4.8.4A.2 Redirection of the UE by the core network

The network that supports CIoT optimizations can redirect a UE between EPC and 5GCN as specified in clause 5.31.3 of 3GPP TS 23.501 [8]. The network can take into account the UE's N1 mode capability or S1 mode capability, the CIoT network behaviour supported and preferred by the UE or the CIoT network behaviour supported by the network to determine the redirection.

NOTE: It is assumed that the network would avoid redirecting the UE back and forth between EPC and 5GCN.

The network redirects the UE to EPC by rejecting the registration request or service request with the 5GMM cause #31 "Redirection to EPC required" as specified in clause 5.5.1.2.5, 5.5.1.3.5 and 5.6.1.5. Upon receipt of reject message, the UE disables the N1 mode capability for 3GPP access as specified in clause 4.9.2 and enables the E-UTRA capability if it was disabled in order to move to EPC.

When there is no ongoing registration procedure or service request procedure for a UE in 5GMM-CONNECTED mode, if the AMF determines to redirect the UE to EPC, the AMF shall initiate the generic UE configuration update procedure to indicate registration requested and release of the N1 NAS signalling connection not requested as described in clause 5.4.4. The network then redirects the UE to EPC by rejecting the registration request as specified in clause 5.5.1.3.5.

The network that supports CIoT optimizations can also redirect a UE from EPC to 5GCN as specified in clause 5.3.19.2 of 3GPP TS 24.301 [15].