rminating Roaming Forwarding. The main difference is that MTRR requires HLR support while MTRF does not. The frequency of authentication, identity request and TMSI reallocation can be controlled by MSS settings. The MSS is able to recognize from CSMT or CSMO flag that Location Update is the initial step of a CSFB call setup, and it can accept the LU directly without any additional intermediate signalling steps such as authentication. Again, MSS support for such optimization is required. The WCDMA measurement procedure consists of search step and level/quality measurement step. In the search step the UE makes code acquisition for the strongest cells of the provided neighbour list. This is followed by the actual measurement level/quality measurement for the strongest cells found in the search step. VoLTE Optimization Typical location update procedure delay in CSFB call setup MSS changed, no MSS changed, with MSS does not MSS does not MTRR or MTRF CSMT/MTRR change, with change, no authentication and authentication and identity request identity request Figure 14.5 Location Update delay in CSFB to 3G call setup From radio access point it is advisable to use fast signalling channel to minimize message transfer delay in the air interface. For 2G this means early allocation of FACH for signalling. For 3G signalling bearers are recommended to be mapped to HSPA or DCH 13.6 kbps. Depending on if MSS change is required, authentication configuration, MSS support of CSMx flag, the Location Update signalling may take from around a couple of hundred mil- liseconds to 4 seconds or even longer (e.g. MTC where MSS changes, CSMT flag not supported and re-paging is required). The most efficient way to reduce the time spent on call setup signalling is to align Location Area Codes in LTE and the target RAT. In this case the UE will not need to make LU and will directly proceed to service request or paging response. A common encountered concern is that skipping LU in the beginning of the call setup might result in paging degradation for