Interworking between WLANs and 3G - Part 3: Loose coupling

The IP network selection is based on a parameter called WLAN access point name (W-APN) similar to the APN parameter used in GPRS. The UE indicates the desired IP network with W-APN. The network authorizes the request, or verifies that the user has the right to use the W-APN.

After selecting the IP network, appropriate tunnels are established to route the user data to the selected IP network. The tunnel is terminated in the home operator packet data gateway (PDG). The PDG is similar to the GGSN used in GPRS. The W_i reference point between the PDG and the remote network is similar to the G_i reference point used between GGSN and the remote IP networks in GPRS. In the visited 3GPP network, the WLAN access gateway (WAG) is required to implement tunneling. The reference points W_n, W_p, W_u, and W_i are used to convey the user data plane, and W_g and W_m are used for control (see Figure 22.16).

22.8.3 3GPP-based Charging for WLAN
The WLAN charging architecture is shown in Figure 22.17. Charging information about WLAN is collected at the WLAN access network and forwarded to the 3GPP visited and home networks. The AAA server in the home 3GPP network authorizes each user's access to a WLAN.

Before authorizing a prepaid user to access the WLAN for direct Internet access, the 3GPP AAA server makes a credit reservation from the user's prepaid amount in the OCS (online charging system) over the W_o reference point. The 3GPP AAA server monitors the received accounting information from the WLAN access network. When the downloaded credit is exhausted a new credit request from OCS is triggered to cover the forthcoming accounting reports from the WLAN access network. At the termination of the WLAN connection, the 3GPP AAA server returns any unused credit back to the OCS.

After authorization to access the WLAN access network is completed, a user-specific accounting session is established between the WLAN access network and the 3GPP home network. The accounting session is established with standard AAA accounting signaling, and the reference point for this signaling is W_b.

At the establishment of the accounting session the 3GPP AAA server indicates to the WLAN a suitable set of accounting criteria, such as an accounting unit (e.g., amount of transferred kilobytes) and reporting threshold to be utilized. After the accounting session establishment the WLAN collects accounting information and reports it to the 3GPP AAA server over the W_b reference point.

All associated IP flows traverse through the PDG; thus, more accurate and service-specific charging information can be collected at the PDG. The resource consumption by each IP flow can be monitored and collected internally at the PDG. For charging of the traversing IP flows, the PDG is also connected to the OCS by the G_y reference point and to the CG (charging gateway) by the G_z reference point. At the establishment of a certain IP flow via the PDG, the PDG requests credit for IP flow charging from the OCS over the G_y reference point in a similar way as the 3GPP AAA server does over the W_o reference point for WLAN access charging.

22.8.4 Session Mobility
In the loose coupling, mobile IP is used to provide session mobility across GPRS and WLAN. This is in contrast to the tight coupling approach in which the GPRS mobility management procedure is used. When the UE moves from GPRS to WLAN, it performs a mobile internet protocol (MIP) registration via the FA that resides in the WLAN. The FA completes the registration with the HA to be used as a forwarding address for the packet destined to the UE. The FA then associates the CoA with that of the UE and acts as a proxy on behalf of the UE for the life of the registration. This way, the UE retains its IP address when it moves from the WLAN to GPRS.

References:
3. Blunk, L., and Vollbrecht, J. PPP Extensible Authentication Protocol (EAP). IETF RFC 2284, March 1998.

4. Haverinen, H., and Salowey, J. Eds. EAP SIM Authentication. IETF draft-haverinenpppext-eap-sim-10.txt, February 2003.

Printed with permission from Morgan Kaufmann, a division of Elsevier. Copyright 2007. "Wireless Communications & Networking" Vijay Garg. For more information about this title and other similar books, please visit www.elsevierdirect.com.

Related links:
Interworking between WLANs and 3G - Part 1: Introduction, interworking objectives & approaches | Part 2: Tight coupling approach
An introduction to networking - Part III: Network and Transport layers