Wireless Local Area Network

Description

Wireless Local Area Network (WLAN), predominantly based on IEEE 802.11 standards (Wi-Fi), is a fundamental radio access technology for local area connectivity. Within the 3GPP ecosystem, WLAN is not defined as a 3GPP RAT itself, but its integration with 3GPP cellular networks is extensively standardized. This integration, known as WLAN Interworking or Access Network Discovery and Selection Function (ANDSF) in earlier releases, and later as Non-3GPP Interworking or Access Traffic Steering, Switching and Splitting (ATSSS) in 5G, allows User Equipment (UE) to utilize WLAN for IP connectivity, often in conjunction with cellular access.

Architecturally, 3GPP defines several interworking scenarios. For 3G and 4G, the core network interacts with WLAN via specific interfaces like S2a (for trusted WLAN access to the Packet Data Network Gateway - PGW) using protocols such as GTP or PMIPv6. Network entities like the ePDG (evolved Packet Data Gateway) provide secure IPsec tunnels for untrusted WLAN access. The ANDSF provides policies to the UE for network selection. In 5G, the Non-3GPP InterWorking Function (N3IWF) connects untrusted non-3GPP access (like public Wi-Fi) to the 5G Core, while trusted WLAN connects directly to the AMF/UPF. The 5G core treats WLAN as another access type, enabling seamless authentication (via 5G-AKA or EAP-AKA'), unified policy control, and session continuity.

How it works: A dual-mode UE discovers available WLAN networks and, based on operator policies (from ANDSF or UE policies), may attach to a WLAN Access Point. For trusted access, the UE authenticates using EAP-SIM/AKA/AKA' credentials, gaining IP connectivity routed through the cellular core network. User plane traffic can be offloaded locally at the WLAN (Local Breakout) or routed back to the core (Home Routed). Key 3GPP specs define authentication, mobility, policy control, and quality of service mechanisms to create a unified experience. Its role is to provide increased capacity, improve user data rates in dense areas, reduce cellular network congestion, and enable fixed-mobile convergence.

Purpose & Motivation

The integration of WLAN into 3GPP standards was motivated by the explosive growth of Wi-Fi and the need for cellular operators to leverage unlicensed spectrum to offload data traffic from congested macro cellular networks. Early cellular-WLAN interworking aimed to provide simple internet access, but the purpose evolved to offer seamless, secure, and policy-controlled access to operator services, creating a combined cellular-WLAN service fabric.

Historically, initial work in Release 6 defined loose coupling for basic interworking. Later releases addressed limitations like lack of seamless mobility, inconsistent security, and poor user experience when switching networks. The evolution towards tighter integration, with EPC and 5GC treating WLAN as a trusted access, solves problems of authentication transparency (using the same SIM credentials), seamless session continuity (e.g., via IFOM or ATSSS), and consistent application of operator policies and charging. This allows operators to manage WLAN as an integral part of their heterogeneous network (HetNet) strategy, improving overall network efficiency and user satisfaction.