me SDL case study, a simplified version of V.76, a protocol standardized by the ITU (International Telecommunications Union). I have written this book because, after using ObjectGeodeTM and Tau SDL SuiteTM (both from Telelogic) for more than ten years, I felt that the numerous powerful features available in such SDL tools needed to be clearly explained to a wide audience, instead of being confined to severe technoweenies. Readers who want to practise the exercises described in the book must contact the SDL tool vendors (see their updated list on www.sdl-forum.org), who generally provide free licenses for evaluation or cheap licenses for universities. The first versions of ObjectGeode and Tau SDL Suite including a simulator, named Geode and SDT at that time, have been released around 1989. However, to my knowledge, 13 years after that release, this is the first book published on validation of SDL systems by simulation. Very few other commercial tools or languages provide such a range of features for the validation and development of communications systems and software. Some may question the need for this book, as the SDL tools have their own documentation. The answer is that the documentation of each tool, assuming one of your colleagues has not taken it away, contains thousands of pages, which is not always organized to present first the basic simulation notions and then to introduce progressively more advanced features. In the book, every notion presented is illustrated by a hands-on systematic example, which has been actually executed on the two simulation tools, with direct explanations. Although this book describes how to validate telecommunication systems, it can be used to validate the behavior of other kinds of real-time systems that can be modeled by communicating state machines. I hope that this book will reveal to students or managers the power of SDL simulation, and will help designers and developers in the validation of their SDL models. Laurent Doldi Foreword 'Better', 'faster' and 'cheaper' are the 