r as well as the end user's profile. It performs many of the same functions that the Home Location Register (HLR) and the Authentication Center (AUC) provide in GSM networks. All the elements described SO far are involved in the control and operation of the network. The actual services are provided by SIP Application Servers (AS). These AS can operate in a variety of modes, in order to provide different types of services to the end user. These modes are the SIP Proxy mode, the SIP User Agent (UA) mode (i.e., it acts as an endpoint), or the Back-to-Back User Agent (B2BUA) mode (i.e., it acts as two SIP user agents joined together). A service such as voice-mail would normally be implemented in the SIP UA mode, whereas a click-to-dial ser- vice would normally be implemented as a B2BUA SIP AS. The SIP protocol is used by the AS to communicate with the SCSCF and end user, but the AS uses the Diameter protocol to communicate with the HSS in a safe and secure manner. The Diameter protocol is an extendable protocol that has been designed to mainly pro- vide authentication, authorization, and accounting (AAA) services [7]. The core IMS network uses Diameter to provide these functions (Figure 8.1). Generic SIP AS Diameter ICSCF SCSCF User agent PCSCF User agent Figure 8.1 The standard IMS architecture. Service Delivery Platforms 8.2.2 Service Triggering in the IMS All nonuser originating services in the IMS are hosted on AS. An example of a user originating service is a normal call placed by the end user. If a more advanced ser- vice is required, it is performed by the AS. The SCSCF directs user's requests to different AS depending on certain conditions. Filters containing these conditions are compared against any incoming SIP requests, which are forwarded to the appro- priate AS if a match is found. These filters are the iFC. The iFCs are vital to the operation of the network, and are stored on a per user basis in the HSS. Only SIP requests that create a dialogue or are a stand-alone request trigger these filters. Thus, t