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In the implementation and deployment of a high-speed wide area public digital network, the most challenging part is the link between subscriber and network -- the digital subscriber line. With billions of potential endpoints worldwide, the prospect of installing new cable for each new customer is daunting. Instead, network designers have sought ways of exploiting the installed base of twisted-pair wire that links virtually all residential and business customers to telephone networks. These links were installed to carry voice-grade signals in a bandwidth from 0 to 4 kHz. However, the wires are capable of transmitting signals over a far broader spectrum -- 1 MHz or more. ADSL is the most widely publicized of a family of new modem technologies designed to provide high-speed digital data transmission over ordinary telephone wire. ADSL is now being offered by a number of carriers and is defined in an ANSI standard.
The term "asymmetric" refers to the fact that ADSL provides more capacity downstream (from the carrier's central office to the customer's site) than upstream (from customer to carrier). ADSL was originally targeted at the expected need for video-on-demand and related services. This application has not materialized. However, since the introduction of ADSL technology, the demand for high-speed access to the Internet has grown. Typically, users require far higher capacity for downstream than for upstream transmission. Most user transmissions are in the form of keyboard strokes or transmissions of short e-mail messages, whereas incoming traffic, especially web traffic, can involve large amounts of data and include images or even video. Thus, ADSL provides a perfect fit for the Internet requirement.
ADSL uses frequency-division modulation (FDM) in a novel way to exploit the 1-MHz capacity of twisted pair. There are three elements of the ADSL strategy:
In addition to ADSL, there are a number of related schemes, generically referred to as xDSL, which also make full use of the twisted-pair bandwidth to provide high-speed digital service.
ADSL: Standards, Implementation, and Architecture, by Charles Summers, provides the briefest treatment of ADSL, but is useful in placing ADSL within the context of a total implementation. The book begins with a short, intuitive treatment of analog and digital communications that highlights some of the key design issues. Then there is a good survey of alternative DSL techniques. Chapter 3 covers the ADSL transmission technology. At only 20 pages, the chapter necessarily sticks to a concise overview. If you are looking for a detailed examination of ADSL, you will not find it here. However, the remainder of the book looks at other communications design issues that must be addressed in implementing ADSL. This include the communications software that rides on top of the ADSL physical layer, the buffering required at each end of the line, signaling protocols, and interface chipset design. The book also introduces the topic of ATM over ADSL and looks at the issues involved there. If you are looking for this type of broad treatment, covering related issues as well as ADSL design, ADSL: Standards, Implementation, and Architecture is a good choice.
Implementing ADSL, by David Ginsburg, also provides a relatively brief technical treatment of ADSL transmission technology while placing ADSL in a broader context. The first chapter provides a business justification for ADSL and also gives an overview of xDSL standardization and history. Chapter 2 provides about 20 pages on the technology of ADSL transmission. In some ways, the treatment is more technical than that of Summers. However, the treatment is so concise that it only provides a flavor of how ADSL works. Readers with a strong background in data communications will get much more from this section than other readers. This chapter also looks at the use of ATM and IP over ADSL and competently summarizes the design issues. The next chapter looks at the architecture and infrastructure required at the service provider to support ADSL, including interface equipment, management, and buffering requirements. Chapter 4 describes an ADSL implementation from the point of view of the services provided. The book then looks at actual implementation examples.
ADSL/VDSL Principles: A Practical and Precise Study, by Dennis Rauschmayer, and DSL: Simulation Techniques and Standards Development, by Walter Chen, go into much more detail on the ADSL physical layer. Rauschmayer's book is excellent in providing the technical background that motivates the various xDSL schemes. Following an overview chapter, the book provides a thorough analysis of the twisted-pair environment, including physical and electrical characteristics, transmission line parameters, and crosstalk. The next chapter looks at the actual subscriber line parameters, followed by an entire chapter devoted to the subject of crosstalk. After all of these preliminaries, Rauschmayer launches into DSL technology. Chapter 5 provides an analysis of the theoretical capacity of ADSL and VDSL in a crosstalk environment. As elsewhere in the book, the treatment is detailed and mathematical--not easy reading, but readable. The next chapter looks at the basic techniques of DSL transmission, including QAM, CAP, multitone modulation, forward error correction, scrambling, and equalization. This is followed by a detailed look at the specific ADSL standard.
Technically, Chen's book is on a par with that of Rauschmayer. Both books require a reasonable background in telecommunications and the basic math involved. Chen’s book, as its length suggests, provides the most detail. The first part of the book, covering almost 300 pages, looks at the fundamentals of the subscriber loop transmission environment and at the basic signal processing techniques, such as echo cancellation. Part Two begins with some of the basic techniques common to all of the DSL approaches, and then covers the principal DSL techniques, including ADSL, devoting about 40 or 50 pages to each.
Thus, the first two books provide a good overview of ADSL and discuss the overall context in which ADSL operates. For this overview purpose, either book is recommended. The last two books go into far more detail. Both of these books are well organized and thorough. Again, either book serves the purpose, although Chen's book provides the most detail.
-- William Stallings