth needs of this application have highlighted the limited Internet access speeds available to residential users. Even at 28.8 Kilobits per second (Kbps)—the fastest residential access commonly available at the time of this writing—the transfer of graphical images can be frustratingly slow. This report examines two enhancements to existing residential communications infrastructure: Integrated Services Digital Network (ISDN), and