Digital Telephony / Edition 3

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From the reviews of the Second Edition . "The book stresses how systems operate and the rationale behind their design, rather than presenting rigorous analytical formulations . [It provides] the practicality and breadth essential to mastering the concepts of modern communications systems." -Telecommunication Journal In this expanded new edition of his bestselling book, telephony expert John Bellamy continues to provide telecommunications engineers with practical, comprehensive coverage of all aspects of digital telephone systems, while addressing the rapid changes the field has seen in recent years. Bellamy discusses the near-complete conversion to digital technology in telephone networks worldwide, examines both existing and emerging technologies, and explores the intricacies of carrying voice over data networks as well as the use of telephone networks for carrying data for Internet access. He emphasizes system design, implementation, and application, but also correlates the practice to communications theory. With 30 percent new material, Digital Telephony, Third Edition features:
* Clear explanations on how to overcome problems associated with the replacement of old analog technology with new digital technology
* A new chapter on digital mobile telephone technology
* New material on how, data networks support voice communication
* A new chapter on digital subscriber access technologies
* More than 300 graphs illustrating concepts
* Examples from the U.S. network as well as ITU public telephone networks

*An Instructor's Manual presenting detailed solutions to all the problems in the book is available from the Wiley editorial department.

Bellamy discusses the near-complete conversion to digital technology in telephone networks worldwide, examines both existing and emerging technologies, and explores the intricacies of carrying voice over data networks as well as the use of telephone networks for carrying data for Internet access. He emphasizes system design, implementation, and application, but also correlates the practice to communications theory.

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Editorial Reviews

From the Publisher
"...strongly recommended for academic libraries serving electrical engineering and telecommunications programs including those owning an earlier edition..." (E-Streams, Vol. 4, No. 5, 2001)

"...provides the reader with the opportunity to review the entire area of telephony and digital telephony in particular." (New Books and Multimedia, January 2001)

From The Critics

"...strongly recommended for academic libraries serving electrical engineering and telecommunications programs including those owning an earlier edition..." (E-Streams, Vol. 4, No. 5, 2001)

"...provides the reader with the opportunity to review the entire area of telephony and digital telephony in particular." (New Books and Multimedia, January 2001)

Intended for graduate electrical engineers, this book covers all elements of digital voice communications technology and networks, stressing the application and operational aspects of communication system design. The third edition adds chapters on digital mobile telephone and digital subscriber access technologies. Annotation c. Book News, Inc., Portland, OR (
John Wiley & Sons Publishing
From a review of the Second Edition --
"The book stresses how systems operate and the rationale behind their design, rather than presenting rigorous analytical formulations…(readers) will find…the practicality and breadth essential to mastering the concepts of modern communications." -- Telecommunications Journal
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Product Details

Meet the Author

JOHN C. BELLAMY, PhD, is a consultant based in Texas and a senior member of the IEEE.
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Read an Excerpt

1 Background And Terminology

Beginning in the 1960s, telecommunications in the United States and around the world began undergoing radical changes in several different areas. First, the conventional analog telephone network was being called upon to provide many new and different services, most of which emanated from the data processing industry. Second, the marketplace and the regulatory agencies in the United States stimulated competition in both old and new areas of traditionally monopolistic services. Third, digital technology emerged to implement many of the fundamental transmission and switching functions within the U.S. telephone network and other networks around the world. The main purpose of this book is to describe the design, application, and operational aspects of this new digital equipment. As background, the technology of the analog telephone network is reviewed to provide a framework for the introduction of digital equipment.

It must be emphasized that the introduction of digital technology into the telephone network was motivated by desires to improve the quality, add new features, and re-duce the costs of conventional voice services. Digitization of the network did not arise from the needs of the data processing industry for better data transmission services. Indeed, most of the digital technology introduced into the network was initially inaccessible to data traffic, except through analog channels. Of course, a digital network is a natural environment for data communications services. As more of the network became digitized, more support for direct use of the facilities became available for data applications. Initially, direct digital access existed only for relatively high-end business applications. It was not until facilities of the Integrated Services Digital Network (ISDN) became available that end-to-end, switched digital channels could be used by individual subscribers for both voice and data. By the late 1990s numerous other approaches to providing digital access to digital facilities became available, primarily for Internet access. These various digital access technologies are described in Chapter 11.

As a point of historical reference, Figure 1.1 has been included to show that the idea of integrated voice and data is not new. This figure depicts a concept of a German inventor named Phillip Reis [1] to add voice communications to the prevailing means of electrical communications at the time-the telegraph. Reis developed the equipment in the 1860s and died in 1874-two years before Alexander Graham Bell received his patent for the telephone. As indicated, the figure implies alternate use of the wires for voice or data communications (i.e., integrated transmission). Reis actually used the telegraph attachment to signal information pertaining to voice tests, an indication of inadequate voice quality.

To implement simultaneous voice and telegraph communications, the telephone in Figure 1.1 would have to have been digital. Because of technology limitations at the time, such an implementation was impossible and telephone systems necessarily evolved with analog technology. One hundred years later the situation changed significantly. Telephone equipment developers and service providers had an abundance of new technology, and they were challenged with how to make effective use of it.

