ptimization E-UTRAN MSC server/ RNS/BSS (source) (source) (target) (target) (target) Measurement reports HO decision Handover required Bearer split SRVCC PS to as request Prepare handover request Relocation request/handover request Forward relocation request Relocation request/handover request Forward relocation response Relocation request ack/handover request ack Prepare handover response Relocation request ack/handover request ack Establishment of circuit connection between the target MSC and the MGW associated with the MSC Server Initiation of session transfer SRVCC PS CS response Handover command Execution of the session transfer HO from E-UTRAN command procedure. The downlink flow of VoIP packets is switched towards UE tunes to the CS access leg at this point UTRAN/GERAN Source IMS access leg release Handover detection SES (Handover complete) Relocati on/handover complete ISUP Answer SRVCC PS to CS complete acknowledge Bearer deletion TMSI reallocation Location update to Relocation complete/handover complete Forward relocation complete HSS/HLR Forward relocation complete ack Release resources Bearer update Session deletion Figure 14.37 SRVCC from EUTRAN with PSHO [9] Once the MME receives the HO required message from eNodeB it performs PS bearer splitting and separates the QCI1 voice bearer from the other data bearers and initiates their relocation towards MSC server and SGSN. The MME sends a single received transparent container to both the target CS domain and the target PS domain. So the CS and PS relocation procedures are split and are performed in parallel but separately in different CS core network and PS core network. In case the target system is incapable of both CS and PS, only the voice bearer is transferred and PS bearers are suspended. Voice Bearer Transfer Preparations in Core Network The MME initiates the PS to CS HO procedure for QCI1 bearer by sending a SRVCC PS to CS Request to the MSC server which then sends Prepare Handover Request message to the target MSC. The target MSC sends Relocation