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ISDN vs. Cable
The Internet is a network of networks that interconnects computers around the world, supporting both business and residential users. In 14, 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 56 Kilobits per second (Kbps)the fastest residential access commonly available at the time of this writingthe transfer of graphical images can be frustratingly slow. My paper 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.
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 paper describes the Internet technology involved in that connection. This paper does not discuss other aspects of Internet technology in detail. 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. Telephonys well-defined and stable service requirements are reflected in the -- 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 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. 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.
• Cables 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.
• Telephonys 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 cables 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.
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 CAT 5 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 40 times the bit rate of the familiar 56 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.
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 0% more cost per subscriber. When this reference model is altered to target 4 Mbps service to individuals instead of organizations at 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 $0.0 for Individual 4 Mbps, $0.60 for Organizational 4 Mbps, and $ 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.
The paragraph above breaks 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 prices show that subscriber equipment is the most significant component of average cost. When subscribers are willing to pay for their own equipment, the access providers capital costs are low. This business model has been successfully adopted by SBC, but it is foreign to the cable industry. 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.
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