Non-3GPP Access to 5G Terminal (UE) Registration and Connection Management (1)

5G (NR) allows terminals (UEs) to access the system via trusted non-3GPP, untrusted non-3GPP, and W-5GAN systems; for this purpose, 3GPP defines the following in TS23.501:

I. Registration Management

For terminals (UEs) accessing the 5G system via W-5GAN, the corresponding term is 5G-RG, while for FN-RG it corresponds to W-AGF. For N5CW terminals (UEs) accessing the 5GC via a trusted WLAN access network, the corresponding term is TWIF. When accessing via non-3GPP, the terminal (UE) and AMF should enter the RM-DEREGISTERED state as follows:

• - After an explicit deregistration procedure is performed at both the UE and AMF;
• - After the network's non-3GPP implicit deregistration timer expires at the AMF;
• - After the UE's non-3GPP deregistration timer expires at the UE. ---Assuming sufficient time is allowed for the UE to reactivate the UP connection of an established PDU session, regardless of whether the session was established via 3GPP or non-3GPP access.

II. Terminal (UE) Access

• When a UE registers via non-3GPP access, it starts a UE non-3GPP deregistration timer based on the value received from the AMF during the registration process when entering the non-3GPP access CM-IDLE state.
• In non-3GPP access mode, the AMF runs a network non-3GPP implicit deregistration timer. When the CM state of the registered UE changes to CM-IDLE via non-3GPP access mode, the network non-3GPP implicit deregistration timer will start at a value greater than the UE non-3GPP deregistration timer value.
• For UEs registered via non-3GPP access mode, access point changes (e.g., WLAN AP changes) should not cause the UE to perform the registration process.
• The UE should not provide 3GPP-specific parameters (e.g., indications of MICO mode preferences) during registration via non-3GPP access mode.

III. Upon successful connection management, a UE accessing the 5GC via non-3GPP will transition to CM-CONNECTED (non-3GPP access). Specifically:

• For untrusted non-3GPP access to the 5GC, the non-3GPP access connection corresponds to an NWu connection.
• For trusted access to the 5GC, the non-3GPP access connection corresponds to an NWt connection.
• For N5CW devices accessing the 5GC via a trusted LAN, the non-3GPP access connection corresponds to a Yt' connection.
• For wired access to the 5GC, the non-3GPP access connection corresponds to Y4 and Y5 connections.

***A UE will not establish multiple non-3GPP access connections to the 5GC simultaneously; non-3GPP access connections can be released through an explicit deregistration procedure or an AN release procedure.