Access Traffic Steering

Description

Access Traffic Steering (ATS) is a fundamental procedure in the 5G System (5GS) architecture, introduced in 3GPP Release 16 as part of the Access Traffic Steering, Switching, and Splitting (ATSSS) feature. It operates between one 3GPP access network (e.g., NR or LTE) and one non-3GPP access network (e.g., Wi-Fi or wireline). The primary function of ATS is to determine the most suitable access network for a new service data flow (SDF) and to transfer the traffic of that flow over the selected access. This decision is made by the User Equipment (UE) based on policies and rules provided by the network, specifically the Access Traffic Steering Function (ATSSS) rules and the Measurement Assistance Information (MAI).

The architecture supporting ATS involves several key components. The Policy Control Function (PCF) generates ATSSS rules that define the steering mode (e.g., active-standby, smallest delay, load balancing) and the priority of access networks. These rules are sent to the Session Management Function (SMF), which then provisions them to the UE via the Access and Mobility Management Function (AMF) using Non-Access Stratum (NAS) signaling. The UE also receives measurement assistance information to perform access performance measurements, such as Round-Trip Time (RTT) and access availability. The UE's ATSSS layer, which sits above the IP layer, enforces the steering decisions by directing the SDF to the appropriate access network.

ATS supports several steering modes, each with distinct behavior. In Active-Standby mode, one access is designated as active and the other as standby; traffic is steered to the active access unless it becomes unavailable. In Smallest Delay mode, the UE measures the RTT of both accesses and steers the flow to the one with the lowest delay. In Load Balancing mode, traffic is distributed across both accesses based on a percentage split defined by the network. In Priority-based mode, the UE steers traffic to the highest priority access that is available. These modes can be combined and are specified in 3GPP TS 23.501 and TS 29.512.

The ATS procedure is closely tied to the concept of Multi-Access PDU (MA PDU) sessions, which allow a single PDU session to use both 3GPP and non-3GPP accesses simultaneously. The UE establishes an MA PDU session and receives ATSSS rules that govern how traffic should be steered. When a new SDF is initiated, the UE evaluates the ATSSS rules and selects the appropriate access. The steering decision can be dynamic, adapting to changing network conditions based on measurements. This ensures that the user experience is optimized, for example, by using Wi-Fi for high-bandwidth applications when available and cellular for low-latency services.

In summary, Access Traffic Steering is a core capability of the 5G multi-access framework, enabling intelligent and policy-driven selection of access networks for individual service data flows. It enhances user experience by leveraging the strengths of different access technologies and provides operators with tools to manage network resources efficiently. Its evolution from Release 16 to Release 20 reflects ongoing refinements in terminology and scope, particularly the shift from "data flow" to "service data flow" to align with 5G QoS concepts.

Purpose & Motivation

Access Traffic Steering was created to address the growing need for seamless integration of multiple access technologies in 5G networks. Before its introduction, users could connect to either cellular or Wi-Fi, but there was no standardized mechanism to intelligently steer individual traffic flows across both accesses simultaneously. This led to suboptimal user experiences, such as dropped connections when moving between accesses or inefficient use of available bandwidth. ATS solves this by providing a policy-driven framework that allows the network to control which access is used for each service data flow, ensuring continuity and optimal performance.

The motivation for ATS stems from the proliferation of diverse access technologies and the desire to leverage them collectively. Mobile operators often own both cellular and Wi-Fi networks, and they wanted a way to offload traffic to Wi-Fi without sacrificing service quality. Additionally, new use cases like augmented reality, cloud gaming, and industrial IoT require ultra-reliable and low-latency communication, which can benefit from the redundancy and diversity offered by multiple accesses. ATS enables such use cases by allowing traffic to be steered to the best access based on real-time conditions.

Furthermore, ATS is a key enabler of the 5G vision of a unified, multi-access network. It allows the 5G core to treat 3GPP and non-3GPP accesses as integral parts of the same network, providing a consistent service experience. This is particularly important for fixed-mobile convergence, where operators can integrate wireline and wireless accesses. By standardizing ATS, 3GPP ensures interoperability between different vendors and enables a competitive ecosystem for multi-access solutions.