ISDN vs. Cable Modems
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 cable television networks upgraded to pass bi-directional digital traffic (Cable Modems). It analyzes the potential of each enhancement to deliver Internet access to residential users. It validates the hypothesis that upgraded cable networks can deliver residential Internet access more cost-effectively, while offering a broader range of services.
The research for this report consisted of case studies of two commercial deployments of residential Internet access, each introduced in the spring of 1994:
Continental Cablevision and Performance Systems International (PSI) jointly developed PSICable, an Internet access service deployed over upgraded cable plant in Cambridge, Massachusetts;
Internex, Inc. began selling Internet access over ISDN telephone circuits available from Pacific Bell. Internex's customers are residences and small businesses in the "Silicon Valley" area south of San Francisco, California.
2.0 The Internet
When a home is connected to the Internet, residential communications infrastructure serves as the "last mile" of the connection between the home computer and the rest of the computers on the Internet. This section describes the Internet technology involved in that connection. This section does not discuss other aspects of Internet technology in detail; that is well done elsewhere. Rather, it focuses on the services that need to be provided for home computer users to connect to the Internet.
ISDN and upgraded cable networks will each provide different functionality (e.g. type and speed of access) and cost profiles for Internet connections. It might seem simple enough to figure out which option can provide the needed level of service for the least cost, and declare that option "better." A key problem with this approach is that it is difficult to define exactly the needed level of service for an Internet connection. The requirements depend on the applications being run over the connection, but these applications are constantly changing. As a result, so are the costs of meeting the applications' requirements.
Until about twenty years ago, human conversation was by far the dominant application running on the telephone network. The network was consequently optimized to provide the type and quality of service needed for conversation. Telephone traffic engineers measured aggregate statistical conversational patterns and sized telephone networks accordingly. Telephony's well-defined and stable service requirements are reflected in the "3-3-3" rule of thumb relied on by traffic engineers: the average voice call lasts three minutes, the user makes an average of three call attempts during the peak busy hour, and the call travels over a bidirectional 3 KHz channel.
In contrast, data communications are far more difficult to characterize. Data transmissions are generated by computer applications. Not only do existing applications change frequently (e.g. because of software upgrades), but entirely new categoriessuch as Web browserscome into being quickly, adding different levels and patterns of load to existing networks. Researchers can barely measure these patterns as quickly as they are generated, let alone plan future network capacity based on them.
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