5G Terminal (UE) Reachability Definition and Limitations

I. Reachability In mobile communication networks, UE reachability refers to the network's ability to locate a terminal device (UE) to transmit data, which is particularly important for UEs in an idle state. It involves states such as CM-IDLE, modes such as MICO (Mobile Initiated Connection Only), and the process by which the UE or network (AMF, UDM, HSS) notifies other parties when the UE is active or has access to specific services (e.g., SMS or data). During this process, data is buffered, and the terminal (UE) is paged when necessary to achieve terminal power saving (PSM/eDRX). 3GPP defines it in TS23.501 as follows;

II. CM-IDLE State For non-3GPP access networks (untrusted, trusted non-3GPP access networks) and W-5GAN, where the UE corresponds to 5G-RG in the W-5GAN case and W-AGF in the case of supporting FN-RG. For N5CW devices accessing 5GC via a trusted WLAN access network, their UEs correspond to TWIF. Specifically,

• the UE cannot page via a non-3GPP access network.
• If the UE state in the AMF is CM-IDLE or RM-REGISTERED for the non-3GPP access network, there may be PDU calls where the last route was through the non-3GPP access network and user plane resources are lacking. If the AMF receives a message from the SMF containing a non-3GPP access type indication, corresponding to a PDU session of a UE in the CMIDLE state of non-3GPP access, and this UE has registered for 3GPP access in the same PLMN as the non-3GPP access, then regardless of whether the UE is in the CM-IDLE or CM-CONNECTED state on the 3GPP access, it can execute network-triggered service requests via the 3GPP access. In this case, the AMF will provide indication that the process is related to non-3GPP access (as described in Section 5.6.8) – the UE's behavior upon receiving such a network-triggered service request is specified in Section 5.6.8.

III. CM-CONNECTED State for non-3GPP access networks (untrusted, trusted non-3GPP access networks) and W-5GAN, where the UE corresponds to 5G-RG in the case of W-5GAN and W-AGF in the case of FN-RG support. For N5CW devices accessing 5GC through a trusted WLAN access network, the UE corresponds to TWIF. A UE in the CM-CONNECTED state is defined where:

• the AMF knows the UE's position at the N3IWF, TNGF, TWIF, and W-AGF node granularities.
• When the UE is unreachable from the perspective of N3IWF, TNGF, TWIF, and W-AGF, i.e. when the non-3GPP access connection is released, N3IWF, TNGF, TWIF, and W-AGF will release the N2 connection.