16.1.6 Bearer model for PDN connectivity service with GTP based S2a

23.4023GPPArchitecture enhancements for non-3GPP accessesRelease 18TS

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 16.1.6.1. 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 16.1.6.2

16.1.6.1 Single point-to-point connection UE-TWAN

Figure 16.1.6.1-1: Two Unicast S2a bearers mapped in a single Point-to-Point connection (GTP based S2a)

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.

NOTE: The WLCP bearer between the UE and the TWAG carries all S2a bearers traffic of the PDN connection. The TWAG routes uplink packets to the different bearers 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 TWAG shall discard the uplink data packet.

16.1.6.2 Multiple point-to-point connection UE-TWAN

Figure 16.1.6.2-1: Two Unicast S2a bearers mapped to separate WLCP bearers (GTP based S2a)

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 [19].

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 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 TS 23.401 [4] clause 4.7.2.2).

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.

Furthermore, the UE shall use 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 [4]) and IEEE Std. 802.11-2012 [64]. 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.