Serving Wireless Local Area Network

Description

The Serving WLAN is a concept defined in 3GPP specifications for interworking between Wireless Local Area Networks (WLANs) and 3GPP cellular networks. It refers to the interworking WLAN that the user is currently connected to, which can be either a visited WLAN (when the user is roaming) or a home WLAN (when the user is in their home network's coverage). This concept is part of the broader effort to integrate WLAN access into the 3GPP ecosystem, allowing users to seamlessly use WLAN for accessing mobile services.

The Serving WLAN is defined in specifications such as 22.234 (Requirements on 3GPP system to Wireless Local Area Network (WLAN) interworking) and 29.161 (Interworking between the Public Land Mobile Network (PLMN) supporting packet based services and Packet Data Networks (PDN)). The concept emerged in Release 6 as part of the initial work on WLAN interworking, which aimed to allow 3GPP subscribers to access their home network services via WLAN access. The Serving WLAN can be a WLAN that is operated by the home operator (home WLAN) or by a visited network operator (visited WLAN), and it may be integrated with the 3GPP network to varying degrees.

The architecture for WLAN interworking involves several key components. The WLAN Access Network (WLAN AN) provides the radio access for the user equipment (UE) to connect to the WLAN. The WLAN AN connects to the 3GPP network through an interworking function, such as the Packet Data Gateway (PDG) in early releases or the evolved Packet Data Gateway (ePDG) in later releases. The Serving WLAN is identified by the WLAN Access Point (AP) and the associated network to which the UE is connected. The 3GPP network uses this information to authenticate the user, authorize access, and establish connectivity to the home network services.

When a UE connects to a Serving WLAN, it may perform authentication using 3GPP credentials (e.g., SIM-based authentication) through protocols like EAP-AKA or EAP-SIM. The Serving WLAN then provides IP connectivity to the 3GPP network, which may involve establishing a secure tunnel (e.g., IPsec) to the PDG or ePDG. This allows the UE to access services such as IMS, internet, and operator-specific services as if it were connected via cellular access. The Serving WLAN concept enables seamless mobility between WLAN and cellular access, supporting handovers and service continuity.

The Serving WLAN can be classified based on its relationship to the home network. A home WLAN is directly operated by the home operator or has a trusted relationship with it, allowing for simpler interworking. A visited WLAN is operated by a different operator, requiring roaming agreements and more complex interworking procedures. The 3GPP specifications define various scenarios for WLAN interworking, including trusted and untrusted WLAN access, each with different security and connectivity requirements.

The concept of Serving WLAN was particularly relevant in Releases 6 through 12, during which 3GPP developed and refined WLAN interworking. In later releases, the focus shifted to more advanced interworking with LTE and 5G, such as LTE-WLAN Aggregation (LWA) and 5G WLAN interworking, where the WLAN can be integrated at the radio access level. However, the Serving WLAN remains a foundational concept for understanding how 3GPP networks interact with WLANs to provide ubiquitous connectivity and offload traffic from cellular networks.

Purpose & Motivation

The Serving WLAN concept was introduced to address the growing demand for high-speed data services and the need to offload traffic from cellular networks. WLAN technology, with its widespread availability and high data rates, offered a complementary access technology that could enhance the user experience and reduce the load on cellular infrastructure. However, integrating WLAN with 3GPP networks required a standardized approach to authentication, authorization, and connectivity, which the Serving WLAN concept provides.

Before the formalization of WLAN interworking, users could connect to WLANs independently, but there was no seamless integration with cellular services. This meant that users could not easily access their home network services, such as IMS or operator-specific services, via WLAN, and there was no support for mobility between WLAN and cellular access. The Serving WLAN concept, along with the associated interworking architecture, solved these problems by defining how a WLAN can act as an access network for 3GPP services, with the necessary security and policy controls.

The introduction of the Serving WLAN also enabled new business models for operators, such as offering WLAN access as part of a subscription bundle, and facilitated roaming between WLAN and cellular networks. It laid the groundwork for later developments like Wi-Fi calling and LTE-WLAN aggregation, which further integrated WLAN into the cellular ecosystem. The concept was motivated by the need to provide a consistent user experience across different access technologies and to leverage the ubiquity of WLAN to improve coverage and capacity.