97In this chapter, an overview of the operation of the SIP protocol will be given,
followed by a discussion of the basic functions of the SIP protocol. Example
call flow diagrams are used throughout to illustrate the protocol. The refer-
ences at the end make a reading list for the details of the protocol.
What Makes SIP Special
We will try here to provide a summary for readers who have no special inter-
est in protocols, why SIP has the capabilities to redefine communications, as
they migrate from the telephone network to the Internet. As you will see, SIP
combines the features of the Advanced Intelligent Network (AIN) from the
telecom world for fixed and mobile telephony, with Internet features for
e-mail, web, transactions, and entertainment. To illustrate SIP concepts, we
will introduce the notion of a SIP-enabled IP network.