information source for xviii Introduction to Telecommunications Network Engineering most of us and we use it daily in the office and at home. Its use for various kinds of commercial service is expanding, and interactive services, including entertainment, are becoming richer. Most new and evolving network tech- nologies for future telecommunications are also based on data communica- tion concepts, especially Internet technology. Examples of these are packet-switched second and third generation cellular systems. Also the core of the fixed telecommunications network will gradually evolve to packet- switched networks carrying both data and speech traffic. Here we try to emphasize this development. For future development all opinions and comments concerning the book are welcome. You may send them directly to the author at tarmo.antta- lainen@evitech.fr. For those readers who will use this book as training mate- rial, please contact the author for additional teachers' instructional material. Acknowledgments I want to thank my wife Pirjo and my children Heini, Sini, and Joni for their patience and understanding while I was writing the book. I am indebted to my colleagues Matti Puska and Tero Nurminen for their valuable proposals regarding the development of the book. I also want to thank my students for their helpful contributions and Espoo-Vantaa Institute of Technology for the opportunity to complete this project. Introduction to Telecommunications 1.1 What Is Telecommunications? Telecommunications has been defined as a technology concerned with communicating from a distance, and we can categorize it in various ways. Figure 1.1 shows one possible view of the different sections of telecommuni- cations. It includes mechanical communication and electrical communica- tion because telecommunications has evolved from a mechanical to an electrical form using increasingly more sophisticated electrical systems. This is why many authorities such as the national post, telegraph, and telephone (PTT) companies are involved in tele