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

[Part 1 begins with a look at interworking objectives and requirements, as well as interworking schemes to connect WLANs and 3G networks. Part 2 examines the tight coupling interworking approach, where the WLAN is connected to the 3GPP (GPRS) core network in the same way as any other radio access network.]

22.8 System Description with Loose Coupling
Figure 22.11 shows the system architecture with loose coupling. The WLAN is coupled with the GPRS network in the operator's IP network. Note that, in contrast to tight coupling, the WLAN data traffic does not pass through the GPRS core network but goes directly to the operator's IP network.

In this architecture, SIM-based authentication can be supported in both the GPRS network and the WLAN to gain access to the operator's services. The architecture also supports integrated billing, via the billing mediator, in a common billing system. The WLAN may be owned by a third party, with roaming/mobility enabled via a dedicated connection between the operator and the WLAN, or over an existing public network, such as the Internet.

Loose coupling utilizes standard IETF-based protocols for authentication, accounting, and mobility. Roaming can be enabled across all types of WLAN implementations, regardless of who owns the WLAN, solely via roaming agreements.

The WLAN access network connects to the GPRS data network like a different type of radio access network for interworking. This allows for the support of the legacy WLAN access networks, which commonly support RADIUS/ DIAMETER protocols in the WLAN access network. This approach defi nes some new interfaces with well-defined functions and commonly used protocols. The new interfaces are discussed below (see Figure 22.17).

• W_b/W_r interface. This interface connects the WLAN access network with the visited 3GPP data network or the home 3GPP data network. The W_r interface transports authentication, authorization, and other related information. The W_b interface transports charging related information. The W_r interface logically connects the WLAN capable user to the AAA server, which resides in the cellular operator home network. The WLAN capable user is authenticated and authorized by the AAA server. The WLAN related subscription information for the user are stored in home location register/home subscriber server (HLR/HSS). The extensible authentication protocol (EAP) is used for this purpose between the WLAN capable user and the AAA server. To accommodate the existing WLAN access networks, which support RADIUS or DIAMETER, the W_r/W_b interface uses DIAMETER protocol toward the AAA server. The principle of authentication is mutual authentication. Two methods of mutual authentication are currently defined, EAP/SIM and EAP/AKA. The EAP/AKA is used for subscribers with USIM and EAP/SIM is used for subscribers with SIM. The existing SIM subscribers' authentication in cellular networks is not based on mutual authentication. This is the reason that the authentication methods are different for SIM- and USIM-based WLAN users.

• W_n interface. This interface transports tunneled WLAN user data toward the packet data gateway in the home network and vice versa. The W_n interface is used to transport tunneled data between the home packet data gateway in the home network and the visited data border gateway in the visited network if the WLAN access network is not directly connected to the home network. It is also possible that the packet data is directly routed by the WLAN access network to the external IP network. This is the reason that this interface is service specific. If the packet data is routed by the packet data gateway then there are two ways of transporting the user packet data to the packet data gateway. One method is to establish a secure tunnel between the WLAN access network and the packet data gateway. This method is called network based tunneling as the WLAN user is not involved. The other method establishes a direct secure tunnel between the WLAN user client and the packet data gateway. This method is referred to as client based tunneling. The W_r interface is used to inform the WLAN access network about tunneling related information.

• W_x interface. This interface connects the AAA server with HLR/HSS. The AAA server retrieves the authentication vectors over this interface from the HLR/HSS. The AAA server also retrieves the WLAN access-related subscriber information using this interface. This interface is also used by the AAA server to register itself for an authorized WLAN-capable user with the HLR/HSS. This interface also helps the AAA server to get an indication of subscription-related changes from the HLR/HSS. The AAA server generates temporary identifiers for the WLAN user for security. The temporary identifiers are used as far as possible over the WLAN radio access network by the WLAN user. This interface is quite similar to the mobile application part (MAP) G_r interface defined between SGSN and HLR/ HSS. This interface is based on the MAP or DIAMETER protocol.

• W_f interface. This interface connects the AAA server with the 3GPP charging control function or charging gateway function. This interface transports charging data toward the 3GPP charging control/gateway function. The charging data is collected by the AAA server from either the packet data gateway over the W_m interface or the W_b interface from the WLAN access network or both. This interface is based on DIAMETER or GPRS tunneling protocol (GTP).

• W_o interface. This interface connects the AAA server with the 3GPP on-line charging system for credit control checks for the WLAN-capable user. This interface is based on DIAMETER protocol.

• W_m interface. This interface connects the AAA server with the packet gateway for transport of charging, related information and tunneling related information to the AAA server from packet data gateway. This interface is based on DIAMETER protocol.

• W_i interface. This interface connects the packet data gateway with the packet data network. The packet data network may be an external public or private data network or an operator's internal packet data network. The protocol for this interface is dependent upon the packet data network.