om the Path header of the SIP REGISTER request; - the IP address of Bob's UA entity results from registration. 3) Upon receipt of the SIP INVITE message, the ATCF entity reserves the resource with the ATGW entity providing the anchor point for the RTP stream. The ATCF entity transfers to the P-CSCF entity the SIP INVITE request whose associated SDP message, sent to Bob's UA entity, is that of the ATGW entity to replace the one received from the domain HomeA. net: - the IP address (200.1.1.2) is replaced by that of the ATGW entity (200.2.2.1); - the port number (8899) is replaced by the value communicated by the ATGW entity (10124). 4) The SIP 183 Session Progress response from Bob's UA entity contains the characteristics of the RTP stream in the SDP message body: - IPv4 address (192.2.2.1); - port number (4528). 5) Upon receipt of the provisional response SIP 183 Session Progress, the ATCF entity configures the ATGW entity and replaces the SDP message body received from Bob's UA entity by the one received from the ATGW entity: - the IP address (192.2.2.1) is replaced by that of the ATGW entity (200.2.2.2); - the port number (4528) is replaced by the value communicated by the ATGW entity (6544). Service Centralization and Continuity 9.2.2.4. PS-CS inter-system handover The PS-CS inter-system handover procedure is given in Figure 9.15. Server UTRAN RRC MeasurementReport S1-AP HANDOVER REQUIRED GTPv2-C SRVCC PS-CS REQUEST RELOCATION REQUEST RELOCATION REQUEST ACK GTPv2-C SRVCC PS-CS RESPONSE S1-AP HANDOVER COMMAND RRC ConnectionReconfiguration Handover execution HANDOVER TO UTRAN COMPLETE GTPv2-C SRVCC PS-CS COMPLETE NOTIFICATION GTPv2-C SRVCC PS-CS COMPLETE NOTIFICATION ACK GTPv2-C DELETE SESSION REQUEST GTPv2-C DELETE SESSION RESPONSE S1-AP UE CONTEXT RELEASE COMMAND Figure 9.15. PS-CS inter-system handover 1) The decision to perform a handover from the EPS network in PS mode to the UMTS or GSM network in CS mode is taken by the eNB on which the mobile is connected. This decision is taken based on measurement repo