Initial registration to 5GC.
The N5CW device associates with the selected trusted WLAN and the EAP authentication procedure is initiated.
The N5CW device provides its Network Access Identity (NAI). The Trusted WLAN Access Point (TWAP) selects a Trusted WLAN Interworking Function (TWIF), e.g. based on the received realm and sends an AAA request to the selected TWIF.
If the N5CW device has not registered over 3GPP access to 5GC of the selected PLMN when the above procedure is initiated, then the NAI includes the SUCI as specified in clause 28.7.7 of TS 23.003
. For example, NAIemail@example.com.
If the N5CW device has registered to 5GC over 3GPP access to 5GC of the selected PLMN (i.e. it is also a 5G UE) when the above procedure is initiated, then the NAI includes the 5G-GUTI assigned to N5CW device over 3GPP access. This enables the TWIF in step 4a below to select the same AMF as the one serving the N5CW device over 3GPP access.
The NAI provided by the N5CW device in step 2b indicates that the N5CW device wants "5G connectivity-without-NAS"
towards a specific PLMN (i.e. the PLMN selected in step 0). For example, with NAI=<5G-GUTI>@nai.5gc-nn.mnc123.mcc45.3gppnetwork.org, the N5CW device indicates that it wants "5G connectivity-without-NAS"
(5gc-nn) to the PLMN with MCC=45 and MNC=123.
The TWIF creates a 5GC Registration Request message on behalf of the N5CW device. The TWIF uses default values to populate the parameters in the Registration Request message, which are the same for all N5CW devices. The Registration type indicates "Initial Registration".
The TWIF selects an AMF (e.g. by using the 5G-GUTI in the NAI, if provided by the N5CW device) and sends an N2 message to the AMF including the Registration Request, the User Location and an AN Type.
The AMF triggers an authentication procedure by sending a request to AUSF indicating the AN type.
An EAP authentication procedure takes place between the N5CW device and AUSF. Over the N2 interface, the EAP messages are encapsulated within NAS Authentication messages. The type of EAP authentication procedure is specified in TS 33.501
After a successful authentication, the AUSF sends to AMF the EAP-Success message and the created SEAF key. The AMF derives an AN key from the received SEAF key.
The NAS Security Mode Command (SMC) is sent from the AMF to the TWIF. The selected NAS security algorithms of integrity protection and ciphering are set to NULL.
The AMF sends an N2 Initial Context Setup Request and provides the AN key to TWIF. In turn, the TWIF derives a Pairwise Master Key (PMK) from the AN key and sends the PMK key and the EAP-Success message to the Trusted WLAN Access Point, which forwards the EAP-Success to the N5CW device. The PMK is the key used to secure the WLAN air-interface communication according to IEEE Std 802.11 . A layer-2 or layer-3 connection is established between the Trusted WLAN Access Point and the TWIF for transporting all user-plane traffic of the N5CW device to TWIF. This connection is later bound to an N3 connection that is created for this N5CW device.
Finally, the AMF sends a Registration Accept message to TWIF. At this point, the N5CW device is connected to the WLAN Access Network and is registered to 5GC.
PDU Session Establishment.
The TWIF creates a PDU Session Establishment Request message on behalf of the N5CW device and sends this message to AMF. This may be triggered by receiving an IP configuration request (e.g. DHCP Offer/Request) from the N5CW device. The TWIF may use default values to populate the parameters in the PDU Session Establishment Request message, but may also skip some PDU session parameters and let the AMF or the SMF determine these parameters based on the N5CW device subscription information received during the registration procedure. This way, default PDU session parameters can be used per N5CW device.
The value of the PDU Session id provided by TWIF in step 20c shall always be the same. It will be a value reserved for the PDU sessions requested by the TWIF and it will be different from the values that can be used by the N5CW device when requesting a PDU session over 3GPP access. This way, the PDU session id provided by the TWIF cannot be the same with the PDU Session Id of any PDU session established by the N5CW device over 3GPP access.
The AMF sends upon request of the SMF an N2 PDU Session Request message to TWIF in order to reserve the appropriate Access Network resources. This N2 message includes the PDU Session Establishment Accept message. In step 21b, the TWIF may reserve WLAN access resources for the user-plane communication between the N5CW device and TWIF. If and how this resource reservation is performed is outside the scope of 3GPP.
After the establishment of the PDU session, the TWIF assigns IP configuration data to N5CW device (e.g. with DHCP). The IP address assigned to N5CW device is the IP address allocated to the PDU session.
User plane communication.
The TWIF binds the N5CW device-specific L2/L3 connection created in step 9g with the N3 connection created in step 21. All user-plane traffic sent by the N5CW device is forwarded to TWIF via the L2/L3 connection and then to UPF via the N3 connection. The TWIF operates as a Layer-2 relay.
The TWIF may receive URSP rules (see TS 23.503
), which indicate the traffic that should be offloaded locally by TWIF (sent outside the PDU session) and the traffic that should be sent inside the PDU session.
The above procedure supports only one PDU session per N5CW device whose parameters are either configured for all N5CW devices in the TWIF or are derived from default values in the N5CW device subscription.