I should note here, however, that the first versions of SIP developed for VoIP are not
robust enough to support the IMS. The 3GPP has added many extensions to SIP to
make this a much more robust protocol, and to support the many functions defined for
security and authorization.
So SIP becomes the new call session control protocol for all things in the IMS network.
As additional processes and procedures are developed and defined, SIP will continue to
Architecture of the IMS 5
play a focal role. For lawful intercept, for example, standards call upon SIP to provide
the needed data regarding a call or session. This is a departure from previous standards
that required switches and network entities to capture call details and provide
this data through a different interface, dedicated to the function of lawful intercept.
For wireless environments, many aspects of SIP had to be modified. For example, in
the GSM network, a subscriber device must register with the network prior to receiving
service. This simple act of registration is how the network maintains the location
of a device for call delivery. It is also how subscribers are authenticated for security
purposes, with the device itself exchanging credentials with the network during the
registration process.
This concept of registration was carried over from the GSM environment into the
IMS domain, and will be evident as you read through the other chapters in this book.
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