) to the mobile and updates the location of the mobile to the HSS entity. 9) The MME entity induces the deletion of the context of audio and video streams at the SGW by sending the message GTPv2-C DELETE SESSION REQUEST, if the PS-PS inter-system handover is not established. Service Centralization and Continuity 10) The SGW and PGW entities carry the message received by the message GTPv2-C DELETE SESSION RESPONSE. 11) The MME entity causes the deletion of the contexts of audio and video streams at the entity eNB by sending the message S1-AP UE CONTEXT RELEASE COMMAND to the eNB entity. After the PS-CS inter-system handover, the CS flow is established between Alice's mobile and the MSC GW entity. The next phase involves the establishment of RTP flows between the MSCGW and ATGW entities. 9.2.2.5. Access transfer The access transfer procedure is described in Figure 8.16. P-CSCF Server SIP INVITE (URI STN-SR, SDP MSC GW) SIP 100 Trying SIP 200 OK (SDP ATGW) SIP ACK SIP INVITE (URI ATU-STI, SDP ATGW) SIP 100 Trying SIP 200 OK (SDP ATGW) SIP ACK SIP BYE SIP BYE SIP 200 OK SIP 200 OK Figure 9.16. Access transfer for service continuity 1) The MSC server entity sends the SIP INVITE request to the ATCF entity. The URI identity of the query contains the STN-SR session number mentioned by the MME in the procedure of the PS-CS inter-system handover. The header Asserted-Identity contains Alice's phone number (MSISDN). INVITE tel +1-237-888-9999 SIP/2.0 P-Asserted-Identity: <tel:+358-50-4821437> 270 VoLTE and ViLTE The body of the SDP message contains the characteristics of the RTP stream provided by the MSC GW entity: - IPv4 address (196.1.1.1); - port number (7236). 2) Upon receipt of the SIP INVITE message, the ATCF entity transfers the SDP message of the MSC GW entity to the ATGW entity and responds with the SIP 200 OK message containing the SDP message with the RTP flow characteristics provided by the ATGW entity: - IPv4 address (200.1.1.1); - port number (5238). The next phase aims at informing the SCC AS entity about the