on of mobile multimedia services in next-generation networks. IMS is a service infrastructure that relies on Session Initiation Protocol (SIP) to establish and maintain call control. IMS is an internationally recognized standard that defines a generic architecture that will offer VoIP and other multimedia ser- vices in wireline and wireless applications. Because voice is still the primary appli- cation for wireless telephony today, by adopting SIP as the signaling protocol, wireless service providers have a standard that works well for both voice and data. In fact, VoIP has now become the foundation of almost every service provider's next-generation network architecture, including wireless, wireline, and cable/MSP operators. As we examine the role of VoIP in these providers' networks, it is clear that it constitutes a vital part of their strategies for differentiation, reduced costs, and increased competitiveness. The IMS architecture is divided into three layers: transport (or access), con- trol, and service (or application) (refer to Figure 10.2 in Chapter 10). IMS assumes that each user is associated with a home network, and it supports the concept of roaming across other wired or wireless networks. IMS also includes a policy engine and an authentication, authorization, and accounting (AAA) server for operator control and security. Rather than the vertically integrated service stacks of today, as Figure 16.2 shows, IMS calls for a single support structure for all services: one billing system, one set of operational support system (OSS) ele- ments, and one subscriber database, accessed by applications that are created and delivered on-the-fly to any number of endpoints, as defined by the user. It is a vision of seamless, contextual personal communications that means not only new revenue-generating services for carriers but also a radical reduction in oper- ational complexity. The IMS architecture is composed of many components, operating at various layers, as illustrated for next-generation wireline networks in