Interworking WLAN

Description

Interworking WLAN (I-WLAN) is a concept introduced in 3GPP Release 6 that defines how Wireless Local Area Networks (WLANs) can interwork with 3GPP systems, such as UMTS and later LTE. The primary goal is to allow mobile devices to access 3GPP services (e.g., IP-based services, IMS) via WLAN access networks, while maintaining authentication, authorization, and accounting (AAA) through the 3GPP core network. This interworking is specified in 3GPP TS 22.234 (requirements) and TS 29.161 (interworking between WLAN and 3GPP networks).

The architecture for I-WLAN involves several key components: the WLAN Access Network (WLAN AN), which includes WLAN access points and possibly a WLAN Access Gateway (WAG); the Packet Data Gateway (PDG), which acts as a gateway between the WLAN and the 3GPP packet core; and the 3GPP AAA server, which authenticates and authorizes the user. The WLAN UE (User Equipment) connects to the WLAN AN, which then tunnels traffic to the PDG over the Wu interface. The PDG then connects to the 3GPP core network (e.g., GGSN in UMTS) via the Gi interface. This allows the UE to access external packet data networks (PDNs) and operator services.

Interworking can be categorized into two modes: direct IP access and 3GPP IP access. In direct IP access, the UE obtains an IP address from the WLAN and accesses the Internet directly, without traversing the 3GPP core. In 3GPP IP access, the UE establishes a tunnel to the PDG, and all traffic goes through the 3GPP core, enabling access to operator services and consistent policy enforcement. The latter mode requires the UE to support specific procedures for tunnel establishment, such as IKEv2 for IPsec tunnel setup.

Authentication and security are critical aspects. The UE authenticates to the 3GPP AAA server using EAP-AKA or EAP-SIM, leveraging the SIM/USIM credentials. This ensures that only authorized users can access the 3GPP services via WLAN. The AAA server also provides the PDG with the necessary information to authorize the tunnel establishment. Security is further ensured by IPsec encryption between the UE and the PDG.

I-WLAN was a significant step towards integrating heterogeneous access technologies, paving the way for more advanced interworking in later releases, such as the Evolved Packet Core (EPC) and the introduction of untrusted non-3GPP access in Release 8. It allowed operators to offload traffic from cellular networks to WLAN, improving capacity and coverage, especially in indoor environments. Although the term "Interworking WLAN" is no longer actively used in later releases (it was replaced by more specific terms like "WLAN interworking" or "non-3GPP access"), the foundational concepts remain relevant in today's 5G networks, where WLAN integration is achieved via Access Traffic Steering, Switching, and Splitting (ATSSS).

Purpose & Motivation

The primary purpose of Interworking WLAN is to enable mobile operators to extend their service offerings over WLAN access networks, thereby improving coverage and capacity without deploying additional cellular infrastructure. Before I-WLAN, WLAN and cellular networks operated independently, requiring users to manage separate subscriptions and authentication mechanisms. This fragmentation limited the ability to provide seamless services across both access types. I-WLAN solves this by defining a standardized architecture that allows a single subscription to be used for both cellular and WLAN access, with unified authentication and policy control.

Another motivation was to leverage the high bandwidth and low cost of WLAN for data offloading, especially in hotspots with high traffic demand. By integrating WLAN with the 3GPP core, operators can offload data traffic from their cellular networks, reducing congestion and improving user experience. This is particularly important as mobile data usage grows exponentially. I-WLAN also enables access to operator-specific services (e.g., IMS, enterprise applications) over WLAN, ensuring a consistent service experience regardless of the access technology.

Historically, I-WLAN was introduced in Release 6 as part of the broader effort to support multiple access technologies in 3GPP. It built upon earlier work on WLAN interworking in Release 5, but Release 6 provided a more complete architecture with the PDG and AAA server. Subsequent releases refined the interworking, leading to the evolved packet core (EPC) in Release 8, which introduced more flexible and scalable non-3GPP access support. The concepts from I-WLAN are still visible in today's 5G standards, where WLAN is integrated as a non-3GPP access under the 5G core network.