Non-3GPP QoS Assistance Information

Description

Non-3GPP QoS Assistance Information (N3QAI) is a concept introduced in 3GPP Release 18 to enhance Quality of Service (QoS) handling for User Equipment (UE) connected via untrusted non-3GPP access, such as Wi-Fi, through the Non-3GPP InterWorking Function (N3IWF). In a pure 3GPP access scenario, the Radio Access Network (RAN) is fully aware of 5G QoS Flows and can map them to appropriate radio bearers. However, when a UE uses a non-3GPP access like Wi-Fi, the underlying access network is typically unaware of 5G QoS constructs. The N3QAI mechanism allows the UE to provide hints to the 5G Core about the QoS characteristics of its uplink traffic, enabling better QoS enforcement at the N3IWF and beyond.

The N3QAI is information sent by the UE, typically as part of the uplink user plane data transmission or via associated control signaling when establishing a PDU session or QoS Flow over non-3GPP access. This information includes parameters that describe the QoS requirements of the application data being sent. These parameters can mirror or derive from 5G QoS characteristics, such as 5G QoS Identifier (5QI), Allocation and Retention Priority (ARP), Guaranteed Flow Bit Rate (GFBR), Maximum Flow Bit Rate (MFBR), or other traffic descriptors. The UE's application layer or its QoS management layer generates this assistance information based on the service requirements.

Upon receiving packets marked with N3QAI from the UE over the NWu IPsec tunnel, the N3IWF can utilize this information. The primary role of the N3IWF is to interpret the N3QAI and perform two key functions: first, to map the incoming traffic to the appropriate 5G QoS Flow within the existing PDU session for the N3 interface towards the UPF; second, to potentially apply local QoS policies or markings on the traffic before forwarding it. This assists the 5G Core's Session Management Function (SMF) and Policy Control Function (PCF) in enforcing the correct QoS policies end-to-end, even though the non-3GPP access segment itself cannot natively support 5G QoS signaling.

Architecturally, N3QAI operates within the user plane path between the UE and the N3IWF. It is a form of QoS signaling that complements the standard N1 NAS signaling used for QoS Flow establishment. Its introduction acknowledges that in many non-3GPP deployments, especially Wi-Fi, the last-hop link characteristics (e.g., Wi-Fi channel conditions, congestion) are opaque to the 5G Core. By providing assistance information from the UE—the endpoint experiencing the local access conditions—the network can make more informed decisions on traffic prioritization and resource allocation, improving the overall consistency of the quality of experience for services like voice, video, or gaming when accessed via Wi-Fi.

Purpose & Motivation

N3QAI was created to address a significant gap in the 5G QoS framework when applied to untrusted non-3GPP access networks. The core problem is the asymmetry of QoS awareness: while the 5G Core defines precise QoS Flows with specific parameters, the non-3GPP access network (e.g., a commercial or residential Wi-Fi router) has no understanding of these 5G-specific constructs. Prior to N3QAI, QoS enforcement for traffic from a UE over non-3GPP access relied heavily on static policies configured at the N3IWF or deep packet inspection (DPI), which could be inefficient, inaccurate, or not dynamically aligned with application needs.

The motivation for N3QAI stems from the increasing reliance on Wi-Fi for delivering high-quality 5G services, especially in indoor and fixed-wireless convergence scenarios. Without a mechanism for the UE to communicate its real-time QoS needs, the network might treat all traffic from a Wi-Fi connection with a default, potentially low-priority QoS profile, degrading the performance of latency-sensitive or high-bandwidth applications. N3QAI solves this by allowing the UE, which has the best knowledge of the application's requirements, to explicitly signal these requirements to the interworking function.

Historically, similar concepts existed in limited forms (like DSCP marking over IPsec), but they were not standardized within the 3GPP architecture for non-3GPP access. N3QAI provides a standardized, 3GPP-aligned method for QoS assistance. It enables more dynamic and accurate QoS mapping, which is essential for supporting advanced 5G services like network slicing, Ultra-Reliable Low Latency Communications (URLLC), and enhanced Mobile Broadband (eMBB) over converged access networks. It represents an evolution from network-centric QoS assumption to a more collaborative, end-point-assisted QoS model for heterogeneous access.