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What is "digital telephony"? To the authors, the term digital telephony denotes the technology used to provide a completely digital telecommunication system from end-to-end. This implies the use of digital technology from one end instru- ment through transmission facilities and switching centers to another end instru- ment. Digital telephony has become possible only because of the recent and on- going surge of semiconductor developments, allowing microminiaturization and high reliability along with reduced costs. This book deals with both the future and the present. Thus, the first chapter is entitled, "A Network in Transition." As baselines, Chapters 2 and 11 provide the reader with the present status of teler-hone technology in terms of voice digiti- zation as well as switching principles. The book is an outgrowth of the authors' consulting and teaching experience in the field since the early 1980s. The book has been written to provide both the engineering student and the practicing engineer a working knowledge of the prin- ciples of present and future telecommunication systems based upon the use of the public switched network. Problems or discussion questions have been included at the ends of the chapters to facilitate the book's use as a senior-level or first- year graduate-level course text. Numerous clients and associates of the authors as well as hundreds of others have provided useful information and examples for the text, and the authors wish to thank all those who have so contributed either directly or indirectly.