This book describes digital telephone technology from two perspectives. The first perspective describes individual equipments or subsystems and technical reasons for transitions from conventional analog equipment to seemingly less natural digital counterparts. Thus, one purpose of this book is to describe how digital technology improves and expands the capabilities of various subsystems within voice telephone net-works. Another purpose of the book is to describe the ultimate benefits derived when an entire network is implemented with digital techniques. A great degree of synergism exists when individual systems are designed into one cohesive network utilizing digital implementations throughout. The synergistic effect benefits conventional voice services and newer services such as the Internet.

Most of the equipment descriptions and design examples presented in this book come from material authored by engineers at AT&T Laboratories (now Lucent Technologies) and other suppliers for the public telephone network. The basic principles, however, are by no means unique to the public telephone network. The concepts and implementation examples are applicable to any communications network: public or private, voice or data. An inherent attribute of a digital network is that it can, to a large extent, be designed independently of its application.

Terminals, Transmission, and Switching

The three basic elements of a communications network are terminals, transmission systems, and switches. The first part of this chapter provides an overview of these elements as implemented in analog telephone networks. Then, the last part of this chapter provides a brief overview of digital implementations within the analog network. Following a detailed discussion of the motivation for digital implementations in Chapter 2, the next four chapters describe the operation and design of the basic elements of a digital voice network. Chapter 3 discusses digital voice terminals and the most common algorithms used to convert analog voice signals into digital bit streams. Chapter 4 presents the basics of digital transmission systems. Fundamentals of digital switching follow in Chapter 5. Basic digital modulation techniques and their application to point-to-point digital microwave and digital cellular systems are described in Chapter 6. A discussion of various synchronization and control considerations for digital networks is provided in Chapter 7. Chapter 8 describes fiber optic transmission systems and the synchronous multiplexing standard (SONET). Chapter 9 discusses the basic architecture and operation of prevailing digital cellular systems in use in the United States and around the world.

The main emphasis of the first nine chapters involves circuit switching as traditionally implemented for voice telephone networks. A circuit-switched network is one that assigns a complete end-to-end connection in response to each request for service. Each connection, with its associated network facilities, is held for the duration of the call. Chapter 10 describes a different type of network, generically referred to as a packetswitched network, that is particularly suited to servicing data traffic. Included in Chapter 10 is a discussion of Asynchronous Transfer Mode (ATM), a form of a packet-switched network. Chapter 11 discusses various technologies and systems for achieving direct digital access to a digital network (voice or data). The last chapter presents the basics of traffic theory: the fundamental mathematics for analyzing and predicting telecommunications network performance.

1.1 Telecommunications Standard Organizations

Prior to the breakup of the Bell System on January 1, 1984, telecommunications standards in North America were essentially established by the dominant equipment designer and supplier to the Bell System: Bell Telephone Laboratories and Western Electric. Independent telephone companies that provided local service to the 20% of the country not covered by the Bell System relied on the U.S. Independent Telephone Association [USITA; later referred to as the U.S. Telephone Association (USTA)] to formulate and disseminate standards, particularly for interconnecting with the Bell System.

In anticipation of the divestiture of the Regional Bell Operating Companies (RBOCs) from AT&T, the Exchange Carriers Standards Association (ECSA) was formed in 1983 as a nonprofit trade association to represent the interests of all exchange carriers (RBOCs and independents). In February 1984 the ECSA sponsored the establishment of the T1 standards committee to formulate new interconnection standards for the U.S. national network. The TI committee is accredited by the American National Standards Institute (ANSI) to ensure that standards approvals follow principles of openness. Thus TI committee standards are designated as ANSI Tl.nnndate (T1 stands for Telecommunications standards entity number 1). Table 1.1 lists the major subcommittees within TI and the respective responsibilities.

Other organizations in North America that establish standards related to telecommunications are the Electronic Industries Association (EIA), the Institute of Electrical and Electronic Engineers (IEEE), and Bell Communications Research (Bellcore). Bellcore was an organization chattered to establish standards and qualify equipment for the RBOCs. Bellcore has since been reorganized as Telcordia Technologies. The IEEE is most known for its data communications standards listed in Table 1.2 but has also established numerous standards for measuring and characterizing telecommunications equipment.

Most of the world outside of North America relies on international telecommunications standards committees established under the auspices of the International Telecommunication Union (ITU). In the past, two major entities within the ITU were established: the International Telegraph and Telephone Consultative Committee (CCITT) and the International Radio Consultative Committee (CCIR). CCITT established recommendations for telephone, telegraph, and data transmission circuits and equipment. CCIR was concerned with coordinating the use of the radio spectrum. CCITT and CCIR activities are no longer identified as being distinct from the ITU. CCITT has become ITU-T and CCIR is now ITU-R. In the United States, use of the radio spectrum is controlled by the Federal Communications Commission (FCC).

