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
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.
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
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.
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
3.1 Cable Technology
Residential cable TV networks follow the tree and branch architecture. In each
community, a 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
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.
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 lon
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.
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.
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
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
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.
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.
Cable-based Internet access can provide the same average bandwidth and higher
peak bandwidth more economically than ISDN. For example, 500 Kbps Internet
access over cable can provide the same average bandwidth and four times the peak
bandwidth of ISDN access for less than half the cost per subscriber. In the
technology reference model of the case study, the 4 Mbps cable service is
targeted at organizations. According to recent benchmarks, the 4 Mbps cable
service can provide the same average bandwidth and thirty-two times the peak
bandwidth of ISDN for only 20% more cost per subscriber. When this reference
model is altered to target 4 Mbps service to individuals instead of
organizations, 4 Mbps cable access costs 40% less per subscriber than ISDN. The
economy of the cable-based approach is most evident when comparing the per-
subscriber cost per bit of peak bandwidth: $0.30 for Individual 4 Mbps, $0.60
for Organizational 4 Mbps, and $2 for the 500 Kbps cable servicesversus close
to $16 for ISDN. However, the potential penetration of cable-based access is
constrained in many cases (especially for the 500 Kbps service) by limited
upstream channel bandwidth. While the penetration limits are quite sensitive to
several of the input parameter assumptions, the cost per subscriber is
surprisingly less so.
Because the models break down the costs of each approach into their separate
components, they also provide insight into the match between what follows
naturally from the technology and how existing business entities are organized.
For example, the models show that subscriber equipment is the most significant
component of average cost. When subscribers are willing to pay for their own
equipment, the access provider’s capital costs are low. This business model has
been successfully adopted by Internex, but it is foreign to the cable industry.
As the concluding chapter discusses, the resulting closed market structure for
cable subscriber equipment has not been as effective as the open market for ISDN
equipment at fostering the development of needed technology. In addition,
commercial development of both cable and ISDN Internet access has been hindered
by monopoly control of the needed infrastructurewhether manifest as high ISDN
tariffs or simple lack of interest from cable operators.