 even if they have already deployed IMS for VoLTE services. One of the main reasons for this is the patchy coverage of the 5G network in the initial phase of 5G SA deployments. Therefore, they choose to continue to provide voice services with VoLTE even if the 5G device is connected to 5GC. One way of achieving this requires IMS connectivity to both 5GC and the EPS. IMS connectivity for 5GC is just required for SIP signaling between the 5G device and the IMS. The following diagram depicts a sample network architecture for EPS fallback. EPS fallback allows fallback to 4G and conducts a VoLTE call instead of a VoNR call. 5G Core Network - 5GC P-CSCF Release with Redirection Inter System Handover 4G Core Network - EPS Figure 8.13 - EPS fallback network architecture In this architecture, the user equipment (UE) is connected to 5GC through 5G RAN, and it has the default QoS flow for SIP signaling towards the IMS. When the UE initiates an IMS voice call, it will be released to 4G RAN by gNB with RRC Release with Redirection or with Inter-System Handover. When the UE attaches to the 4G network, the IMS voice call setup procedure for VoLTE continues. Radio access technology fallback Radio Access Technology (RAT) fallback is very similar to EPS fallback. In RAT fallback, Next- Generation eNBs (ng-eNBs) are connected to the 5GC together with gNBs. Remember that ng-eNBs provide 4G RAN coverage while it is connected to 5GC by supporting 5GC protocols. RAT fallback allows fallback from 5G RAN to 4G RAN and to conduct a VoLTE call instead of a VoNR call in case of patchy coverage in the 5G network. The following diagram shows a sample network architecture for RAT fallback: Interworking and voice call continuity 5G Core Network - 5GC P-CSCF Release with Redirection Inter System Handover ng-eNB Figure 8.14- RAT fallback network architecture In this architecture, a similar procedure to EPS fallback is in place. When the UE is connected to 5GC through 5G RAN, it has the default QoS flow for SIP signaling towards the IMS. When the 