ctions: - it separates the bearer dedicated to the voice from the other media carrying no voice; - it initializes, via the Sv interface, the e-SRVCC procedure for voice handover to the target cell of the GSM or UMTS network; - it coordinates the handover from the PS mode to the CS mode for voice and possibly the handover of the PS mode to the PS mode for the other streams. The MSC server entity of the 2G GSM or 3G UMTS network is also affected by the e-SRVCC function by performing the following functions: - it ensures the availability of resources in the 2G GSM or 3G UMTS network before executing the handover; - it coordinates the execution of the handover and the transfer of the telephone communication. The transfer of the telephone signaling involves transferring, on one hand, SIP signaling exchanged between the mobile and the ATCF entity, while on the other hand, the signaling consisting of call management exchanged between the mobile and the MSC server and SIP exchanged between MSC server and ATCF entity. The voice transfer involves transferring, on one hand, the RTP stream established between the mobile and the ATGW entity, while on the other hand, the CS bearer between the mobile and the MSC GW entity and the RTP stream established between the MSC GW entity and the ATGW entity. 9.2.1.2. Architecture for emergency call The functional architecture of the e-SRVCC function in the case of an emergency call is given in Figure 9.11. The emergency access transfer function (EATF) ensures continuity of service when the mobile performs the PS-CS inter-system handover. The EATF entity constitutes the anchor point of the SIP signaling and acts as a B2BUA entity. Service Centralization and Continuity In the case of roaming, the E-CSCF (Emergency CSCF) and EATF entities are located in the visited network. GERAN UTRAN 2G/3G Server GTPv2-C Sv interface S1-AP GTPv2-C Radio Bearer Bearer Bearer Figure 9.11. Functional architecture for emergency call control plane SIP flow is transmitted to the public safety answering point (P