Access Traffic Switching

Description

Access Traffic Switching (ATSw) is a critical procedure within the 5G System's Access Traffic Steering, Switching, and Splitting (ATSSS) framework, introduced in 3GPP Release 16. Unlike Access Traffic Steering, which selects an access for a new flow, ATSw deals with the migration of an existing service data flow from one access network to another. The procedure ensures that all traffic of the ongoing flow is moved without interruption, preserving the continuity of the service. This is particularly important for applications that require session continuity, such as voice calls, video streaming, or online gaming, where a change in access network should not cause a drop or degradation.

ATSw operates between one 3GPP access (e.g., NR or LTE) and one non-3GPP access (e.g., Wi-Fi). The decision to switch can be triggered by various factors, such as changes in access availability, performance degradation, or network policies. The UE, guided by ATSSS rules and measurement assistance information, initiates the switch. The ATSSS rules, provided by the PCF and delivered via the SMF and AMF, define the conditions under which a switch should occur. For example, a rule might specify that if the RTT on the current access exceeds a threshold, the UE should switch to the alternative access.

The switching procedure involves the UE's ATSSS layer, which manages the mapping of the service data flow to the access network. When a switch is triggered, the UE redirects the flow's packets from the source access to the target access. To maintain continuity, the UE may use mechanisms such as buffering or make-before-break, depending on the steering mode and the capabilities of the accesses. The 5G core network, specifically the UPF, supports the switch by ensuring that the user plane path is updated accordingly. The UPF may also assist in reordering packets to prevent out-of-order delivery during the transition.

ATSw is closely related to the concept of Multi-Access PDU (MA PDU) sessions, which allow a single PDU session to span both 3GPP and non-3GPP accesses. The MA PDU session provides the anchor for the service data flow, and the switching occurs within the context of this session. The UE and network coordinate to ensure that the QoS and policy requirements of the flow are maintained after the switch. This includes updating the QoS flows and ensuring that the target access can support the required QoS.

The procedure is specified in 3GPP TS 23.501 and TS 29.512. It supports various scenarios, including switching due to coverage loss, load balancing, or operator preferences. The evolution of the definition from Release 16 to Release 20 shows a refinement in terminology, with the term "data flow" being replaced by "service data flow" in Release 19 to align with 5G QoS terminology. This change reflects the maturation of the 5G specifications and the need for precise language.

In summary, Access Traffic Switching is a vital mechanism for ensuring seamless service continuity in multi-access 5G networks. It allows ongoing flows to be moved between accesses without disruption, enhancing reliability and user experience. Its integration with ATSSS and MA PDU sessions makes it a cornerstone of 5G's multi-access capabilities.

Purpose & Motivation

Access Traffic Switching was developed to address the challenge of maintaining service continuity when a user's traffic needs to be moved from one access network to another. In previous generations, switching between accesses often resulted in session breaks or required complex application-level mechanisms. ATSw provides a standardized, network-controlled solution that ensures seamless migration of ongoing flows, which is essential for real-time services like VoNR, video calls, and cloud gaming. It solves the problem of service interruption during access changes, which is particularly important in heterogeneous networks where users may move between cellular and Wi-Fi coverage.

The motivation for ATSw comes from the need to support mobility and load balancing across multiple accesses without degrading user experience. For example, when a user walks out of Wi-Fi coverage, their ongoing video stream should seamlessly switch to cellular without buffering. Similarly, operators may want to offload traffic from cellular to Wi-Fi to relieve congestion, and ATSw enables this without dropping sessions. This is a key enabler for fixed-mobile convergence and for providing a consistent service experience across different access technologies.

Furthermore, ATSw is part of the broader ATSSS framework, which aims to make the 5G core access-agnostic. By allowing flows to be switched between accesses, it provides flexibility and resilience. It also supports advanced use cases like ultra-reliable low-latency communication (URLLC) by enabling redundant paths and fast failover. The standardization of ATSw ensures interoperability and allows operators to deploy multi-access solutions from multiple vendors.