G/3G mobile networks: - anchoring the traffic is carried by the SGSN entity that connects to the SGW entity if the feature Direct Tunnel is not available; - anchoring the traffic is carried by the RNC entity that connects to the SGW entity if the feature Direct Tunnel is available. The functional architecture for the PS-PS inter-system handover is described in Figure 7.10. Handover E-UTRAN Source S1-MME LTE-Uu GERAN UTRAN Figure 7.10. PS-PS inter-system handover functional architecture The S3 interface is the reference point between the SGSN and MME entities: - this interface supports GTPv2-C (GPRS Tunnel Protocol Control) signaling; - this interface allows the exchange of messages related to the management of PS-PS inter-system handover. The S4 interface is the reference point between the SGW and SGSN entities: - this interface is deployed if the feature Direct Tunnel is not available; - this interface supports GTPv2-C signaling for the control plane and GTP-U (GPRS Tunnel Protocol User) tunneling for the traffic plane; - GTPv2-C signaling ensures the construction of S4 bearer built between the SGW and SGSN entities. The S12 interface is the reference point between the SGW and RNC entities: - this interface is deployed if the feature Direct Tunnel is available; - this interface supports the GTP-U tunneling for the traffic plane. VoLTE and ViLTE 7.4.2. Procedure The procedure for PS-Ps inter-system handover is shown in Figure 7.11: - the preparation phase includes messages 1 to 8; - the execution phase includes messages 9 and 10; - the completion phase includes messages 11 to 21. During the preparation phase, a temporary bearer is constructed to route the incoming data: - the temporary bearer may be direct if the data is routed directly from the eNB entity to the 2G/3G radio access network; - the temporary bearer may be indirect if the incoming data passes through the SGW and SGSN entities. The procedure for PS-PS inter-system handover is elaborated with the following assumptions in hand: - the temporary bearer b