 UE implementing ICS function functional architecture The SCC AS entity combines the SIP signaling arising from the mobile (via the Gm interface) and from the MSC server entity (via the I2 interface) and acts as a back-to-back user agent (B2BUA), providing an anchor point for incoming and outgoing calls: - in the case of an outgoing call, it ends the dialogue with the mobile and the MSC server entity and initiates a new dialogue with the downstream entity. The SCC AS entity is the first application server invoked by the serving call session control function (S-CSCF). - in the case of an incoming call, it ends the dialogue with the upstream entity and initiates a dialogue with the MSC server entity and the mobile. The SCC AS entity is the latest application server invoked by the S-CSCF entity. The SCC AS entity provided the terminating access domain selection (T-ADS) that can route an incoming call to the appropriate mobile network: 2G or 3G network in CS mode, 3G or 4G network in PS mode (Figure 9.2). The SCC AS entity queries the home subscriber server (HSS) to obtain the address of the mobility management entity (MME) and the service GPRS support node (SGSN). The HSS entity asks the SGSN and MME entities to recover network and mobile capacities. Service Centralization and Continuity - NAS (ESM) S6a (DIAMETER) T-ADS (DIAMETER) NAS (SM) (MAP) S6d (DIAMETER) (DIAMETER) NAS (CM) I2 (SIP) Mx / Mi (SIP) Server Figure 9.2. Functional architecture for TADS The MSC server entity ensures registering the mobile from the SCC AS and S-CSCF entities. The MSC server entity provides the translation of the telephone signaling for the establishment of a communication: - call management (CM), a part of non-access stratum (NAS), at CS side; - SIP signaling, at IMS side via the I2 interface. The MSC server entity provides the control of the MSC GW entity for the establishment of the CS bearer, on one side, and the real-time transport protocol (RTP) stream on the side of the IP network. The functional architecture of the ICS functio