What is "digital telephony"? To the authors, the term digital telephony de­ notes the technology used to provide a completely digital point-to-point voice communication system from end to end. This implies the use of digital technol­ ogy from one end instrument through the transmission facilities and switching centers to another end instrument. Digital telephony has become possible only because of the recent and ongoing 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, 3, and 10 provide the reader with the present status of telephone technology in terms of voice digitization as well as switching principles. The book is an outgrowth of the authors' continuing engineering education course, "Digital Telephony," which they have taught since January, 1980, to attendees from business, industry, government, common carriers, and tele­ phony equipment manufacturers. These attendees come from a wide variety of educational backgrounds. but generally have the equivalent of at least a bachelor's degree in electrical engineering. The book has been written to provide both the engineering student and the practicing engineer a working knowledge of the principles of present and future voice communication 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.