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 writingthe transfer of graphical images can be
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,
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
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’
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
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
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 materializes.
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 Internet
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 community
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.
3.1.1. Technological evolution
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 long-
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 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.
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.