PDU Sequence Number within a PDU Set

Description

The PDU Sequence Number (PSN) is a field within the protocol data unit (PDU) header that identifies the order of PDUs within a PDU Set. A PDU Set is a group of PDUs that belong together and are treated as a single unit for QoS and delivery purposes, such as a video frame or an application-level message. The PSN is used by the receiver to detect losses, reorder PDUs, and reassemble the original data. It is introduced in 3GPP Release 19 as part of the enhancements for extended reality (XR) and media services, where multiple PDUs constitute a single application data unit.

In the 5G protocol stack, the PSN is likely carried in the SDAP (Service Data Adaptation Protocol) layer or a new sublayer, as indicated by its specification in 38.415 (SDAP) and 26.822 (media handling). The PSN allows the network to perform PDU Set-based QoS treatment, where the entire set is prioritized and delivered with a certain reliability and latency. For example, in XR, a video frame may be encoded into multiple PDUs; if one PDU is lost, the whole frame may be undecodable. The PSN helps the receiver know which PDUs belong to which frame and their order, enabling efficient retransmissions or error concealment.

Architecturally, the PSN is assigned by the sender (e.g., the UE or the UPF) and is included in the header of each PDU. The receiver uses the PSN to place the PDU in the correct order within the PDU Set. The PSN may also be used for duplication detection and for synchronization across multiple flows. In the 5G core, the PSN can be used by the UPF to enforce QoS policies per PDU Set, such as marking packets for differentiated treatment. The PSN is part of the PDU Set Information that may be exchanged between network functions.

Key components include the PSN field itself, which is typically a counter that increments for each PDU within a set, and the PDU Set boundaries, which may be indicated by a start/end marker or by the PSN range. The PSN must be large enough to avoid wraparound within the maximum set size. In 3GPP, the exact format and size of the PSN are defined in the relevant specifications. The PSN enables the network to support new QoS parameters like PDU Set Error Rate (PSER) and PDU Set Delay Budget (PSDB), which are crucial for XR and other advanced services.

The role of PSN in the network is to facilitate the efficient and reliable delivery of PDU Sets, which is essential for applications that require high data rates and low latency, such as augmented reality and cloud gaming. By providing a sequence number within a set, the network can better manage resources, prioritize critical data, and improve user experience. As 5G evolves towards 6G, PSN will be a key enabler for immersive media and other time-sensitive applications.

Purpose & Motivation

The PDU Sequence Number (PSN) was introduced to address the limitations of traditional per-PDU QoS handling in 5G networks. In earlier releases, QoS was applied on a per-packet basis, which is inefficient for applications that generate multiple PDUs that are interdependent, such as video frames in XR. If one PDU is lost, the entire frame may be corrupted, but the network would treat each PDU independently, leading to wasted resources and poor user experience. The PSN enables the network to treat a group of PDUs as a single unit, allowing for more intelligent scheduling, retransmission, and error recovery.

Historically, 3GPP QoS frameworks focused on per-flow treatment, with parameters like 5QI, GBR, and MBR. However, with the rise of XR and media services, it became clear that a more granular approach was needed. The concept of PDU Sets was introduced in Rel-18 studies and formalized in Rel-19. The PSN is a key part of this framework, as it allows the receiver to reassemble the set and the network to enforce set-level QoS. This solves the problem of head-of-line blocking and enables efficient use of network resources by dropping or deprioritizing entire sets when necessary.

The PSN also supports advanced features like packet duplication and multi-path transmission, where PDUs of the same set may be sent over different paths. The receiver can use the PSN to merge the duplicates and reconstruct the set. This is particularly important for URLLC and industrial IoT applications. By standardizing the PSN, 3GPP ensures interoperability and provides a foundation for future enhancements in 6G.