5.5 Non-3GPP access specific aspects

23.5013GPPRelease 18System architecture for the 5G System (5GS)TS

5.5.0 General

This clause describe the specific aspects for untrusted non-3GPP access, trusted non-3GPP access and W-5GAN access.

5.5.1 Registration Management

This clause applies to Non-3GPP access network corresponding to the Untrusted Non-3GPP access network, to the Trusted Non-3GPP access network and to the W-5GAN. In the case of W-5GAN the UE mentioned in this clause corresponds to 5G-RG or to the W-AGF in the case of FN-RG. In the case of N5CW devices access 5GC via trusted WLAN access networks, the UE mentioned in this clause corresponds to TWIF.

The UE shall enter RM-DEREGISTERED state and the AMF shall enter RM-DEREGISTERED state for the UE on non-3GPP access as follows:

– at the UE and at the AMF, after performing an Explicit Deregistration procedure;

– at the AMF, after the Network non-3GPP Implicit Deregistration timer has expired.

– at the UE, after the UE non-3GPP Deregistration timer has expired.

NOTE: This is assumed to leave sufficient time to allow the UE to re-activate UP connections for the established PDU Sessions over 3GPP or non-3GPP access.

Whenever a UE registered over non-3GPP access enters CM-IDLE state for the non-3GPP access, it starts the UE non-3GPP Deregistration timer according to the value received from the AMF during a Registration procedure.

Over non-3GPP access, the AMF runs the Network non-3GPP Implicit Deregistration timer. The Network non-3GPP Implicit Deregistration timer is started with a value longer than the UE’s non-3GPP Deregistration timer, whenever the CM state for the UE registered over non-3GPP access changes to CM-IDLE for the non-3GPP access.

For a UE that is registered over Non-3GPP access, a change of the point of attachment (e.g. change of WLAN AP) shall not lead the UE to perform a Registration procedure.

A UE shall not provide 3GPP-specific parameters (e.g. indicate a preference for MICO mode) during registration over a non-3GPP access.

During registration procedure the AMF may determines whether the serving N3IWF is appropriate based on the slices supported by the N3IWFs as specified in clause 6.3.6.

5.5.2 Connection Management

This clause applies to Non-3GPP access network corresponding to the Untrusted Non-3GPP access network, to the Trusted Non-3GPP access network and to the W-5GAN. The UE mentioned in this clause corresponds to the 5G-RG in the case of W-5GAN and to the W-AGF in the case of FN-RG. In the case of N5CW devices access 5GC via trusted WLAN access networks, the UE mentioned in this clause corresponds to TWIF.

A UE that successfully establishes a Non-3GPP Access Connection to the 5GC over a Non-3GPP access transitions to CM-CONNECTED state for the Non-3GPP access.

In the case of Untrusted Non-3GPP access to 5GC, the Non-3GPP Access Connection corresponds to an NWu connection.

In the case of Trusted access to 5GC, the Non-3GPP Access Connection corresponds to an NWt connection.

In the case of N5CW devices access 5GC via trusted WLAN access networks, the Non-3GPP Access Connection corresponds to an Yt’ connection.

In the case of Wireline access to 5GC, the Non-3GPP Access Connection corresponds to a Y4 connection and to Y5 connection.

A UE does not establish multiple simultaneous Non-3GPP Access Connection to the 5GC.

The Non-3GPP Access Connection is released either as a result of an Explicit Deregistration procedure or an AN Release procedure.

In the case of Untrusted Non-3GPP access, Trusted Non-3GPP access and W-5GAN access to 5GC, the N3IWF, TNGF, TWIF and W-AGF may in addition explicitly release the NWu, NWt, Yt’, Y4 and Y5 signalling connection due to NWu, NWt, Yt’, Y4 and Y5 connection failure, respectively. In the case of NWu and NWt, the release may be determined by the "dead peer detection" mechanism in IKEv2 defined in RFC 7296 [60]. In the case of Y4 and Y5 the release may be detected for example by lost of synchronisation of physical link, lost of PPPoE session, etc. Further details on how NWu, NWt, Yt’, Y4 and Y5 connection failure is detected is out of scope of 3GPP specifications.

For W-5GCAN, the W-AGF explicitly releases the N2 connection due to Y4 or Y5 connection failure, as determined by the "dead peer detection" mechanism in DOCSIS MULPI [89].

The release of the Non-3GPP Access Connection between the UE and the N3IWF, TNGF, TWIF or W-AGF shall be interpreted as follows:

– By the N3IWF, TNGF, TWIF and W-AGF as a criterion to release the N2 connection.

