Description
The PDU Set Sequence Number (PSSN) is a protocol mechanism introduced in 3GPP Release 18 as part of the 5G NR protocol stack, specifically within the Service Data Adaptation Protocol (SDAP) and Packet Data Convergence Protocol (PDCP) layers. A PDU Set is a collection of one or more Protocol Data Units (PDUs) that correspond to a single Application Data Unit (ADU) generated by a higher-layer application. For URLLC services like industrial automation, autonomous driving, or remote surgery, the application often generates data units (e.g., a control command, a sensor reading) that must be delivered completely, in order, and with extremely high reliability and low latency. The PSSN is assigned to each PDU Set to enable the receiver to reassemble the original ADU correctly, even if the individual PDUs within the set arrive out of order or are lost and retransmitted.
Operationally, when an application generates an ADU, the SDAP/PDCP layer at the transmitter segments it (if necessary) into multiple PDUs, forming a PDU Set. A unique PSSN is assigned to this set. This PSSN is included in the header of each PDU belonging to that set. At the receiver side, the PDCP layer uses the PSSN to group incoming PDUs back into their original sets. It can then detect missing PDUs within a set and request retransmissions if needed, and finally deliver the complete ADU to the upper layer in the correct sequence. This is more efficient than per-PDU sequencing for applications where the atomic unit of meaning is the ADU, not the individual PDUs.
The PSSN works in conjunction with existing sequence numbering mechanisms like the PDCP Sequence Number (SN), which provides per-PDU ordering and duplication detection. The PSSN operates at a higher granularity, managing the integrity of the logical group. This is particularly important for services using packet duplication over multiple paths (e.g., via carrier aggregation or dual connectivity), where PDUs from the same set might take different routes and arrive at different times. The receiver uses the PSSN to reorder PDUs from different paths before reconstructing the ADU, ensuring the application receives a consistent and complete data unit.
Purpose & Motivation
PSSN was created to address the stringent requirements of Ultra-Reliable Low-Latency Communication (URLLC) services in 5G Advanced (Release 18 and beyond). Traditional per-PDU sequencing in PDCP, while effective for bulk data transmission, is not optimal for applications where the meaningful data unit is a larger Application Data Unit (ADU) that may be segmented into multiple PDUs. In scenarios like industrial control or vehicular coordination, losing or misordering even one PDU within an ADU can render the entire ADU useless, causing application failure.
The previous approach relied on higher-layer protocols or applications to handle ADU reassembly and integrity, which could introduce additional latency and complexity. PSSN moves this functionality into the lower layers (PDCP/SDAP), allowing for faster and more efficient processing closer to the radio interface. It solves the problem of ensuring the complete and ordered delivery of ADUs over potentially unreliable wireless links with packet duplication and multi-connectivity. By explicitly identifying all PDUs belonging to the same logical set, the network can optimize retransmissions and reordering, directly contributing to meeting the 99.9999% reliability and sub-1ms latency targets for critical URLLC applications.
Classification
Detected Changes Across Releases
from 3GPP Change RequestsSpecific changes extracted from the „Change history“ tables of 3GPP specifications (55 CRs across 5 releases). Complements the general historical overview above with the evidence-based evolution of this function.
In Release 16, the PDU Set Sequence Number (PSSN) function was introduced to enable RAN and UPF nodes to handle gaps in a QoS flow's data stream, specifically for managing missing PDU Sets. This was part of broader enhancements for QoS monitoring, particularly for URLLC, and for improved PDCP data volume measurements. The PSSN mechanism allows the network to identify and process sequences of PDUs as a set, supporting more efficient handling of applications like extended reality (XR) streams.
- Add use case and definitions of PDCP data volume measurements TS 28.552CR0036
- Add measurements of PDCP data volume in DC-scenarios TS 28.552CR0046
- Add use case for PDCP end user throughput measurements TS 28.552CR0052
- Add PDCP data volume measurements for EE TS 28.552CR0055
- Add Number of PDU session creation in HR roaming scenario TS 28.552CR0112
- PDCP Data Volume measurements update TS 28.552CR0160
+ 12 more changes
In Release 17, the PDU Set Sequence Number (PSSN) function was enhanced to enable network nodes to handle gaps in the PSSN for a specific QoS flow, allowing the UPF and RAN to manage missing PDU Sets. This provides support for applications like XR streams using RTP with end-to-end encryption, where the sender can group tiles as a PDU Set. The release also clarified the optionality of certain header fields for retransmitted PDUs, noting that the PSN and NPDS may not be necessary in those cases.
