ardization, I have heard this and many other misconceptions several times. That was the main reason that pushed me to write this book. This book intends to clarify the philosophy behind SIP. In order to have a good understanding of a protocol such as SIP, it is necessary to be able to answer three simple questions: what, how, and why. This book deals with all three of them, but sets a special focus on the last one: why. The reason for doing SO is from my own experience of speak- ing with several engineers and programmers. I was surprised to meet peo- ple that have a great knowledge about what SIP does and how it does it, but who did not understand the philosophy behind the protocol. They did not know why SIP was designed like it was. They understood the proto- col details but were missing the overall picture. The trees did not let them see the forest. The why part is also very useful for business managers who do not need to know the protocol details in depth, but need to under- stand why to use a particular technology in their products. It would be sad if SIP was used just because it is fashionable and not for all its good features. In order to understand why SIP is a good signaling protocol, it necessary to know the paradigm behind it and compare it with another paradigm to see its advantages. That is why Chapter 1, "Signaling in Copyright 2002 The McGraw-Hill Companies, Inc. Click Here for Terms of Use. Preface the Circuit Switched Network," provides an introduction to traditional telephone signaling. This brief introduction helps the reader understand why a paradigm shift was needed, and the advantages and disadvantages of the Internet paradigm. An introduction to packet switching and to IP has been added at the beginning of Chapter 2, "Signalling in the Packet-Switched Network." This introduction is intended for those professionals who have experience in the telecom world and are trying to jump into the new datacom tech- nologies. They will find advantages and disadvantages of packet switch- ing networks and 