rchy, having limited national markets, and SO on). The following sections define the main aspects of the IMS functions and its two most important accompanying protocols, SIP and Diameter. 3.3.1 IMS Architecture The IMS is the main signaling system in NGN, but it has been standardized by 3GPP (3G Partnership Project) and also accepted by the other SDOs in its "common" version (3GPP Release 8). In general, the IMS is access independent, SO the same system can be included for both fixed and mobile access networks. It is primarily targeted at services that require end-to-end signaling, which include QoS-enabled VoIP and IPTV. Figure 3.4 shows the IMS architecture with its functional entities. Note that the words "function" (in 3GPP terminology) and "functional entity" (in NGN terminology) refer to the same functional entities of the IMS. The main functional entities in the IMS architecture are three types of call session control functions (CSCFs): Proxy-call session control function (P-CSCF). This is the first contact point on the IMS side for the user equipment (e.g. mobile terminal, fixed terminal). P-CSCF acts as a SIP proxy node and it interacts with the admission control subsystem to provide authorization of media components that can be provided with appropriate QoS. In 3.3 Service Architecture in NGN IMS services IMS core I-CSCF S-CSCF P-CSCF H.248 Resource and admission control function (RACF) Telecom operator's IP network Internet and (mobile/fixed access network packet-switched and core networks) networks User data Diameter protocol SIP (Session initiation protocol) Control data AS - Application servers MRFC - Multimedia resource function controller BGCF - Breakout gateway control function MRFP - Multimedia resource function processor HSS - Home subscriber server NACF - Network attachment control function I-CSCF - Interrogating call session control function P-CSCF - Proxy call session control function IBCF - Interconnection border control function PLMN - Public land mobile network IMS - IP multimedia subsyst