Section: 5.31.3 Selection, steering and redirection between EPS and 5GS
Content: The UE selects the core network type (EPC or 5GC) based on the broadcast indications for both EPC and 5GC, and the UE's EPC and 5GC Preferred Network Behaviour. Networks that support NB-IoT shall broadcast an indication whether N3 data transfer is supported or not in system information. When the UE performs the registration procedure it includes its Preferred Network Behaviour (for 5G and EPC) in the Registration Request message and the AMF replies with the 5G Supported Network Behaviour in the Registration Accept message. If the UE supports any of the CIoT 5GS Optimisations included in 5GC Preferred Network Behaviour, then when the UE performs an Attach or TAU procedure and the UE includes its EPC Preferred Network Behaviour then the UE shall also include its 5GC Preferred Network Behaviour. In networks that support CIoT features in both EPC and 5GC, the operator may steer UEs from a specific CN type due to operator policy, e.g. due to roaming agreements, Preferred and Supported Network Behaviour, load redistribution, etc. Operator policies in EPC and 5GC are assumed to avoid steering UEs back and forth between EPC and 5GC. To redirect a UE from 5GC to EPC, when the UE sends a Registration Request or Service Request, the AMF sends a Registration Reject or Service Reject with an EMM cause value indicating that the UE should not use 5GC. The UE disables N1 mode and re-enables S1 mode, if it was disabled. The UE then performs either an Attach or TAU in EPC as described in clause 5.17.2. To redirect a UE from EPC to 5GC, when the UE requests an Attach or TAU procedure or Service Reject, the MME sends a reject message with an EMM cause indicating the UE should not use EPC. The UE disables S1 mode and re-enables N1 mode, if it was disabled. The UE then registers with 5GC as described in clause 5.17.2. When determining whether to redirect the UE, the AMF/MME takes into account the UE support of S1/N1 mode, respectively, and the UE's Preferred Network Behaviour and the Supported Network Behaviour of the network the UE is being redirected towards. When determining to redirect the UE in 5GMM-CONNECTED mode to EPC, the AMF shall initiate the UE Configuration Update procedure to indicate registration requested and release of the N1 NAS signalling connection not requested, then the AMF redirects the UE to EPC by rejecting the subsequent Registration Request, see TS 24.501[ Non-Access-Stratum (NAS) protocol for 5G System (5GS); Stage 3 ]  [47]. If after redirection the UE cannot find a cell supporting connectivity, the UE may re-enable the disabled N1/S1 mode and then perform Registration, Attach or TAU.