This new book explains how telecommunications networks work. It uses straightforward language supported by copious block - schematic diagrams so that non-engineers and engineers alike can learn about the principles of fixed and mobile telecommunications networks carrying voice and data. The book covers all aspects of today's networks, including how they are planned, formed and operated, plus next generation networks and how they will be implemented. After an introductory chapter on telephony, the book briefly describes all of today's networks - PSTN, mobile, cable television, the Internet, etc. - and considers how they interconnect. Individual chapters then consider the principles, technologies and network structures relating to transmission, circuit switching, signalling and control, data (including voice-over-IP) networks, and mobile networks. The important subject of numbering and addressing for telephony and IP is then covered. The book concludes with a chapter designed to pull everything together, considering architecture, quality of service and performance, operations, network evolution and next generation networks.