Access Traffic Steering, Switching, and Splitting

Description

ATSSS is a comprehensive framework standardized in 3GPP Release 16 and beyond that enables a User Equipment (UE) to establish and utilize a single Protocol Data Unit (PDU) Session across multiple access networks simultaneously. These access networks can include 3GPP access types (like 5G NR or LTE) and non-3GPP access types (like Wi-Fi or fixed networks). The core architectural principle involves the ATSSS Function, which resides in the User Plane Function (UPF) and the UE, working in coordination with the Session Management Function (SMF) in the control plane.

The system operates through three fundamental traffic handling mechanisms: Steering, Switching, and Splitting. Steering involves selecting the most appropriate access network for a new traffic flow based on policies (e.g., send latency-sensitive traffic to 5G, bulk download to Wi-Fi). Switching involves dynamically moving an ongoing flow from one access to another, typically for service continuity if the quality of the current access degrades. Splitting involves distributing the packets of a single IP flow across multiple access networks, which can be done at the packet level (MPTCP-based) or the IP layer (ATSSS-LL-based), to aggregate bandwidth and enhance reliability.

Key components include the ATSSS Control Function (part of the SMF), which manages ATSSS policies and rules, and the ATSSS User Plane Function (within the UPF and UE), which executes the actual traffic distribution. The framework relies on the N4 interface for the SMF to provision these rules to the UPF. Policies are derived from the Policy Control Function (PCF) and can consider real-time access network conditions, user subscription, and application requirements. The UE's ATSSS capability is negotiated during PDU Session Establishment.

ATSSS plays a pivotal role in the 5G architecture by realizing true multi-access convergence. It moves beyond simple access selection (like ANDSF/MPTCP) to provide seamless, policy-driven, and granular control over how user traffic utilizes heterogeneous access resources. This enables operators to deliver enhanced Quality of Experience (QoE) by always using the best available network path(s) for a given service, improving overall network efficiency and resource utilization.

Purpose & Motivation

ATSSS was created to solve the fundamental challenge of efficiently and intelligently utilizing multiple available access networks (cellular and non-cellular) that a modern UE can connect to. Prior to ATSSS, solutions like Access Network Discovery and Selection Function (ANDSF) provided policy-based access selection but lacked the ability to use multiple accesses simultaneously for a single session or to dynamically switch/split traffic within a flow. Multi-Path TCP (MPTCP) offered application-layer splitting but required application support and was not tightly integrated with the core network's policy framework.

The primary motivation was to enhance user experience through improved performance, reliability, and seamless mobility. By allowing traffic steering, switching, and splitting, ATSSS ensures that critical applications always use the best available path, can survive the failure of one access type without service interruption, and can aggregate bandwidth from multiple accesses. This is especially important for 5G's vision of supporting diverse services (eMBB, URLLC, mMTC) where requirements for latency, bandwidth, and reliability vary dramatically.

Furthermore, ATSSS provides operators with a standardized, network-controlled mechanism for traffic management across heterogeneous accesses. It enables new business models, such as fixed-mobile convergence, by treating Wi-Fi and 5G as complementary resources under unified policy control. It addresses the limitations of previous fragmented approaches by integrating deeply into the 5G Core's service-based architecture, providing a scalable and flexible framework essential for the future of converged networks.