p to other protocols and technologies. Gonzalo Camarillo does an excellent job walking the fine line between technical completeness and technology overviews. Gonzalo has been an important contributor to the SIP revolution. He is the author of several of the key documents being developed within the standards bodies. He is an active contributor on the mailing lists, and a good teacher for those who have basic questions. It is this combination of technical depth and teaching skills that have resulted in a fine book, which I wholeheartedly recommend to anyone who asks the question, "what is SIP, and why is it important?" As one of the co-authors of SIP, I have dedicated several years of my life to its development. I have seen it grow from an academic technology to a force that is changing the way telecommunications will work in the Copyright 2002 The McGraw-Hill Companies, Inc. Click Here for Terms of Use. xviii Foreword years to come. It is truly impressive that throughout its growth, the fun- damental goals and principles on which the technology was built have not changed. This has happened because of the cadre of people who have come to believe in the vision SIP presents, and who have worked to promote it within their organizations. Gonzalo has captured that vision in a book, and so I encourage you, reader, to turn the page and learn more about what the future of telecommunications will look like. DR. JONATHAN ROSENBERG CO-AUTHOR, SESSION INITIATION PROTOCOL CHIEF SCIENTIST, DYNAMICSOFT INC. CHAPTER Signalling in the Circuit- Switched Network Chapter 1 The telephone network, also known as the Public Switched Telephone Net- work (PSTN), reaches almost every country in the world. We can find tele- phone sets almost in every house, including simple analog phones, more advanced Integrated Services Digital Network (ISDN) phones, cordless phones, cellular phones, and even satellite phones. Among technologies, telephone systems are astonishingly widespread. All these phones have something in common. They use a circuit-sw