o-end design, SIP networks are much more powerful and open to the implemen- tation of new services. Although IMS is SIP based, it includes enhancements and exceptions to the SIP specification, particularly for registration, authentication, and session policy. IMS uses DIAMETER rather than RADIUS for authentication, taking advantage of DIAMETER's additional support for charging and billing functions, such as prepaid calling services. IMS also uses the Common Open Policy Service (COPS) protocol for mobile operators to enforce security and QoS policies across network elements. IMS initially required the use of IPv6 (discussed in Chapter 8), but given the number of transport networks using IPv4, this requirement has been relaxed. IMS terminal devices are centrally and tightly controlled, unlike the IP networks that provide the underlying network transport for IMS. IMS assumes that each user is associated with a home network, and it supports the concept of roaming across other wired or wireless networks. Network services Chapter 10 Next-Generation Networks and applications are implemented in the user's home network. Because visited works act primarily as vehicles for multimedia IP packets sent or received by the roamer, service knowledge is not required in visited networks. IMS also includes a policy engine and an authentication, authorization, and accounting (AAA) server for operator control and security. The IMS Architecture The IMS standard defines a generic architecture that offers VoIP and other multi- media services within wireline and wireless infrastructures. As shown in Figure 10.2, the IMS architecture is divided into three layers: service (or application), control, and transport (or access). The IMS architecture consists of various components, including SIP proxies, media gateways, and various types of servers. The key components, as shown in Figure 10.3, include the following: Service- or application-layer components: Application servers-Application servers are IMS-compliant multime- dia application server