Access Traffic Splitting

Description

Access Traffic Splitting (ATS) is a key feature introduced in 5G to enable the simultaneous use of multiple access networks for a single service data flow. It is part of the broader Access Traffic Steering, Switching, and Splitting (ATSSS) framework defined in 3GPP TS 23.501. ATS specifically refers to the splitting of traffic, where some packets of a service data flow are transmitted over one access (e.g., 3GPP access) and other packets over another access (e.g., non-3GPP access). This is distinct from traffic steering, which selects the best access for a flow, and traffic switching, which moves a flow from one access to another.

ATS operates at the IP flow level and is controlled by the SMF (Session Management Function) in the 5G core network. The SMF configures the UE and the UPF (User Plane Function) with ATSSS rules that determine how traffic should be split. These rules can be based on various criteria, such as the type of application, QoS requirements, or network conditions. The UE and UPF then enforce these rules by distributing packets across the available accesses. To ensure proper delivery, ATS relies on a performance measurement function (PMF) that monitors the quality of each access and provides feedback for dynamic adjustments.

The ATS procedure involves several key components: the ATSSS rules, the Multi-Access PDU (MA PDU) session, and the ATSSS-LL (Low Layer) or MP-TCP (Multipath TCP) functionality. An MA PDU session is a PDU session that can be established over both 3GPP and non-3GPP accesses simultaneously. The UE and UPF use the ATSSS rules to decide how to split the traffic. For example, a rule might specify that 70% of the traffic goes over 3GPP access and 30% over non-3GPP access. The actual splitting can be done at the IP layer (using MP-TCP or QUIC) or at the lower layers (ATSSS-LL).

ATS provides significant benefits. It can increase throughput by aggregating the bandwidth of multiple accesses. It can improve reliability by providing redundancy; if one access fails, the traffic can be sent over the other. It can also enhance user experience by dynamically adapting to changing network conditions. For instance, if the non-3GPP access becomes congested, the SMF can update the ATSSS rules to shift more traffic to the 3GPP access.

The feature was introduced in Release 16 as part of the 5G core network specifications. In Release 19, the definition was refined to refer to 'service data flow' instead of 'data flow', aligning with 5G terminology. ATS is applicable between one 3GPP access and one non-3GPP access, and it is a key enabler for enhanced mobile broadband (eMBB) and ultra-reliable low-latency communication (URLLC) services. It is also relevant for fixed-mobile convergence, where a user can seamlessly use both cellular and Wi-Fi networks.

Purpose & Motivation

Access Traffic Splitting was created to address the growing demand for higher data rates, improved reliability, and seamless connectivity in 5G networks. Traditional networks typically used a single access at a time, and switching between accesses (e.g., from Wi-Fi to cellular) could cause interruptions or degradation. ATS allows the simultaneous use of multiple accesses, aggregating their bandwidth and providing redundancy. This is particularly important for applications that require high throughput or low latency, such as video streaming, cloud gaming, and industrial automation.

The motivation for ATS also comes from the proliferation of multiple access technologies in user devices. Smartphones and other devices often have both cellular and Wi-Fi capabilities. Before ATS, these accesses were used independently, and the device would select one based on policies. ATS enables a more intelligent and dynamic use of these accesses, improving the overall user experience. It also helps operators to better utilize their spectrum and offload traffic to non-3GPP accesses when beneficial.

ATS is part of the broader ATSSS framework, which was introduced in 5G to provide a standardized way to manage traffic across multiple accesses. The framework includes steering, switching, and splitting, each serving different purposes. ATS specifically addresses the need to split traffic for a single flow, which is useful when no single access can meet the requirements, or when redundancy is desired. The development of ATS was driven by the need for a more flexible and efficient use of network resources, and it represents a significant step forward in multi-access connectivity.