om the SCC AS entity with SIP 200 OK message to indicate that Bob picked up the phone. Service Centralization and Continuity 25), 26) The SCC AS entity responds to the SIP INVITE request received from the domain HomeA. net with the SIP 200 OK message to indicate that Bob picked up the phone. 27), 28) The domain HomeA. net acknowledges the SIP 200 OK response with the subsequent SIP ACK request. 29), 30) The SCC AS entity acknowledges the SIP 200 OK response with the subsequent SIP ACK request. 9.2. e-SRVCC function 9.2.1. Functional architecture 9.2.1.1. Architecture for basic call When the mobile has established a call on the 4G evolved packet system (EPS), it is necessary to be able to transfer to the 3G universal mobile telecommunications system (UMTS) or 2G global system for mobile (GSM), if loss of coverage occurs on the 4G EPS network. The enhanced single radio voice call continuity (e-SRVCC) ensures continuity of service when the mobile moves from one network in PS mode to a network in CS mode. The functional architecture of the e-SRVCC function is described in Figure 9.7, for the SIP flow and in Figure 9.8 for the RTP stream. To ensure continuity of service, e-SRVCC function introduces two anchor points in the IMS network: - access transfer control function (ATCF) which provides the anchor point for SIP signaling. The ATCF entity is inserted into the path of SIP signaling between control session CSCF entities, on one hand, the P-CSCF (Proxy-CSCF) entity while, on the other hand, the I-CSCF or the S-CSCF entity; - access gateway transfer (ATGW) that provides the anchor point for the RTP stream. The ATCF entity is located in the visited IMS network in the case of roaming to hide to the nominal IMS network and to the interconnected IMS VoLTE and ViLTE network that the SIP flow has changed IP address, given the move from the PS mode to the CS mode. GERAN DIAMETER UTRAN 2G/3G DIAMETER SIP flow Interconnection interface S / I Server Roaming interface GTPv2-C Sv interface S1-AP GTPv2-C Radio Bearer Bearer Bearer