Access Network Discovery and Selection Policy

Description

The Access Network Discovery and Selection Policy (ANDSP) is a comprehensive policy management framework defined within the 5G System (5GS) architecture to govern how User Equipment (UE) discovers, evaluates, and selects non-3GPP access networks, primarily Wireless Local Area Networks (WLANs/Wi-Fi). It operates as a key component of the Access Network Discovery and Selection Function (ANDSF) legacy concept, now more deeply integrated into the 5G core's Policy Control Framework (PCF). The policy is delivered from the network to the UE, typically via the Non-Access Stratum (NAS) signaling or user plane mechanisms, and is stored locally on the device for execution.

Architecturally, ANDSP policies are structured as a set of rules and preferences defined in an Extensible Markup Language (XML) document or other structured data format. The policy framework includes several key components: Discovery Information, which provides the UE with data on available non-3GPP networks in its vicinity (like SSIDs, HESSIDs, or realm lists); Selection Criteria, which defines the rules and priorities for choosing one network over another; and Validity Conditions, which specify the temporal and spatial scope where the policy is applicable. The UE's ANDSP client interprets these policies, often interacting with the device's operating system and connection manager to execute the network selection decisions.

In operation, the UE uses ANDSP to make intelligent access selection decisions. When multiple access networks are available (e.g., 5G NR, LTE, and Wi-Fi 6), the UE consults the ANDSP rules. These rules can be based on a wide array of factors beyond simple Received Signal Strength Indicator (RSSI). They include the UE's location (via geographic coordinates or PLMN), time of day, the specific application or service being used (mapped via DNN, S-NSSAI, or OSId/OSAppId), required QoS levels, network load conditions, user preferences, and roaming status. For instance, a policy might dictate that for a high-definition video streaming service (identified by its S-NSSAI), the UE should prefer a trusted, high-throughput Wi-Fi network over a cellular connection if available, unless the cellular connection is the home PLMN and the Wi-Fi is an untrusted hotspot.

ANDSP's role is pivotal in enabling seamless mobility and session continuity across heterogeneous access networks. It is a fundamental enabler for Access Traffic Steering, Switching, and Splitting (ATSSS) features defined in 3GPP, allowing the network to steer specific IP flows to the most appropriate access (3GPP or non-3GPP). By moving network intelligence to the UE via policies, ANDSP reduces the need for constant network signaling during discovery phases and allows for more context-aware, efficient, and user-centric connectivity management. It effectively bridges the policy control mechanisms of the 5G core with the actual connectivity decisions made at the edge device.

Purpose & Motivation

ANDSP was created to solve the critical challenge of intelligent and efficient access network selection in increasingly heterogeneous wireless environments. Prior to its standardization, UE behavior for selecting between cellular and Wi-Fi networks was largely simplistic, often based solely on signal strength or user manual intervention. This led to suboptimal network utilization, poor user experience (e.g., a UE sticking to a weak but preferred cellular signal instead of switching to a strong Wi-Fi), and an inability for operators to manage traffic strategically across their multi-access portfolio. The proliferation of Wi-Fi as a crucial complement to cellular networks, especially for data offloading and indoor coverage, necessitated a standardized, policy-driven approach.

The technology addresses the limitations of earlier, non-standardized methods by providing a unified, operator-controlled framework. It allows Mobile Network Operators (MNOs) to define sophisticated business and technical rules for access selection, which are then enforced on the UE. This solves problems such as uncontrolled offloading that could congest certain Wi-Fi networks, inability to prioritize access for specific services (like voice over Wi-Fi), and poor support for seamless mobility between access types. Historically, the concept originated with the ANDSF in 3GPP Release 8 for EPS, but ANDSP in 5G represents an evolution, offering tighter integration with the 5G core's service-based architecture and more granular policy control aligned with network slicing and QoS flows.

Furthermore, ANDSP is motivated by the need to support complex 5G use cases. For enhanced Mobile Broadband (eMBB), it ensures users are always on the best available access for high-throughput applications. For network slicing, it can include slice-specific selection rules, ensuring a UE connects to an access network capable of supporting the required slice. For ATSSS, it provides the foundational policies that guide how traffic is steered or split. Ultimately, ANDSP empowers operators to optimize their overall network resources, improve customer experience, and implement advanced traffic management strategies in a multi-access 5G world.