Section: 5.3.19.2 Redirection of the UE by the core network
Content: 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[ System architecture for the 5G System (5GS) ]  [58]. 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 5GCN by rejecting the attach request, or tracking area update request, or service request with the EMM cause #31 "Redirection to 5GCN required" as specified in clause 5.5.1.2.5, 5.5.1.3.5, 5.5.3.2.5, 5.5.3.3.5 and 5.6.1.5. Upon receipt of reject message, the UE disables the E-UTRA capability as specified in clause 4.5 and enables the N1 mode capability if it was disabled in order to move to 5GCN. The network that supports CIoT optimizations can also redirect a UE from 5GCN to EPC as specified in clause 4.8.4A.2 of 3GPP TS 24.501[ Non-Access-Stratum (NAS) protocol for 5G System (5GS); Stage 3 ]  [54].