 Figure 10.3 in Chapter 10. While Figure 10.3 covers the full set of IMS components, Figure 16.2 is a simplified illustration that incorporates components specific to wireless net- works, showing the connectivity between radio access networks (RANs) and the IMS layers. Chapter 16 Emerging Wireless Applications Internet Radio Access Network Node B Figure 16.2 Moving up the value chain: the wireless IMS architecture As shown in Figure 16.2, the wireless IMS architecture includes the following key components: Service- or application-layer components: Application servers (AS)-The main responsibilities of an application server are to execute service-specific logic and deliver to IMS value- added services, such as push-to-talk, ringback tone, prepaid calling card, multimedia messaging, and multimedia conferencing service logic. Control-layer components: Call session controller function (CSCF)-This element is the central brain of IMS, interacting with the various service platforms. It maintains session state, while interacting with the gateways, service platform ele- ments, and charging functions for the overall orchestration of service delivery. Home subscriber servers (HSSs)-An HSS has the role of the main data storage for all subscriber and service-related data. Multimedia resource function controller (MRFC)-The MRFC acts as a media resource broker between the application server and MRFP The IP Multimedia Subsystem resources, and it can be implemented as part of an application server or as a separate network element. Multimedia gateway controller function (MGCF)-The MGCF, also known as a softswitch or call agent, communicates with the CSCF and controls the connections for media channels in an IMS media gateway. Transport- or access-layer components: Multimedia resource function processor (MRFP)-Also known as the IP media server, the MRFP supports media processing for the application layer, such as audio mixing, DTMF digit collection, content recording and playback, and codec transcoding. However, unlike traditional a