The book is intended to be a reference guide for the technology used in modern telecommunication networks. The material presented in this book is not limited to North American infrastructure, but also contains a detailed description of the methods and international exchange of information. The book describes the terminology, concepts, techniques and apparatus which are used worldwide for data and voice signals. It is written in such a way that the material is clear even to those readers who know little about telecommunications, but the book can be a good technical guidance and experienced engineers.