ected to a standalone 5G network), the following alternatives are available: In idle mode, the UE can reselect the LTE cell, in which case the UE mobility manage- ment (MM) and SM context will be transferred from the 5G core to the EPC. This has been defined both ways, SO when 5G connectivity resumes, the UE can again reselect 5G, and now the context is transferred back from the EPC to the 5G core. Moving from 5G to 4G is shown in Figure 5.15. In the active mode, the handover has been defined in both ways between LTE and 5G, on the basis of the measurements done by the UE, as discussed in Chapters 6 and 7. For services like voice, the same IMS can be used, and there is no need to involve the CS domain in 5G and LTE interworking. An example of active mode handover shown in Figure 5.16. The active mode mobility is normally based on UE measurements, as discussed in Chapter 7. Following this, the network side will make a decision on the need for han- dover. In Figure 5.15, an example of 5G to LTE mobility is shown following the UE measurement report and gNB decisions that inter-system handover to LTE is needed. 3GPP specifications do not support in Release 15 5G interworking with any other radio technologies other than LTE. It is being addressed in Release 16 for voice ser- vice if there is still a need to add interworking with 3G to cover for possible deploy- ments without existing LTE coverage in the area (and only 3G coverage). A Release 15 UE, when running out of 5G coverage and not finding an LTE network can naturally select 3G or even 2G, but there is no support for UE context transfer in such a case or active mode handover possibility directly from 5G to 2G/3G. Once the UE has moved to the LTE side, the existing interworking mechanisms on the LTE side are naturally available including 2G/3G related interworking if the UE supports 2G and 3G. There is no radio level interworking with Wi-Fi defined in Release 15. However the untrusted non-3GPP-related access technologies, such as Wi-Fi, may use the 5G core via th