PC5 QoS Flow Identifier

Description

The PC5 QoS Flow Identifier (PQFI) is a fundamental component within the 3GPP architecture for managing Quality of Service (QoS) on the PC5 interface, which is the reference point for direct communication between User Equipment (UE) without traversing the network infrastructure (sidelink). Each established QoS flow over the PC5 interface is assigned a unique PQFI value. This identifier is used end-to-end between the communicating UEs to distinguish and manage the packet flows according to their specific QoS profiles, which are defined by parameters such as priority, packet delay budget, packet error rate, and data rate.

Architecturally, the PQFI operates within the protocol stack of the UE, specifically at the layer where QoS flows are mapped to data radio bearers or sidelink radio bearers for transmission over the air interface. When a ProSe Per-Packet Priority (PPPP) or a 5G QoS Identifier (5QI) is derived for a PC5 communication session, the corresponding QoS flow is established and tagged with a PQFI. This identifier is then included in the packet headers or associated with the flow context, allowing both the transmitting and receiving UEs, as well as any intermediate relay UEs, to apply the correct QoS treatment, including scheduling, admission control, and resource allocation.

The role of the PQFI is critical in scenarios requiring multiple concurrent services with varying requirements over a single PC5 link. For instance, a vehicle might simultaneously run a safety-critical collision avoidance application (requiring ultra-low latency and high reliability) and an infotainment service (requiring high bandwidth). The PQFI allows the UE to classify packets from these different applications into separate QoS flows, each with its own identifier, ensuring that the stringent requirements of the safety application are not compromised by the bandwidth-intensive infotainment traffic. The management of PQFI values, including their assignment, modification, and release, is governed by procedures defined in 3GPP specifications, often triggered by session establishment or modification requests.

In the broader QoS framework, the PQFI works in conjunction with other identifiers like the PC5 QoS Identifier (PQI), which defines the QoS characteristics template, and the PC5 Link Identifier. The PQFI provides the granularity needed for per-flow QoS enforcement, while the PQI provides the standardized profile. This separation allows for flexible and efficient QoS management, enabling dynamic adaptation to changing service requirements and radio conditions without needing to redefine the entire QoS profile for each packet.

Purpose & Motivation

The PQFI was introduced to address the need for sophisticated QoS management in direct device-to-device (D2D) communications, particularly for Vehicle-to-Everything (V2X) and public safety services standardized from 3GPP Release 16 onwards. Prior to its introduction, sidelink communications in earlier releases had more limited QoS mechanisms, often relying on broad service classes or priorities (like PPPP) without the granular, flow-based differentiation required for advanced use cases. The evolution towards autonomous driving and advanced V2X applications created a demand for supporting multiple, concurrent data streams with diverse and stringent requirements over a single PC5 link.

The primary problem PQFI solves is the inability to distinctly identify and manage individual QoS flows within a PC5 communication session. Without such an identifier, all traffic between two UEs would be treated homogeneously, making it impossible to guarantee the performance for critical services amidst background traffic. PQFI enables the network and the UEs to apply specific QoS policies on a per-flow basis, ensuring that latency-sensitive safety messages are prioritized over non-critical data. This granular control is essential for meeting the performance benchmarks set for advanced V2X services, which are foundational for road safety and traffic efficiency.

Its creation was motivated by the broader 3GPP effort to integrate sidelink communication into the 5G system with a QoS framework comparable in capability to the Uu interface (UE-to-network). It allows the principles of 5G QoS, such as flow-based forwarding and granular resource management, to be extended to direct UE-to-UE links. This ensures a consistent and high-performance user experience across all communication modes, whether the traffic flows through the network or directly between devices, thereby fulfilling the 5G vision of supporting ultra-reliable low-latency communication (URLLC) in D2D scenarios.