2 Table 6.1. RTP flow characteristics The procedure for establishing the session is illustrated in Figure 6.12. To simplify the presentation, responses 100 Trying, 180 Ringing and precondition mechanisms are avoided. 194 VoLTE and ViLTE Alice's network (HomeA net) Bob's network (HomeB.net) S-CSCF I-CSCF SIP INVITE SIP INVITE SIP INVITE SIP INVITE SIP INVITE SIP INVITE SIP 183 SIP 183 SIP 183 SIP 183 SIP 183 SIP 183 SIP 200 SIP 200 SIP 200 SIP 200 SIP 200 SIP 200 SIP ACK SIP ACK SIP ACK SIP ACK Figure 6.12. Interconnection with IMS network session establishment 1) The S-CSCF entity receives from the P-CSCF entity the INVITE message whose associated SDP message contains the characteristics of the RTP streams of Alice's UA entity. INVITE SIP tel:+4687197378 SIP/2.0 Route: sip:scscf1.operatorHA.net; Content-Type application/sdp Content-Length: ( C=IN IP4 192.0.2.1 m=audio 49170 RTP/AVP 96 97 2) The S-CSCF entity detects that Alice's and Bob's UA entities are in different domains and transfers the INVITE message to the BGCF entity. The S-CSCF entity removes its identity from the Route header and performs the ENUM resolution on the URI identity of the destination to which it adds the iotl parameter indicating the direction of the request (homeA-Homeb) Interconnections The BGCF entity selects the IBCF entity and forwards the INVITE message without changing the SDP message. INVITE <sip:+46107197378@operatorHb.net;user=phone;iotl=homeA- homeB> SIP/2.0 Content-Type: application/sdp Content-Length: (...) C=IN IP4 192.0.2.1 m=audio 49170 RTP/AVP 96 97 3) The IBCF entity of the HomeA. net network selects the IBCF entity of the HomeB. net network and forwards the INVITE message including the SDP message which replaces the characteristics of the RTP streams received from Alice's UA entity with those communicated the TrGW entity of the HomeA. net network. INVITE <sip:+46107197378@operatorHb.net;user=phone;iotl=homeA- homeB> SIP/2.0 Content-Type: application/sdp Content-Length: (...) C=IN IP4 178.15.1.1 m=audio 62111 RTP/AVP 96 9