PDU Set Importance

Description

PDU Set Importance (PSI) is a Quality of Service (QoS) attribute defined within the 5G System and Radio Access Network (RAN) specifications. It operates at a granularity above individual packets, applying to a 'PDU Set', which is a logical grouping of Protocol Data Units that share a common importance level relevant to the application's semantics. For instance, in a video stream using Scalable Video Coding (SVC) or in an Extended Reality (XR) application streaming multiple independent video tiles, different tiles or layers can be grouped into PDU Sets with distinct PSI values. The PSI value is an integer that signifies the relative priority of one PDU Set compared to others within the same QoS Flow or Data Radio Bearer.

The architecture for PSI involves extensions across multiple protocol layers and network functions. At the application layer or the Application Function (AF), the importance metadata is generated. This information is conveyed to the 5G Core Network via the Network Exposure Function (NEF) or directly via the Policy Control Function (PCF) using service parameters. The PCF translates this application requirement into a QoS policy that includes the PSI attribute. This policy is then enforced by the Session Management Function (SMF), which maps the QoS Flow carrying the PDU Sets to appropriate Access Network resources.

In the Radio Access Network (RAN), the gNB receives the PSI information associated with the QoS Flow or the specific data packets via control plane signaling (e.g., NGAP, F1AP) and user plane markings (e.g., in GTP-U or SDAP headers). The RAN scheduler utilizes the PSI, alongside other QoS parameters like 5G QoS Identifier (5QI) and Guaranteed Flow Bit Rate (GFBR), to make intelligent scheduling and packet discard decisions. When facing radio resource constraints or congestion, the RAN can prioritize the transmission of PDU Sets with higher importance and may proactively discard packets from lower-importance sets to conserve resources and maintain the quality of the most critical content.

Key components enabling PSI include the QoS Framework in the 5G Core, the SDAP (Service Data Adaptation Protocol) layer in the 5G radio protocol stack for marking and handling PDU Sets, and the RAN intelligence for packet prioritization. Its role is to bridge application-aware semantics with network-level resource management, allowing for more efficient and intelligent use of transport resources, ultimately enhancing user experience for services with heterogeneous data importance within a single stream.

Purpose & Motivation

PSI was created to address the limitations of traditional QoS models, which treat all packets within a flow with uniform priority. Modern applications, particularly immersive media like Augmented Reality (VR), Virtual Reality (VR), and cloud gaming, often generate data streams where not all packets are equally valuable. For example, in a 360-degree video, packets for the tile currently in the user's field of view are more important than those for peripheral tiles. Similarly, base layers in video coding are more critical than enhancement layers. Previous QoS mechanisms lacked a standardized way to convey this intra-flow importance granularity to the network.

The historical context stems from the increasing demand for bandwidth-efficient delivery of high-volume, latency-sensitive media. Without PSI, the network either over-provisions resources for an entire flow (wasteful) or applies blanket packet drops during congestion, which can degrade the most important parts of the content. PSI solves this by providing a standardized, in-band signaling mechanism for application-level importance. This allows the RAN and core network to make context-aware decisions, optimizing resource allocation, reducing unnecessary transmission, and improving overall spectral efficiency and user QoE for advanced services introduced in 5G-Advanced (Release 18 and beyond).