Providing a digested experts' view of IMS, its concepts, architecture, protocols and functionalities, this books answers the question What is IMS? Provides a general description of system concepts, architecture and functionality. Presents an introduction to and description of base protocols. Gives a detailed description of key functionalities. Explains services on top of IMS. Also including architecture diagrams, call flows, comprehensive table of reference points and example protocol messages, this book is an ideal companion for engineers working for telecommunications companies, technical staff and engineers working for regulatory bodies and marketing personnel and students.