Protocol Data Unit Set

Description

The PDU Set is a concept introduced in 3GPP Release 19 to address the specific quality of service (QoS) challenges posed by emerging applications like eXtended Reality (XR), cloud gaming, and immersive video. In these applications, the data generated by the application layer often consists of discrete units—for example, a video frame or a video slice—that are packetized into multiple PDUs (Protocol Data Units) for transmission over the 5G network. The PDU Set aggregates these PDUs that belong to the same application-level unit. The key constraint is that all PDUs within a PDU Set are transmitted within the same QoS Flow, ensuring that they receive consistent QoS treatment, such as the same 5QI, scheduling priority, and packet delay budget.

From an architectural perspective, the PDU Set is a conceptual entity that resides above the individual PDU level but below the QoS Flow level. It is not a protocol layer but rather a grouping mechanism used by the network to apply QoS policies more intelligently. The 5G System (5GS) identifies PDU Sets through marking in the packet headers. In the downlink, the UPF can mark PDUs belonging to a PDU Set based on information from the application (e.g., via the N6 interface) or from the SMF's policy. In the uplink, the UE may mark PDUs based on QoS rules or reflective QoS. The marking typically uses a PDU Set Identifier or a similar field in the GTP-U header or an extension header. This allows the RAN (gNB) to recognize which PDUs form a set and to schedule them together, potentially prioritizing the set as a whole.

The primary motivation for PDU Sets is to enable more efficient and application-aware QoS enforcement. Traditional QoS treats each PDU individually, which can lead to suboptimal resource allocation when a single application unit spans multiple PDUs. For example, if one PDU of a video frame is delayed, the entire frame may be useless, even if the other PDUs arrive on time. By grouping PDUs into a set, the network can ensure that all PDUs of a set are delivered within the required latency budget, or it can discard the entire set if it becomes clear that the deadline cannot be met, thus saving resources. This is particularly important for XR services, where a dropped frame can cause noticeable glitches and user discomfort.

Furthermore, PDU Sets enable advanced features such as PDU Set-based integrity protection and PDU Set-based discard. Integrity protection ensures that the network can verify that all PDUs of a set are correctly received and in order, while discard allows the network to drop entire sets under congestion to free up resources for other critical traffic. These mechanisms are specified in TS 23.501 and TS 38.415, which define the QoS model and the corresponding signaling between the SMF, UPF, and RAN. The PDU Set concept is expected to evolve in future releases to support more sophisticated cross-layer optimization and application-network interaction.

In summary, the PDU Set is a critical enabler for delivering high-quality XR and immersive services over 5G. It bridges the gap between application-level data units and network-level QoS, allowing the 5GS to treat groups of PDUs as a single logical entity for the purpose of scheduling, resource allocation, and error handling. As 5G-Advanced and 6G evolve, PDU Sets are likely to become even more important for supporting real-time interactive applications.

Purpose & Motivation

The PDU Set was created to solve a fundamental mismatch between how applications generate data and how the network handles it. Applications like XR, augmented reality (AR), and cloud gaming produce data in discrete units such as video frames or slices, each of which may be encoded into multiple IP packets. However, the 5G QoS framework traditionally operates on individual packets or flows, without awareness of these higher-level units. This can lead to inefficient resource usage and degraded user experience because the network may prioritize or drop individual packets without considering their importance to the overall application unit.

For example, if a video frame is split into 10 packets and the network drops just one packet due to congestion, the entire frame may become undecodable. The remaining 9 packets are wasted. By grouping these 10 packets into a PDU Set, the network can make smarter decisions: it can either ensure all 10 are delivered within the latency budget or drop all 10 if the deadline is missed, freeing resources for other traffic. This improves spectral efficiency and reduces the impact of congestion on perceived quality.

Additionally, PDU Sets enable the network to apply differentiated QoS to different application units within the same QoS Flow. For instance, an XR application might have I-frames (key frames) that are more critical than P-frames (predicted frames). By marking them as separate PDU Sets, the network can allocate more resources to I-frames while treating P-frames with lower priority. This level of granularity was not possible before Release 19, and it is essential for meeting the stringent requirements of immersive services. The PDU Set concept thus represents a significant step towards application-aware networking in 5G-Advanced.