North American standards and ITU standards have often been incompatible in the past. North American standards established by the Bell System were therefore incorporated into CCITT recommendations as a subset. Due to the need for more international compatibility, the T1 subcommittees and ITU-T committees now work closely together to establish joint standards. A first major example of the joint effort is the standard for synchronous fiber transmission systems referred to as SONET in the United States and Synchronous Digital Hierarchy (SDH) in ITU-T standards.

The International Standards Organization (ISO) is an organization with standards activities in a wide range of subject matters, some of which involve telecommunications. Technical subcommittees within ISO work closely with ITU study groups in formulating ITU recommendations, particularly those related to ISDN protocols that adhere, as much as possible, to the ISO data communications standard for Open Systems Interconnection (OSI) Reference Model...

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Table of Contents

Background and Terminology.

Why Digital?

Voice Digitization.

Digital Transmission and Multiplexing.

Digital Switching.

Digital Modulation and Radio Systems.

Network Synchronization Control and Management.

Fiber Optic Transmission Systems.

Digital Mobile Telephony.

Data and Asynchronous Transfer Mode Networks.

Digital Subscriber Access.

Traffic Analysis.



Answers to Selected Problems.


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As mentioned in the preface of the first two editions, the term digital telephony refers to the use of digital technology in the message path of voice communications networks. In this case the term digital refers to a method of encoding the signal-that is, a form of modulation. Hence digital telephony implies voice transmission and switching applications, not data communications. Although the primary focus of this book is not data communications, this edition contains an expanded treatment of data communications networks, particularly as they relate to providing voice communications services in addition to data communications.

This book covers all aspects of digital voice communications technology and networks. It is not a technical book in the traditional, analytical sense of communications theory. Since numerous books covering communications theory are already available, this book stresses the application and operational aspects of communications system design. Some basic theory is presented in both qualitative and, when appropriate, quantitative terms. The main purpose, however, is to introduce concepts, terminology, and how applications influence implementations. In most cases the concepts are supported by citing example implementations in the U.S. telephone network, although examples from other (ITU) public telephone networks are also provided.

The primary audience for this book are graduate electrical engineers. The electrical engineering student is most capable of appreciating occasional references to communications theory and its influence on the practice. However, because analytical rigor is waived in favor of operational descriptions, less analytically oriented readersshould have no difficulty understanding the principles. Chapter 6 (covering digital radio and modulation) is the most analytical but is easily skipped without losing continuity for the other chapters. Similarly, Chapter 12 (covering traffic analysis) contains numerous equations that are unnecessary for understanding the material in other chapters.

When the first edition of Digital Telephony was written (1980), public telephone networks around the world were primarily implemented with analog technology, but it was clear that digital technology was rapidly taking over. When the second edition was written (1990), the internal portions of the network had, for the most part, been converted to an all-digital network. Then and today (1999) the main remnants of the original analog telephone networks are analog subscriber loops and analog telephones connected to them.

Although Integrated Services Digital Network (ISDN) technology was developed as a means of replacing analog loops to complete the transformation of the network to support end-to-end digital connections, ISDN deployment is below expectations for several reasons. One of these reasons is a growing need for more bandwidth than what is available from a basic rate ISDN subscriber loop (128 kbps). There is currently much activity within the industry to develop new technologies for medium- and highbandwidth digital subscriber access. A new chapter (Chapter 11) has been added to this edition to specifically address alternative technologies for digital subscriber access.

Another relatively recent application of digital technology added to this edition involves digital cellular telephones, which first appeared in the marketplace in the mid-1990s. Digital mobile radio is enabled by the emergence of low-cost, highperformance digital signal processing (DSP) technology for compressing speech signals to low bit rates and for providing sophisticated coding, modulation, and equalization required for digital radios in a bandwidth-constrained mobile application. A complete list of chapter topics is:

Chapter 1: Overview of analog telephone technology followed by an introduction of how digital technology is used to fulfill the same functions

Chapter 2: Discussion of advantages and disadvantages of digital technology for voice communications

Chapter 3: Descriptions of the most common voice digitization algorithms

Chapter 4: Fundamentals of digital wire-line transmission and multiplexing

Chapter 5: Basic concepts and operations of digital switching machines

Chapter 6: Digital modulation and radio fundamentals

Chapter 7: Network synchronization, control, and management requirements

Chapter 8: Fiber optic transmission systems and SONET

Chapter 9: Digital cellular telephone systems

Chapter 10: Data networks

Chapter 11: Digital subscriber access technology

Chapter 12: Fundamentals of traffic analysis for designing networks

The appendices cover the derivation of equations, PCM voice coding relationships, fundamentals of digital communications theory, and traffic tables.

Coppell, Texas
October 1999
johncbellamy @ ieee. org

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  • Anonymous

    Posted June 5, 2000

    A 'state of the art' book on modern communications

    Dr. Bellamy's book on digital telephony is one of the most accurate and complete books on the subject. I have found Digital Telephony an invaluable help while attending postgraduate courses on modern telecommunication systems.

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