Digital-subscriber-line modems are spearheading the race for high-connect speeds in the residential marketplace. There, modern web pages and e-commerce are creating a demand for greater bandwidth. Growing interest has given rise to a number of issues in the design and implementation of systems based on digital subscriber lines (DSLs). These include monthly connect fees, ease and cost of installation, interoperability with central-office (CO) equipment, and the ultimate issue: "How fast will it connect?"
One key distinction, in terms of use and equipment needs, is between consumer-broadband and commercial-broadband service. A high level of activity resides in the commercial segment, where service fees are estimated to be as high as $49 billion annually. Its opportunities appear so attractive that many residential-related issues have been neglected.
The primary differences between commercial and consumer service involve monthly service fees and guaranteed level of service between the two account types. Most commercial accounts are being offered monthly services in the $100 range, with minimum guaranteed download speeds of 768 kbits/s and higher. The typical residential account is being offered rates of $30 to $50 per month for minimum guaranteed download speeds of 384 kbits/s.
The differences aren't merely the result of marketing and positioning, but are derived directly from system issues. Commercial establishments tend to be located relatively near their CO, while the average household is roughly 12.5 kft. away. This 12.5-kft. median distance is significant in terms of available bandwidth. The only way to shorten it is to install more COs in the residential territories. While this would alleviate some of the distance-related bandwidth issues, however, it wouldn't solve the Internet delivery problems that exist in today's implementation.
Local Internet traffic may well be available at rates above 1 Mbit/s. But practical data rates for information traveling from one city to the next appear to be in the 1-Mbit/s range and below. The net result is that today's Internet should be able to consistently supply data in the 1-Mbit/s range. Meanwhile, rapid improvements in backbone capability are being consumed by the growing demand for Internet-related content. With the continued increase in new users, there won't likely be a major increase in the Internet's "practical capability" for residential users during the next few years.
Differences In Wiring
Fundamental differences exist in the distance from the customer premise equipment (CPE) to the CO, as well as with the nature of the local wiring between residential and commercial local loops. The commercial wiring approach is to pull the incoming lines into the telephone closet. The typical home has a main line brought to a debarkation block on the side of the residence. From there, the pattern can be as simple as "straight to the phone" or as complicated as a web of connections and branches. This says nothing of home-improvement-related changes in the wiring and mixed-age wiring issues common in this setting.
Supporting widespread adoption of DSL technology requires easy and straightforward installation, with no surprises like unexpectedly low connect rates. The line between full-rate (8-Mbit/s maximum download) and G.Lite service (1.5-Mbit/s maximum download) is somewhat blurred. So many consumers will expect full-rate connect speeds when they bring a DSL device home for installation. Signal loss through the line, combined with noise sources at both the near and the far ends, tends to limit the connection's available bandwidth. Consumers located more than 10 kft. from the CO will see a significant drop in available bandwidth when compared with the promises seen in much of the promotional material.
The full-rate specification chip sets support a number of protocols. They're also expensive. A modem like the G.Lite-compliant one, which is specifically designed for the residential environment,will cost less and perform better.
The distance from the CO to the CPE site affects signal strength and frequency response. Plus, the current local-loop infrastructure has been installed over many decades. A typical local loop may have several gauges and types of wire between the CO and the CPE site. Because the real-world installed plant of twisted-pair copper isn't uniform, the ITU has specified a number of representative test loops that normalize testing and help ensure interoperability. Several of the ANSI T1.601 test-loop standards are shown in Figure 1.
Test loop #12 is composed of a 7500- ft. 26-gauge segment trailed by a 4500-ft. 24-gauge segment. That second segment is followed by a 1500-ft. 26-gauge one stretching from the CO to the CPE. The characteristics of a given segment can be modeled, as in Figure 2. Each of these sections has a fairly well-defined impedance characteristic that can be approximated using the standard tables for R/C/L/G components and transmission line theory.
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