This book provides comprehensive coverage of the prools of communication systems. The book is divided into four parts. Part I covers the basic concepts of system and prool design and specification, overviews the models and languages for informal and formal specification of prools, and describes the specification language SDL. In the second part, the basic notions and properties of communication prools and prool stacks are explained, including the treatment of the logical correctness and the performance of prools. In the third part, many methods for message transfer, on which specific communication prools are based, are explained and formally specified in the SDL language. The fourth part provides for short descriptions of some specific prools, mainly used in IP networks, in order to acquaint a reader with the practical use of communication methods presented in the third part of the book. The book is relevant to researchers, academics, professionals and students in communications engineering. • Provides comprehensive yet granular coverage of the prools of communication systems • Allows readers the ability to understand the formal specification of communication prools • Specifies communication methods and prools in the specification language SDL, giving readers practical tools to venture on their own