Multi-Access Rule

Description

The Multi-Access Rule (MAR) is a policy construct introduced in 3GPP Release 16 as part of the enhanced Policy and Charging Control (PCC) architecture for the 5G System (5GS). It is a specific type of policy rule provisioned by the Policy Control Function (PCF) and enforced by the Session Management Function (SMF) to control how a UE's traffic is routed across multiple available access networks when the UE is simultaneously connected via both 3GPP access (e.g., 5G NR) and non-3GPP access (e.g., Wi-Fi). A MAR is essentially a set of conditions and actions that determine on which access network a specific IP flow or 5G QoS Flow should be forwarded.

Architecturally, MARs are integral to the 5G PCC framework. The PCF generates MARs based on operator policy, subscriber profile, and potentially real-time input from the Network Data Analytics Function (NWDAF). These rules are then provided to the SMF as part of the PCC rules within a PDU Session context. The SMF is responsible for interpreting and enforcing the MARs. It does this by configuring the appropriate traffic steering actions on the User Plane Function (UPF) and by providing guidance to the UE via the Access Traffic Steering, Switching and Splitting (ATSSS) framework. The MAR contains several key components: a rule identifier, precedence value, service data flow (SDF) filters or QoS Flow identifiers to identify the target traffic, conditions for activation (e.g., access network availability, load thresholds), and the steering action (e.g., steer to 3GPP access, steer to non-3GPP access, split across both).

How MAR works involves dynamic evaluation and enforcement. When a PDU Session is established with ATSSS capabilities, the SMF installs the relevant MARs. As user traffic matches the SDF filters, the SMF (with UPF support) evaluates the MAR's conditions against the current network state. For example, if a MAR states that video traffic should be steered to Wi-Fi if the Wi-Fi signal strength is above a threshold and the 3GPP access is congested, the system continuously monitors these parameters. When conditions are met, the SMF instructs the UPF to mark packets accordingly and may use the ATSSS mechanisms (like MPTCP or GRE tunneling) to direct the flow over the selected access. The UE's ATSSS handler collaborates in this process based on steering mode (UE-assisted or network-assisted). MARs enable per-flow granularity, allowing different applications to be routed over different accesses simultaneously, optimizing performance and resource utilization.

Purpose & Motivation

MAR was created to address the increasing complexity of managing traffic in heterogeneous multi-access 5G networks. With the proliferation of Wi-Fi 6/6E and the desire to use unlicensed spectrum alongside licensed 5G NR, operators needed a sophisticated, policy-driven mechanism to steer traffic optimally across all available paths. Previous multi-access solutions like MAPCON and IFOM in 4G provided foundational capabilities but were often static or required significant UE-centric decision-making. MAR, as part of the 5G ATSSS framework, introduces a network-centric, dynamic, and flow-aware policy control.

The primary problem MAR solves is how to intelligently utilize multiple access connections for a single PDU Session to enhance user experience and network efficiency. Without such rules, traffic distribution might be suboptimal—for instance, overloading the 5G RAN with bulk data while underutilizing available Wi-Fi, or failing to move latency-sensitive gaming traffic to a lower-latency access. MAR allows operators to define rich policies based on a wide range of conditions including access network load, quality (e.g., latency, jitter), cost, subscription type, and application requirements. This enables use cases like always-on connectivity, where critical signaling stays on 5G for reliability while large downloads are offloaded to Wi-Fi, or seamless switching of a video call from 5G to Wi-Fi when moving indoors.

Furthermore, MAR supports the 5G vision of network slicing and service-based architecture. Different network slices may have different MAR policies; for example, an enhanced Mobile Broadband (eMBB) slice might aggressively offload to Wi-Fi, while an Ultra-Reliable Low-Latency Communication (URLLC) slice might keep all traffic on 3GPP access. By integrating with the PCF and NWDAF, MAR policies can be adaptive, using analytics to predict congestion and preemptively steer traffic. This represents a significant evolution from static configurations to a closed-loop, intelligent traffic management system that is essential for delivering consistent quality of experience in a multi-access 5G environment.