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ISDN Vs. Cable Modems

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1399 words

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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 and maintained to
provide the same level of aggregate bandwidth to a neighborhood. The result
should be cost savings and easier upgrades.

' Cable's shared bandwidth approach is more flexible at allocating any
particular level of bandwidth among a group of subscribers. Since it does not
need to rely as much on forecasts of which subscribers will sign up for the
service, the cable architecture can adapt more readily to the actual demand that

' Telephony's dedication of bandwidth to individual customers limits the
peak (i.e. burst) data rate that can be provided cost-effectively. In contrast,
the dynamic sharing enabled by cable's bus architecture can, if the statistical
aggregation properties of neighborhood traffic cooperate, give a customer access
to a faster peak data rate than the expected average data rate.

2.2 Why focus on Internet access?

Internet access has several desirable properties as an application to consider
for exercising residential infrastructure. Internet technology is based on a
peer-to-peer model of communications. Internet usage encompasses a wide mix of
applications, including low- and high-bandwidth as well as asynchronous and
real-time communications. Different Internet applications may create varying
degrees of symmetrical (both to and from the home) and asymmetrical traffic
flows. Supporting all of these properties poses a challenge for existing
residential communications infrastructures.

Internet access differs from the future services modeled by other studies
described below in that it is a real application today, with growing demand.
Aside from creating pragmatic interest in the topic, this factor also makes it
possible to perform case studies of real deployments.

Finally, the Internet's organization as an "Open Data Network" (in the language
of (Computer Science and Telecommunications Board of the National Research
Council, 1994)) makes it a service worthy of study from a policy perspective.
The Internet culture's expectation of interconnection and cooperation among
competing organizations may clash with the monopoly-oriented cultures of
traditional infrastructure organizations, exposing policy issues. In addition,
the Internet's status as a public data network may make Internet access a
service worth encouraging for the public good. Therefore, analysis of costs to
provide this service may provide useful input to future policy debates.

3.0 Technologies

This chapter reviews the present state and technical evolution of residential
cable network infrastructure. It then discusses a topic not covered much in the
literature, namely, how this infrastructure can be used to provide Internet
access. It concludes with a qualitative evaluation of the advantages and
disadvantages of cable-based Internet access. While ISDN is extensively
described in the literature, its use as an Internet access medium is less well-
documented. This chapter briefly reviews local telephone network technology,
including ISDN and future evolutionary technologies. It concludes with a
qualitative evaluation of the advantages and disadvantages of ISDN-based
Internet access.

3.1 Cable Technology

Residential cable TV networks follow the tree and branch architecture. ...

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Keywords: isdn modems, isdn modem speed, isdn vs tdm

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