ISDN VS. Cable Modem

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ISDN VS. Cable Modem

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 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.

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"ISDN VS. Cable Modem." 28 Apr 2017
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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 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.

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.

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-distance network.

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

Full-time connections

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.

5.0 Cost

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 services—versus 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 infrastructure—whether manifest as high ISDN tariffs or simple lack of interest from cable operators.

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