 Figure 9.7. Functional architecture for basic call control plane GERAN UTRAN Interconnection interface BSSAP H.248 RANAP Server GTPCv2-C H.248 Sv interface S1-AP GTPv2-C Radio Bearer Bearer Bearer Figure 9.8. Functional architecture for basic call traffic plane Service Centralization and Continuity The ATGW entity is located in the visited IMS network too in the case of roaming to hide from the interconnected IMS network that the RTP stream has changed its IP address, given the move from the PS mode to the CS mode. The ATGW entity is controlled by the ATCF entity via the H.248 protocol. The ATGW entity may also perform the transcoding of voice if the codecs used, firstly, in the EPS network and, secondly, in GSM or UMTS networks, are different. Figure 9.9 provides an example of the constitution of the RTP stream between Alice's UA in the domain HomeA net, Bob's UA in the domain HomeB. net and ATGW entities. domain HomeA net domain HomeB net RTP flow RTP/flow RTP flow Alice @IP 192.1.1.1 @IP 200.1.1.1 @IP 200.1.1.2 @IP 200.2.2.2 @IP 200.2.2.1 @IP 192.2.2.1 N° port 3456 N° port 11234 N° port 8899 N° port 6544 N° port 10124 N° port 4528 Figure 9.9. RTP flow characteristics for service continuity Figure 9.10 provides an example of the constitution of the RTP stream between MSC GW entity in the domain HomeA net, Bob's UA in the domain HomeB net ATGW entities, after the PS-CS inter-system handover for Alice's mobile. domain HomeA. net domain HomeB net Alice CS flow RTP flow RTP flow RTP flow @IP 196.1.1.1 @IP 200.1.1.1 @IP 200.1.1.2 @IP 200.2.2.2 @IP 200.2.2.1 @IP 192.2.2.1 N° port 7236 N° port 5238 N° port 8899 N° port 6544 N° port 10124 N° port 4528 Figure 9.10. RTP flow characteristics at the end of PS-CS inter-system handover The SCC AS entity implements the mechanisms for the control of the registration of the e-SRVCC function and for the transfer of the session at the PS-CS inter-system handover. VoLTE and ViLTE The MME entity of the EPS network is affected by the e-SRVCC function by performing the following fun