lative to redirect which has to establish signalling connection prior to sending first NAS message. Another less obvious benefit from CSFB with PS handover is that the system is able to collect handover counters at LTE-3G cell pair level which provides some visibility to the CSFB call setup performance at cell level. Call setup time with PS handover can be optimized by shortening the time to trigger to first measurement report and by optimization of target RAT neighbour lists. The latter, while usually amounting to tedious optimization, is important as missing neighbours will result in delayed or absent measurement report. Thus, non-optimized neighbour lists may result in degraded call setup time. Some guard timer should be implemented in the source LTE cell to resort to blind handover or redirect in case of missing measurement report. Comparison of Call Setup Times with Different Methods Figures 14.3 and 14.4 summarize the transition time to target RAT with different methods. With redirect, most of the time is spent on system information acquisition in the target system. With system information provided in RRC release this time can be efficiently minimized; less than 1 second delay from Extended Service Request to initial NAS message is possible for both 2G and 3G. For CSFB to 2G the redirect with system information provides large savings in call setup time which would otherwise be more than 2 seconds longer. On the other hand, if the provided system information is misaligned, the benefit is lost. Depending on the VoLTE Optimization Typical transition time to target RAT with blind redirect Redirect to 3G, UE Redirect to 3G, UE Redirect to 3G, UE Redirect to 3G, SIBs Redirect to 2G, UE Redirect to 2G, SI reads SIBs from air reads SIBs from reads SIBs from air provided in RRC reads SI from air provided in RRC interface cache interface with Release interface Release ESR to RRC Release in LTE RRC Release to Connection Establishment Connection Establishment to Initial NAS msg Figure 14.3 Typical transition time from 