– By the UE as a criterion for the UE to transition to CM-IDLE. A UE registered over non-3GPP access remains in RM-REGISTERED state, unless the Non-3GPP Access Connection release occurs as part of a Deregistration procedure over non-3GPP access in which case the UE enters the RM-DEREGISTERED state. When the UE in RM-REGISTERED transitions to CM-IDLE, the UE non-3GPP Deregistration timer starts running in the UE. The UE non-3GPP Deregistration timer stops when the UE moves to CM-CONNECTED state or to the RM-DEREGISTERED state.

NOTE 1: When moved to CM-IDLE state over one access, the UE can attempt to re-activate UP connections for the PDU Sessions over other access, per UE policies and depending on the availability of these accesses.

NOTE 2: The release of the NWu, NWt, Yt’, Y4 or Y5 at the UE can occur as a result of explicit signalling from the N3IWF, TNGF, TWIF or W-AGF respectively, e.g. IKE INFORMATION EXCHANGE in the case of NWu or as a result of the UE detecting NWu, NWt, Yt’, Y4 or Y5 connection failure, e.g. as determined by the "dead peer detection" mechanism in IKEv2 as defined in RFC 7296 [60] for NWu, NWt and Yt’ or W-5GAN access specific mechanism for Y4 and Y5. Further details on how the UE detects NWu, NWt, Yt’, Y4 or Y5 connection failure is out of scope of 3GPP specifications.

In the case of Non-3GPP access, when the AMF releases the N2 interface, the N3IWF, TNGF, TWIF and W-AGF shall release all the resources associated with the UE including the Non-3GPP Access Connection with the UE and its corresponding N3 resources. A release of the N2 connection by the AMF shall set the CM state for the UE in the AMF to CM-IDLE.

NOTE 3: It is assumed that a UE configured to receive services from a 5GC over non-3GPP access that is RM-DEREGISTERED or CM-IDLE over the non-3GPP access will attempt to establish Non-3GPP Access Connection and transition to CM-CONNECTED state whenever the UE successfully connects to a non-3GPP access unless prohibited by the network to make a N3GPP Access Connection (e.g. due to network congestion).

An UE cannot be paged on Non-3GPP access network.

When a UE registered simultaneously over a 3GPP access and a non-3GPP access moves all the PDU Sessions to one of the accesses, whether the UE initiates a Deregistration procedure in the access that has no PDU Sessions is up to the UE implementation.

Release of PDU Sessions over the non-3GPP access does not imply the release of N2 connection.

When the UE has PDU Sessions routed over the non-3GPP access and the UE state becomes CM-IDLE for the non-3GPP access, these PDU Sessions are not released to enable the UE to move the PDU Sessions over the 3GPP access based on UE policies. The core network maintains the PDU Sessions but deactivates the N3 user plane connection for such PDU Sessions.

5.5.3 UE Reachability

5.5.3.1 UE reachability in CM-IDLE

This clause applies to Non-3GPP access network corresponding to the Untrusted Non-3GPP access network, to the Trusted Non-3GPP access network and to the W-5GAN. The UE mentioned in this clause corresponds to 5G-RG, in the case of W-5GAN or to W-AGF in the case of support of FN-RG. In the case of N5CW devices access 5GC via trusted WLAN access networks, the UE mentioned in this clause corresponds to TWIF.

An UE cannot be paged over Non-3GPP access network.

If the UE states in the AMF are CM-IDLE and RM-REGISTERED for the non-3GPP access, there may be PDU Sessions that were last routed over the non-3GPP access and without user plane resources. If the AMF receives a message with a Non-3GPP Access Type indication from an SMF for a PDU Session corresponding to a UE that is CM-IDLE for non-3GPP access, and the UE is registered over 3GPP access in the same PLMN as the one registered over non-3GPP access, a Network Triggered Service Request may be performed over the 3GPP access independently of whether the UE is CM-IDLE or CM-CONNECTED over the 3GPP access. In this case, the AMF provides an indication that the procedure is related to non-3GPP access, as specified in clause 5.6.8.

NOTE: The UE behaviour upon such network triggered Service Request is specified in clause 5.6.8.

5.5.3.2 UE reachability in CM-CONNECTED

This clause applies to Non-3GPP access network corresponding to the Untrusted Non-3GPP access network, to the Trusted Non-3GPP access network and to the W-5GAN. In the case of W-5GAN the UE mentioned in this clause corresponds to 5G-RG and to W-AGF in the case of support of FN-RG. In the case of N5CW devices access 5GC via trusted WLAN access networks, the UE mentioned in this clause corresponds to TWIF.

For a UE in CM-CONNECTED state:

– the AMF knows the UE location on a N3IWF, TNGF, TWIF and W-AGF node granularity.

– the N3IWF, TNGF, TWIF and W-AGF releases the N2 connection when UE becomes unreachable from N3IWF, TNGF, TWIF and W-AGF point of view, i.e. upon Non-3GPP Access Connection release.