the WAN. Seam- less handoff between fixed and mobile networks is also included. Defined by 3GPP and 3GPP2 (www.3gpp2.org), IMS is a service infrastructure that relies on Session Initiation Protocol (SIP) to establish and maintain call con- trol. IMS is an internationally recognized standard that defines a generic architec- ture for offering VoIP and other multimedia services in wireline and wireless applications. By adopting SIP as the signaling protocol, 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 architec- ture, 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. IMS gives carriers the opportunity to build a single and common IP service infrastructure independent of the access method. Building the infrastructure only once causes the cost per service to go down over time and increases the opportu- nity to offer a more integrated, richer, and more seamless environment. The IMS architecture offers a number of benefits, including enhanced person-to-person communications; improved interaction between media streams, which enables easy integration and interworking of different IP-based services; improved service mobility, which enables services to be offered over various access networks; and the ability of third-party developers and vendors to easily create and integrate new solutions through well-defined APIs and standards. IMS applications include voice telephony, video telephony, multimedia stream- ing, HTTP and TCP/IP browsing, instant messaging, file sharing, gaming, push-to- talk/push-to-media, and presence-based services. Major vendors, including Ericsson, Lucent, Nortel, and Siemens, have made substantial investments in IMS software. Four basic principles are associated with IMS: Access