Two scenarios are supported. In one scenario, a single point-to-point connectivity between the UE and the TWAG is used to transport traffic for the default and all S2a bearer(s). This is described in clause 188.8.131.52. In the second scenario, a separate WLCP bearer is established for the default S2a bearer, and for each dedicated S2a bearer established on the S2a interface. This scenario applies when both the UE and the TWAN supports the establishment of multiple WLCP bearers. This is described in clause 184.108.40.206
For Trusted WLAN access to the EPC, the PDN connectivity service is provided by the point-to-point connectivity between the UE and the TWAG concatenated with S2a bearer(s) between the TWAG and the PDN-GW.
The bearer model of GTP based S2a interface is similar to that of GTP based S5/S8 interface and GTP based S2b interface. The TWAN handles the uplink packets based on the uplink packet filters in the TFTs received from the PDN-GW for the S2a bearers of the PDN connection, in the same way as an ePDG does for GTP based S2b interface.
This scenario applies when the UE and the TWAG support multiple WLCP bearers,
In this scenario, one default S2a bearer is established on the S2a interface when the UE connects to a PDN, and the default S2a bearer remains established throughout the lifetime of the PDN connection to provide the UE with always-on IP connectivity to that PDN. Additional dedicated S2a bearers may be established on S2a for the same PDN connection depending on operator policy. The PGW establishes dedicated S2a bearers on S2a for the same PDN connection based on PCC decisions as specified in TS 23.203.
The TWAG releases the WLCP PDN connection, when the default S2a bearer of the associated PDN connection is released.
The S2a bearer is realized with the following elements:
A GTP tunnel on S2a transports the packets of an S2a bearer between the TWAG and a PDN-GW;
The TWAG stores the mapping between WLCP bearer and the corresponding S2a bearer;
The PDN-GW stores the mapping between downlink packet filters and an S2a bearer.
The TWAG shall establish a WLCP bearer per dedicated bearer as per clause 16.5. The default S2a bearer maps to the initial WLCP bearer. The TWAG shall maintain a 1 to 1 mapping between an S2a bearer and a WLCP bearer.
Additionally, for these UEs, TFTs and dedicated bearer QoS information is conveyed from the TWAG to the UE in WLCP signalling associated with the corresponding WLCP bearer at S2a bearer creation and S2a bearer modification. The bearer QoS information includes information regarding the QoS characteristics of the bearer (i.e. QCI, GBR and MBR).
The TWAG routes uplink packets to the different bearers based on the incoming WLCP bearer and the corresponding S2a bearer. The TWAG may set a DiffServ Code Point for uplink packets based on the receipt of the MPS subscription notification from the UE, as per clause 7.2.4, or based on QCI, and optionally the ARP priority level.
The PDN-GW routes downlink packets to the different bearers based on the downlink packet filters in the TFTs assigned to each of the S2a bearers in the PDN connection, in the same way as the PDN-GW does on GTP-based S5/S8 bearers (see clause 220.127.116.11 of TS 23.401).
The UE routes uplink packets to the WLCP bearer based on the uplink packet filters in the TFTs assigned to the bearers in the PDN connection, in the same way as a UE does for uplink traffic under 3GPP access. If no match is found, the uplink data packet shall be sent via the bearer that does not have any uplink packet filter assigned. If all bearers (including the default bearer for that PDN) have been assigned an uplink packet filter, the UE shall discard the uplink data packet.
To support QoS differentiation,
for downlink packets, if the TWAG sets the DSCP, the TWAG may use the QCI, and optionally the ARP, in WLCP bearer level QoS information to derive the DSCP value for downlink packets.
for uplink packets, the UE shall either use the DSCP value that it received from the TWAG, or the QCI in dedicated bearer QoS information to set the DSCP value for uplink packets. The mapping of QoS class to DSCP could be configurable in the UE and as an example can use the recommended mappings specified in 3GPP (mapping between standardized QCIs and Release 99 QoS parameter value in TS 23.401) and IEEE Std. 802.11-2012 . The UE may also use information included in dedicated QoS information for local aspects of admission control (e.g. application traffic shaping) however this is out of scope for this document.
In order for an Application Function (e.g. the P-CSCF) to be able to determine the NPLI (Network Provided location Information) of an UE in case of a TWAN access, the TWAN shall report over S2a TWAN related Access Network Information at PDN connection establishment, at bearer creation / modification / release and at PDN connection release. Such TWAN related Access Network Information may correspond to a "TWAN Identifier" and/or to a UE Time Zone.
The TWAN Identifier (reported over S2a, Gx, Gy, ...) shall include the SSID of the access point to which the UE is attached and shall include at least one of the following elements, unless otherwise determined by the TWAN operator's policies:
the BSSID (see IEEE Std 802.11-2012 );
civic address information of the AP to which the UE is attached;
line identifier (Logical Access ID see ETSI ES 282 004 ) of the access point to which the UE is attached.
The TWAN Id may also contain the identifier of the operator of the TWAN. When the TWAN is operated by a mobile operator, this corresponds to a PLMN-ID. When the TWAN is not operated by a mobile operator, this corresponds to an operator Name (e.g. in Realm format).