re since in this example it is assumed that the LAC is different from the one it received from MME during attach or TAU. Depending on CS core configuration the MSS may or may not be the same as the one serving the LTE LAC. If MSS is changed, this will increase the Location Update time, and hence also delay the call setup in target RAT. Moreover, depending on the MSS configuration authentication, IMEI checking and TMSI reallocation may be done at this point. After Location Update is accepted, the UE is able to start the actual CS call setup by sending CM Service Request as it would do in a normal mobile-originating call in the target RAT. LTE Small Cell Optimization Extended Service Request Transition to target RAT Location Update Request Authentication, identity request, TMSI reallocation Location Update Accept CM Service Request CS call setup Figure 14.2 Mobile-originating CSFB call signalling where LAC in target RAT is different from LTE LAC, and both LACs served by the same MSS After the CS call is finished, the UE should return back to LTE. This is possible by either UE-controlled normal inter-RAT cell reselection or redirect/handover based on a vendor- proprietary trigger. The main CSFB-specific optimization tasks to be discussed in this section are minimization of call setup time and accelerating the return to LTE. From the example in Figure 14.2 it can be seen that the CSFB call setup time is essentially the CS call setup in target RAT plus Location Update time, the time to transfer the UE from LTE to the target RAT and connection setup time in LTE (if UE is idle). On the other hand, call setup failure rate is the same as CS call setup failure rate in the target RAT plus the probability that Extended Service Request (connection setup) or the transition to target RAT fails. Neither the CSFB call setup time nor the CSFB call setup success probability can be better than the corresponding performance in the target RAT. However, the gap can be minimized by proper design choices and optimization. 14.3.2 CSFB Cal