Recently, a few wireless operators, in cooperation with WLAN providers and NEMs, have concluded trials to successfully demonstrate seamless roaming. 3GPP also released a series of specifications on WLAN and PLMN interworking in late 2004. Figure 13-3 illustrates WLAN networks and interworking with the PLMN in a simplified form.
WLAN UE
WLAN UE is user equipment such as a laptop computer or PDA equipped with a SIM/USIM card or UICC (universal integrated circuit card). The UICC is a physically secured device that can be inserted and removed from the terminal equipment. It contains one or more applications, one of which is always USIM. WLAN UE may support WLAN-only access or WLAN and 3GPP radio access. The functions of a WLAN-only UE include:
■ Associating with an interworking WLAN (I-WLAN). An I-WLAN is a WLAN that interworks with a 3GPP System.
■ WLAN access authentication using the extensible authentication protocol (EAP).
Figure 13-3 WLAN-PLMN interworking.
■ Constructing a network access identifier (NAI). The NAI is used to identify the WLAN UE to the AAA server in EAP messages. NAI is derived from the IMSI.
■ Obtaining a local IP address.
In addition to above functions, the following functions are required to support a WLAN 3GPP IP access-enabled WLAN UE.
■ Constructing a WLAN access point name (W-APN) to identify the external IP network.
■ Initiating procedures to resolve W-APN to a packet data gateway (PDG) and establishing a secure tunnel to PDG.
■ Obtaining a remote IP address.
In case of roaming, WLAN UE also performs VPLMN selection.
AAA server
The AAA server supports authentication, authorization, and accounting (AAA) for a PLMN subscriber using WLAN access provided by a partner WLAN operator. The AAA server resides in the 3GPP-compliant PLMN, as shown in Figure 13-3. The AAA server communicates with the HLR/HSS to retrieve necessary authentication and subscription information. It communicates with the WLAN access network (WLAN AN) to authorize or deny a PLMN subscriber access to WLAN. Subsequent changes in user profiles (if any) are also communicated to the WLAN-AN.
In cases where the user wishes to use packet switched data services by the HPLMN or the VPLMN, using WLAN access, the AAA server, in addition to its main functions communicates with the packet data gateway (PDG) to pass the service authorization information.
AAA proxy functions may collocate with an AAA server in the same node. AAA proxy is responsible for relaying AAA information between WLAN and the AAA server, filtering, enforcing policies based on the roaming agreement between the WLAN and PLMN service providers, and reporting charging and accounting information for each user to the VPLMN offline charging system.
Packet data gateway
The packet data gateway resides in the user’s HPLMN or the selected VPLMN. The PDG is applied only if a user is a PLMN subscriber and wishes to use the packet switched services offered by the PLMN or access Internet/intranet via PLMN. For a roamer, W-APN and the subscription determine whether the services will be provided by the HPLMN or the VPLMN. The PDG contains the routing information for WLAN users accessing PLMN services and is responsible for data transfer between the PDN and the WLAN user. The PDG performs data screening based on defined rules to filter out unsolicited and unauthorized traffic. It is also responsible for generating charging information for the usage.
