Preface The field of data networks has evolved over the last fifteen years from a stage where networks were designed in a very ad hoc and technology-dependent manner to a stage where some broad conceptual understanding of many under- lying issues now exists.The major purpose of this book is to convey that con- ceptual understanding to the reader. Previous books in this field broadly separate into two major categories.The first,exemplified by Tannenbaum [Tan81]and Stallings [Sta85],are primarily descriptive in nature,focusing on current practice and selected details of the operation of various existing networks.The second,exemplified by Kleinrock [Kle76],Hayes [Hay84],and Stuck and Arthurs [StA85],deal primarily with performance analysis.This book,in contrast,is balanced between description and analysis.The descriptive material,however,is used to illustrate the underly- ing concepts,and the analytical material is used to provide a deeper and more precise understanding of the concepts.We feel that a continuing separation between description and analysis is unwise in a field after the underlying con- cepts have been developed;understanding is then best enhanced by focusing on the concepts. The book is designed to be used at a number of levels,varying from a senior undergraduate elective,to a first year graduate course,to a more ad- vanced graduate course,to a reference work for designers and researchers in the field.The material has been tested in a number of graduate courses at M.1.T. and in a number of short courses at varying levels.The book assumes some xiliPreface The field of data networks has evolved over the last fifteen years from a stage where networks were designed in a very ad hoc and technology-dependent manner to a stage where some broad conceptual understanding of many under￾lying issues now exists. The major purpose of this book is to convey that con￾ceptual understanding to the reader. Previous books in this field broadly separate into two major categories. The first, exemplified by Tannenbaum [Tan81] and Stallings [Sta85], are primarily descriptive in nature, focusing on current practice and selected details of the operation of various existing networks. The second, exemplified by Kleinrock [Kle76], Hayes [Hay84], and Stuck and Arthurs [StA85], deal primarily with performance analysis. This book, in contrast, is balanced between description and analysis. The descriptive material, however, is used to illustrate the underly￾ing concepts, and the analytical material is used to provide a deeper and more precise understanding of the concepts. We feel that a continuing separation between description and analysis is unwise in a field after the underlying con￾cepts have been developed; understanding is then best enhanced by focusing on the concepts. The book is designed to be used at a number of levels, varying from a senior undergraduate elective, to a first year graduate course, to a more ad￾vanced graduate course, to a reference work for designers and researchers in the field. The material has been tested in a number of graduate courses at M.I.T. and in a number of short courses at varying levels. The book assumes some