5G QoS Identifier

Description

The 5G QoS Identifier (5QI) is a fundamental mechanism in 5G System (5GS) architecture for managing Quality of Service. It's a scalar value ranging from 1 to 255, where standardized values (1-89) have predefined QoS characteristics defined in 3GPP specifications, while dynamic values (90-254) can be assigned with operator-specific QoS parameters. Each 5QI value maps to a specific QoS profile containing five key parameters: Resource Type (GBR, Delay Critical GBR, or Non-GBR), Priority Level, Packet Delay Budget (PDB), Packet Error Rate (PER), and Averaging Window (for GBR flows only).

When a Protocol Data Unit (PDU) Session is established, the 5G Core Network (5GC) assigns one or more QoS Flows identified by their 5QI values. The Access and Mobility Management Function (AMF) communicates these QoS requirements to the Radio Access Network (RAN) via the N2 interface. The RAN then maps each QoS Flow to appropriate Data Radio Bearers (DRBs) using QoS Flow to DRB mapping rules. This hierarchical approach separates QoS control (in 5GC) from bearer management (in RAN), providing flexibility and scalability.

The 5QI mechanism works through standardized signaling procedures. During PDU Session Establishment or Modification, the Session Management Function (SMF) determines the appropriate 5QI based on the service requirements and subscriber profile. The SMF sends this information to the User Plane Function (UPF) for packet marking and to the RAN via the AMF. In the user plane, packets are marked with QoS Flow Identifiers (QFIs) derived from 5QI values, enabling consistent QoS treatment across network nodes. The RAN uses these markings to apply appropriate scheduling, admission control, and link layer configurations.

Key architectural components involved in 5QI implementation include the Policy Control Function (PCF), which provides policy rules containing 5QI assignments; the SMF, which enforces these policies; the UPF, which performs packet marking and rate policing; and the gNB, which implements radio resource scheduling based on 5QI parameters. The system supports both reflective QoS, where the UE can derive QoS rules from downlink traffic, and explicit QoS signaling via NAS and RRC protocols.

5QI plays a critical role in enabling network slicing and service differentiation. Different network slices can use different 5QI values to achieve their specific performance requirements. The standardized 5QI values cover a wide range of services including conversational voice, live streaming, autonomous driving, industrial automation, and massive IoT applications. This standardized approach ensures interoperability between different vendors' equipment and consistent QoS experience for end users.

Purpose & Motivation

5QI was created to address the limitations of previous QoS mechanisms in 4G/LTE networks, particularly the QCI (QoS Class Identifier). While QCI served well for 4G services, it lacked the granularity and flexibility needed for 5G's diverse use cases including ultra-reliable low-latency communications (URLLC), enhanced mobile broadband (eMBB), and massive machine-type communications (mMTC). The 4G system's bearer-based QoS model was too rigid for 5G's service-based architecture and network slicing requirements.

5QI solves several key problems: First, it provides finer granularity for delay-critical services with specific values for industrial automation, intelligent transport systems, and remote control applications. Second, it introduces the Delay Critical GBR resource type specifically for URLLC services requiring both guaranteed bitrate and strict latency bounds. Third, 5QI enables more efficient resource utilization through improved priority handling and the separation of QoS control from bearer management.

The historical context for 5QI development includes the need to support vertical industry requirements identified in 3GPP Study Items like TR 22.891 and TR 22.804. These studies revealed that previous QoS mechanisms couldn't adequately support services with conflicting requirements operating simultaneously on the same device, such as augmented reality (requiring high bandwidth) and vehicle-to-everything communication (requiring ultra-low latency). 5QI provides the foundation for meeting these diverse requirements through standardized yet flexible QoS profiles.