ISDN vs. Cable Modems

1.0 Introduction

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

· 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 categories—such as Web browsers—come 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.

The one generalization that does emerge from studies of both local and wide-
area data traffic over the years is that computer traffic is bursty. It does
not flow in constant streams; rather, "the level of traffic varies widely over
almost any measurement time scale" (Fowler and Leland, 1991). Dynamic bandwidth
allocations are therefore preferred for data traffic, since static allocations
waste unused resources and limit the flexibility to absorb bursts of traffic.

This requirement addresses traffic patterns, but it says nothing about the
absolute level of load. How can we evaluate a system when we never know how
much capacity is enough? In the personal computing industry, this problem is
solved by defining "enough" to be "however much I can afford today," and relying
on continuous price-performance improvements in digital technology to increase
that level in the near future. Since both of the infrastructure upgrade options
rely heavily on digital technology, another criteria for evaluation is the
extent to which rapidly advancing technology can be immediately reflected in
improved service offerings.

Cable networks satisfy these evaluation criteria more effectively than telephone
networks because:

· Coaxial cable is a higher quality transmission medium than twisted
copper wire pairs of the same length. Therefore, fewer wires, and consequently
fewer pieces of associated equipment, need to be installed