Network Simple, no Minor Interworking in IMS + VoIP support requirements interworking interworking radio and core in radio + requirements requirements network SRVCC Terminal Complex with Simple with Simple with single VoLTE support requirements dual radios single radio radio required but simple with single radio LTE coverage No need for full No need for full No need for full Good LTE coverage requirement LTE coverage LTE coverage LTE coverage preferred to avoid many SRVCCs Simultaneous CS voice + LTE CS voice + HSPA LTE VoIP + LTE voice + data Voice setup time Equal to CDMA Equal to CDMA CS voice setup time Faster than CS voice CS voice CS voice + 1-2 seconds For the second and third cases the response from MME determines whether the UE will establish mobile-originating call using PS or CS voice. For mobile-terminating calls the CS/PS domain selection is made by IMS and HSS. In the first three cases the UE tries to attach to CS voice services. The attach request is relayed by MME to MSC Server over the SGs interface. In the attach request MME includes the Location Area Code for the UE, based on an operator-configurable LAC-TAC mapping table. From this point on, the MSS knows the location of the UE at MME level and can send paging to the correct MME over the SGs interface. In this section, the emphasis is on the performance optimization aspects of CSFB; the interested reader can find further SGs protocol details and background information in [2] and [3]. Figure 14.2 is a simplified signalling diagram of a mobile-originating CSFB call in the case where the LAC of the target RAT is different from LTE LAC. UE initiates the call by sending Extended Service Request to MME (RRC signalling not shown). The transition to target RAT can be made via RRC release with redirect, via PS handover or (in case of 2G target RAT) inter-RAT cell change order. The decision of target RAT and how to move the UE there depend on eNodeB configuration and UE radio capability. Once the UE is in target RAT, it initiates Location Update procedu