We dedicate this book to students 0/ computer science who want to under stand why the radio channel is at the heart of mobile communications, be it data or voice, and to students 0/ communications engineering looking for an introduction to the structure of the Internet protocols, and their application to mobile radio. We also welcome the practicing engineer' working for mobile network operators and infrastructure suppliers, and the application program mer looking for the underlying reasons for the problems they encounter in their daily work. Given that convergence of the information technology and communica tions worlds has been talked about for almost 20 years, there is still surpris ingly little knowledge among specialists of each others' fields. Data communi cations over radio, notably Internet access, however, requires fuH understand ing of both fields. Internet protocols have been designed for fixed networks. There bit errors are so rare that interrupted transmission almost inevitably means congestion. In contrast, the radio channel constitutes a bottleneck for mobile data communications in that it introduces bit errors even at modest data rates. This book is based on the doctoral thesis of one of us (M. Taferner).