This book presents a unified approach to developing accurate discrete-time models of communication and computer networks, using discrete-time queuing theory and approximation techniques to obtain solutions. The first chapter outlines the basic aims and philosophy of the book and introduces discrete-time queues at the simplest possible level. The chapters that follow cover probability theory and discrete-time Markov chains, delve into discrete-time queues and queuing networks, and focus on applications for satellite and local area networks. In addition, the book gives examples of specific modeling techniques, and covers handling statistically different users, finite channel delays, timing delays, and unsolvable traffic equations.