Interworking Wireless Local Area Network

Description

Interworking Wireless Local Area Network (I-WLAN) is a comprehensive 3GPP framework, initially introduced in Release 6, that defines how a User Equipment (UE) can access 3GPP network services through a WLAN access network. It is not merely a Wi-Fi offload technology but a full interworking system that integrates WLAN into the 3GPP core network ecosystem. The architecture involves several key network elements: the WLAN Access Network (WLAN AN), the WLAN Access Gateway (WAG), the Packet Data Gateway (PDG), and the 3GPP AAA (Authentication, Authorization, and Accounting) Server.

The core of I-WLAN operation is based on establishing an IPsec tunnel between the UE and the PDG located in the 3GPP home network. This tunnel, established over the WLAN access and any intervening IP networks, provides a secure connection for the UE to access 3GPP packet-switched services and the Internet. The process begins with WLAN access authentication, which can be based on EAP-SIM/AKA methods, leveraging the UE's SIM/USIM credentials and involving the 3GPP AAA Server. Following successful authentication, the UE initiates a tunnel establishment procedure with a PDG, resulting in a secure IPsec tunnel. All user plane traffic is then routed through this tunnel to the PDG, which acts as the gateway to external Packet Data Networks (PDNs), including the Internet and 3GPP operator services like IMS.

I-WLAN supports two primary connectivity models: 'Direct IP Access' and '3GPP IP Access'. Direct IP Access allows the UE to reach the Internet directly via the WLAN, with the 3GPP network primarily involved in access authentication and charging. 3GPP IP Access is the more integrated model where the UE's traffic is tunneled to the PDG in the home network, allowing it to access 3GPP services (like IMS) and be assigned an IP address from the home operator's pool. The framework also includes mechanisms for charging, policy control, and mobility, although the initial focus was on nomadic or static access rather than seamless handover. I-WLAN laid the essential groundwork for the later, more seamless integration of non-3GPP access defined for EPS and 5GS.

Purpose & Motivation

I-WLAN was developed in the mid-2000s (Release 6) to address the growing proliferation of WLAN (Wi-Fi) hotspots and the desire of mobile operators to integrate this unlicensed spectrum technology into their service offerings. Prior to I-WLAN, using Wi-Fi for cellular data was largely an unmanaged 'offload' with no integration with the operator's core network, leading to a disjointed user experience, separate subscriptions, and no unified security or charging. Operators sought to leverage their existing subscriber identity (SIM), authentication infrastructure, and billing systems to offer integrated WLAN services.

The I-WLAN standards solved this by providing a secure, operator-controlled method for subscribers to access both the Internet and 3GPP services via WLAN. It allowed operators to offer a unified service bundle, use SIM-based authentication for WLAN access, apply consistent charging policies, and maintain a secure connection back to the home network. While I-WLAN did not initially support seamless mobility (handover) between 3GPP and WLAN, it established the critical architectural principles—like the use of EAP-AKA for authentication, tunneling to a home network gateway (PDG), and the central role of the AAA server—that were later evolved and refined. It addressed the key problem of how to treat untrusted, non-3GPP IP access networks as an extension of the trusted mobile core, a concept that became fundamental to the Evolved Packet Core (EPC) with Trusted/Untrusted Non-3GPP Access and later to the 5G Core.