erent Optical Communication Systems in 1995 and Nonlinear Optical Networks in 1998. Introduction From the very first step in the field of telecommunications, every relevant development has brought about a change in people's lifestyles. The introduction of fiber optics and of the digital mobile phone (the GSM) are examples in this regard. Within a brief period of about 10 years, fiber optics have increased the capacity of com- mercial transmission systems from 0.5 to 1280 Gbit/s, creating an abundance of capacity on which the Internet is based. The diffusion of GSM has transformed the telephone from a location-based communica- tion to a mobile form of personal communication, completely changing the way in which people communicate. Internet and GSM, among the other prominent developments in the field of telecommu- nications, have shaped our lifestyles and how we interact, to the extent that sometimes our society is called "the communication society." The complex and pervasive services that today are offered by telecommunication carri- ers are based on the telecommunication infrastructure: the network. This network is per- haps the most complex infrastructure deployed by man. It is pervasive, reaching almost all parts of the world, and uses a wide set of different technologies. As a natural consequence, managing a telecommunication network is a very complex task and to develop and produce equipment or software for the telecommunication net- work requires a high level of technological and of industrial expertise. The aim of this book is to identify the unitary design within this complexity, which allows the network to function, to evolve, and, finally, to satisfy the ever-changing needs of customers. 1.1 Book Content and Organization Even though the scope of the book is to give a unitary view of telecommunications under a technical point of view, it is simply impossible to include in a single book all the various elements that combined form the telecommunication network. Thus, some choices have to be made. The first 