 subsequent terminating calls. However, [5] requires that CSFB-capable UE will make the LU after the call has been released even when the LACs are the same. 4 Figure 14.5 gives some idea of the Location Update delay for different CS core network configurations. The values should be considered typical ones rather than definitive; both radio and core network parameter configuration as well as radio conditions will cause variations to the procedure duration. The values shown assume 3G and 13.6 kbps signalling radio bearer. For 2G access, the delay will be larger. It is worth noting that by appropriate CS core network configuration it is possible to make considerable savings in CSFB call setup time. Specifically, coordinated Location Area planning for LTE and the target system is especially important. Further savings are possible by reducing the frequency of authentication and IMEI checking (identity request) procedures. Call Setup Signalling The third phase of the CSFB call setup is the call setup signalling. The duration of this phase is basically the same for ordinary CS call in the target system. Optimization is possible by using fast signalling connection. Also if the UE falls back to the same MSS, reducing the frequency of authentication and IMEI checking will reduce the setup duration by around half a second. 5 4 In normal inter-RAT reselection from LTE to another RAT a CSFB-capable UE will make LU even if the LACs are the same. The 3GPP 24.008 option to postpone LU after the release of the CS call is to optimize the CSFB call setup time. Authentication and identity request may have been done already earlier during Location Update. LTE Small Cell Optimization As mentioned in [2], for mobile-terminating call, the call setup time perceived by the calling party can be made to appear shorter by connecting the alerting tone as soon as the UE has responded to CS paging in LTE and MME has acknowledged the paging to MSS. The perceived time savings are almost the same as the duration of the end-to-end call setup less L