xiv Preface In contrast to typical architecture and computing systems books, this book follows a structure analogous to those of typical networking books. Following the OSI reference model for network protocols, a model that has been proven highly valuable from an education point of view, we classify and present network systems and their designs. Later in the book, we also discuss specific components of network systems, similarly to books on system architecture. Since this book is the first systematic effort to present the architecture of the complete range of network systems as a whole, we focus on promoting key con- cepts for all types of network systems. We attempt to present the main architec- tures and designs of systems and components that cover this area. Clearly, the book does not and cannot cover all existing material in this area, which has experienced fast growth in recent years. Instead, the book focuses on major con- cepts. In addition to presenting state of the art, we have tried to demonstrate the progress in the field during the last couple of decades through specific examples that illustrate the improvements of technology and indicate the path of evolution for network systems in the future.