The IMS is not a service in itself; on the contrary, it is a SIP-based architecture for enabling an advanced IP service and application on top of the PS network. IMS provides the necessary means for invoking services; this functionality is called "service provision". IMS service provisioning contains three fundamental steps:
Define possible service or service sets.
Create user-specific service data in the format of initial filter criteria when a user orders/modifies a subscription.
Pass an incoming initial request to an AS.
Item (1) is not addressed in this book because it is up to operators and service providers to define what kind of services they are willing to offer their subscribers. The other two steps are described next.
Whenever a user obtains an IMS subscription and her subscription contains some value-added services or an operator is willing to utilize ASs as part of its IMS infrastructure, they need to create service-specific data. These service-specific data are part of the user's user profile. More precisely, service-specific data are represented as initial filter criteria. Hereafter, we only concentrate on initial filter criteria. Section 3.12 describes how initial filter criteria fit into a user profile. When constructing initial filter criteria an operator needs to consider these questions:
What is a trigger point?
What is the correct AS when the trigger point is met?
What is the priority ...