transcoding and NAPT or NAPT-PT translation of the RTP streams under the control of the IBCF entity. Alice's visited network (VisitedA.net) Alice's home network (HomeA. net) P-CSCF IBCF-1 IBCF-2 S-CSCF TrGW-1 TrGW-2 IBCF-3 RTP flow SIP flow TrGW-4 TrGW-3 IBCF-4 P-CSCF IBCF-6 IBCF-5 Bob 's visited network (VisitedB.net) Bob's home network (HomeB net) Figure 8.2. Roaming applied to the IMS network: nominal routeing The IBCF entity of Alice's home network (Bob's home network, respectively) consists of two IBCF-2 and-3 IBCF instances (IBCF-4 and IBCF-5, respectively). The IBCF entity of Alice's home network (Bob's home network, respectively) controls the TrGW-2 entity (TrGW-3, respectively). Roaming interfaces between the home network and the visited network are provided by: - Ici interfaces required for SIP flow between IBCF-1 and IBCF-2 instances for Alice's network and between IBCF-5 and IBCF-6 instances for Bob's network; - Izi interfaces for the RTP stream between TrGW-1 and TrGW-2 entities for Alice's network and between TrGW-3 and TrGW-4 entities for Bob's network. 226 VoLTE and ViLTE The interfaces for the interconnection between IMS networks performing, on the one hand, the outgoing call and, on the other hand, the incoming call is provided by: - Ici interface between IBCF-3 and IBCF-4 instances for the SIP flow; - Izi interface between the TrGW-2 and TrGW-3 entities for the RTP stream. The functional architecture of roaming applied to the IMS network is described in Figure 8.3, when the RTP flows does not pass through the home network of the caller, corresponding to the optimal media routeing (OMR). Alice's visited network (VisitedA.net) Alice' home network (HomeA. net) P-CSCF IBCF-1 IBCF-2 S-CSCF TrGW-1 TrGW-2 IBCF-5 IBCF-4 IBCF-3 SIP flow RTP flow TrGW-4 TrGW-3 IBCF-6 P-CSCF IBCF-8 IBCF-7 Bob' visited network (VisitedB.net) Bob's home network (HomeB.net) Figure 8.3. Roaming applied to IMS network: optimal routeing The IBCF entity of Alice's visited network consists of three instances, IBCF-1, IBCF-4 and I