The 5G Core Network supports connectivity of UEs via non-3GPP access networks, e.g. WLAN access networks.
Only the support of non-3GPP access networks deployed outside the NG-RAN is described in this clause.
The 5G Core Network supports both untrusted non-3GPP access networks and trusted non-3GPP access networks (TNANs).
An untrusted non-3GPP access network shall be connected to the 5G Core Network via a Non-3GPP InterWorking Function (N3IWF), whereas a trusted non-3GPP access network shall be connected to the 5G Core Network via a Trusted Non-3GPP Gateway Function (TNGF). Both the N3IWF and the TNGF interface with the 5G Core Network CP and UP functions via the N2 and N3 interfaces, respectively.
A non-3GPP access network may advertise the PLMNs for which it supports trusted connectivity and the type of supported trusted connectivity (e.g. "5G connectivity"). Therefore, the UEs can discover the non-3GPP access networks that can provide trusted connectivity to one or more PLMNs. This is further specified in clause 6.3.12
(Trusted Non-3GPP Access Network selection).
The UE decides to use trusted or untrusted non-3GPP access for connecting to a 5G PLMN by using procedures not specified in this document. Examples of such procedures are defined in clause 18.104.22.168
When the UE decides to use untrusted non-3GPP access to connect to a 5G Core Network in a PLMN:
the UE first selects and connects with a non-3GPP access network; and then
the UE selects a PLMN and an N3IWF in this PLMN. The PLMN/N3IWF selection and the non-3GPP access network selection are independent. The N3IWF selection is defined in clause 6.3.6.
When the UE decides to use trusted non-3GPP access to connect to a 5G Core Network in a PLMN:
the UE first selects a PLMN; and then
the UE selects a non-3GPP access network (a TNAN) that supports trusted connectivity to the selected PLMN.
In this case, the non-3GPP access network selection is affected by the PLMN selection.
A UE that accesses the 5G Core Network over a non-3GPP access shall, after UE registration, support NAS signalling with 5G Core Network control-plane functions using the N1 reference point.
When a UE is connected via a NG-RAN and via a non-3GPP access, multiple N1 instances shall exist for the UE i.e. there shall be one N1 instance over NG-RAN and one N1 instance over non-3GPP access.
A UE simultaneously connected to the same 5G Core Network of a PLMN over a 3GPP access and a non-3GPP access shall be served by a single AMF in this 5G Core Network.
When a UE is connected to a 3GPP access of a PLMN, if the UE selects a N3IWF and the N3IWF is located in a PLMN different from the PLMN of the 3GPP access, e.g. in a different VPLMN or in the HPLMN, the UE is served separately by the two PLMNs. The UE is registered with two separate AMFs. PDU Sessions over the 3GPP access are served by V-SMFs different from the V-SMF serving the PDU Sessions over the non-3GPP access. The same can be true when the UE uses trusted non-3GPP access, i.e. the UE may select one PLMN for 3GPP access and a different PLMN for trusted non-3GPP access.
The PLMN selection for the 3GPP access does not depend on the PLMN that is used for non-3GPP access. In other words, if a UE is registered with a PLMN over a non-3GPP access, the UE performs PLMN selection for the 3GPP access independently of this PLMN.
A UE shall establish an IPsec tunnel with the N3IWF or with the TNGF in order to register with the 5G Core Network over non-3GPP access. Further details about the UE registration to 5G Core Network over untrusted non-3GPP access and over trusted non-3GPP access are described in clause 4.12.2
and in clause 4.12a.2 in TS 23.502
It shall be possible to maintain the UE NAS signalling connection with the AMF over the non-3GPP access after all the PDU Sessions for the UE over that access have been released or handed over to 3GPP access.
N1 NAS signalling over non-3GPP accesses shall be protected with the same security mechanism applied for N1 over a 3GPP access.
User plane QoS differentiation between UE and N3IWF is supported as described in clause 5.7
and TS 23.502, clause 4.12.5
. QoS differentiation between UE and TNGF is supported as described in clause 5.7
and TS 23.502, clause 4.12a.5