16.1.2 High level functions

23.4023GPPArchitecture enhancements for non-3GPP accessesRelease 18TS

The detailed functional split within a Trusted WLAN Access Network (TWAN) is not in the scope of 3GPP. Nevertheless, the procedures specified in the subsequent sections assume the following functions in the TWAN:

– A WLAN Access Network (WLAN AN). WLAN AN includes a collection of one or more WLAN access points. An access point terminates the UE’s WLAN IEEE 802.11 link defined in IEEE Std 802.11-2012 [64].

– A Trusted WLAN Access Gateway (TWAG). This function terminates S2a.

– When the TWAN provides access to EPC in Transparent Single-Connection mode or in Single-Connection mode, it forwards packets between the UE-TWAG point-to-point link and the S2a tunnel for that UE. The association in the TWAN between UE-TWAG point-to-point link and S2a tunnel is based on the UE MAC address.

– When the TWAN provides access to EPC in Multi-Connection mode, it forwards user plane packets between the UE-TWAG point-to-point link corresponding to a specific WLCP bearerand the associated S2a tunnel for that UE. The UE’s MAC address and a TWAG’s MAC address that is assigned for a specific WLCP bearer are used to identify the point-to-point link between the UE and its serving TWAG, which corresponds to the S2a tunnel for the associated WLCP bearer.

– When the TWAN provides access to EPC in Multi-Connection mode, the WLCP signalling is used between the UE and the TWAG.

– A Trusted WLAN AAA Proxy (TWAP). This function terminates STa. It relays the AAA information between the WLAN Access Network and the 3GPP AAA Server or Proxy in case of roaming. It establishes the binding of UE subscription data (including IMSI) with UE MAC address on the WLAN Access Network. If L2 attach triggers are used, it informs the TWAG of L2 attach events. It is aware of UE L2 Detach from the WLAN Access Network and informs the TWAG of L2 Detach events. It provides the TWAG with UE subscription data during initial attach or at UE subscription data modification.

Figure 16.1.2-1: Trusted WLAN Access Network functional split

A per-UE point-to-point link between the UE and the TWAG is required when traffic for that UE is routed via S2a. Additionally, in Multi-Connection mode, one point-to-point link between an UE and its serving TWAG is required for transporting user plane traffic for every PDN connection or S2a bearer. The UE’s MAC address and an associated TWAG’s MAC address are used to identify the point-to-point link between the UE and its serving TWAG that is associated to a specific PDN connection or S2a bearer. In particular it is assumed that the WLAN AN enforces upstream and downstream forced-forwarding between the UE’s WLAN IEEE 802.11 association and the TWAG. The aspects of point-to-point link described in RFC 5213 [8] and RFC 5844 [17] also apply to the point-to-point link between UE and TWAG. The implementation of the point-to-point link, including how and when it is setup, is out-of-scope of 3GPP.

NOTE 1: In TSCM from the UE’s perspective the SWw reference point appears as a shared medium / link as any other IEEE 802.11 WLAN and thus the UE can use the subnet prefix / mask and the default GW address for its packet routing decisions. The point-to-point nature of the link is realized by the TWAN enforcing that packets sent from, and received by the UE are respectively forwarded to, and forwarded by the TWAG.

In SCM and MCM from the UE’s perspective an EPC routed PDN connection over the SWw reference point appears as a point-to-point link similar to how it is in 3GPP access. Shared link parameters such as netmask and default router IP address are not used in these modes.

NOTE 2: Gxa interface is not used for S2a-PMIP in Trusted WLAN within this Release of the specification. No policy interworking solution based on S9a is defined for Fixed Broadband access interworking via S2a within this Release of the specification.

NOTE 3: Whether multiple TWAN functions are mapped to a single entity, or a single TWAN function is distributed among multiple entities is out-of-scope of 3GPP.

In order to support EPC access through S2a over Trusted WLAN the following functions shall be supported by the UE:

– WLAN specifications as per IEEE Std. 802.11-2012 [64].

– 3GPP-based network access authentication with EPC over WLAN as defined in clause 4.9.1, using IEEE Std 802.1X-2004 [65].

– IPv4 and/or IPv6 support:

– For IPv4: IETF RFC 791 [66], IETF RFC 2131 [28]

– For IPv6: IETF RFC 2460 [67], IETF RFC 4861 [38], and IETF RFC 4862 [58]

Three different modes of operation are distinguished: Transparent Single-Connection mode, Single-Connection mode and Multi-Connection mode. The UE and the network negotiate the mode of operation as part of the authentication procedure based on extensions to the EAP-AKA’, (IETF RFC 5448 [72]) signaling between the UE and the network.

The Single-Connection mode only supports NSWO or a single PDN connection at a given time over a Trusted WLAN. On the other hand, the Multi-Connection mode supports simultaneous one or more PDN connections and/or NSWO over Trusted WLAN. Multi-Connection mode for GTP based S2a supports multiple WLCP bearers between UE and TWAG. Both Single-Connection mode and Multi-Connection modes support IP address preservation between 3GPP and Trusted WLAN access and PDN connectivity to a non-default APN.

The Single-Connection mode does not require additional protocols than EAP-AKA’ in order to establish NSWO or PDN connectivity.

The multi connection mode uses a specific protocol (WLCP, specified in clause 16.1.4A3.1) after the access authentication procedure to trigger PDN connection establishment / release.

The negotiation of connection mode is further detailed in clause 16.4.A.1.

In Transparent Single-Connection mode, handover-indicator from the UE, APN indication from the UE and PCO via WLAN are not supported. As a consequence the following features are not supported: handover between TWAN and 3GPP access with IP address preservation; connectivity to a non-default APN; UE initiated connectivity to additional PDN.

When the TWAN supports emergency services, it shall be configured with emergency configuration data as defined in clause 4.5.7.2.1. The TWAN notifies the UE whether it supports emergency services by sending a related indication to the 3GPP AAA server, which relays this information in EAP signalling sent to the UE.