- Add PLMN granularity for number of active UEs measurements TS 28.552CR0284
- Add PLMN granularity for RRC connection number measurements TS 28.552CR0294
- New measurements for the number of attempted and successfully resumed DRBs TS 28.552CR0303
- Add Time-domain average Maximum Scheduled Layer Number for MIMO scenario TS 28.552CR0322
- Mean number of DRBs undergoing from User Plane Path Failures TS 28.552CR0369
- Correction for the Number of Active Ues measurements TS 28.552CR0309
+ 3 more changes
In Release 18, the specification work for the PDU Set Sequence Number (PSSN) function focused on enabling RAN nodes to handle gaps in the PSSN for a stream within a QoS flow, particularly for managing missing PDU Sets. This involved defining how the network handles retransmitted PDUs, where certain header fields like the PDU Set Size (PSSize) and Number of PDUs in the PDU Set (NPDS) were identified as unnecessary. The enhancements ensure robust handling of PDU Set marking for applications like extended reality (XR) streams, especially when using RTP end-to-end encryption.
- New measurement DL PDCP buffered throughput per UE per DRB. TS 28.552CR0398
- Add performance measurement related with number of notifications for NWDAF data collection TS 28.552CR0431
- Add performance measurement related with number of subscriptions for NWDAF data collection TS 28.552CR0432
- New measurement Number of Idle State RRC release TS 28.552CR0440
- Add number of MA PDU sessions for ATSSS TS 28.552CR0447
- Add Total number of UL PDCP SDU Packets for split gNB deployment scenario TS 28.552CR0451
+ 9 more changes
In Release 19, the PSSN function was enhanced to enable the RAN and UPF to handle gaps in the PDU Set Sequence Number for a stream within a QoS flow, which is critical for managing missing PDU Sets in applications like extended reality (XR). This specifically addresses the need for nodes to process and compensate for these sequence gaps to maintain stream integrity. The release also introduced refinements for handling retransmitted PDUs, where certain PDU Set marking header fields like PSSize and NPDS are deemed unnecessary.
- New performance measurements for Number of UE Capability enquiry and information messages TS 28.552CR0564
- New performance measurements for Number of PDU Session Establishment Requests and Rejects TS 28.552CR0565
- Rel-19 CR 28.552 Add a new measurement related to number of failed NWDAF roaming analytics service subscriptions TS 28.552CR0655
- Rel-19 CR TS28.552 Enhance Number of Active UEs related measurements for RedCap UEs TS 28.552CR0686
- Rel-19 CR TS 28.552 Update number of active UEs related measurements TS 28.552CR0706
- Rel-19 CR TS 28.552 Add new filter for PDCP PDU data volume measurements for RedCap service TS 28.552CR0709
+ 3 more changes
In Release 20, the PSSN (PDU Set Sequence Number) function was enhanced to enable network nodes like the UPF and RAN to handle gaps in the PSSN sequence for a stream within a QoS flow, which is critical for managing missing PDU Sets. This allows for more robust handling of extended reality (XR) streams, particularly when using RTP end-to-end encryption. Furthermore, the specification clarified the handling of retransmitted PDUs, noting that certain PDU Set marking header fields like the PDU Sequence Number (PSN) within a set are not necessary for retransmissions.
- Rel-20 CR 28.552 Add a new measurement related to the number of roaming data collection service subscriptions received by RE-NWDAF TS 28.552CR0717
- Rel-20 CR 28.552 Add a new measurement related to the number of roaming data collection service notifications generated by RE-NWDAF TS 28.552CR0718
- Rel-20 CR 28.552 Add new measurements related to the number of successful and failed roaming data collection service subscriptions received by RE-NWDAF TS 28.552CR0719
- Rel-20 CR TS 28.552 Add Number of UE related the SSB beam Index (Maximum) TS 28.552CR0752
Explore further
Broader topics and technologies where PSSN plays a role.
Defining Specifications
3GPP specifications that define or reference PSSN, with the latest known release. Sourced from the 3GPP document catalog — see methodology.
| Specification | Title | Release |
|---|---|---|
| TS 26.822 vj20 | 5G RTP Configurations Study Phase 2 | Rel-19 |
| TS 28.552 vk10 | 5G Performance Management Measurements | Rel-20 |
| TS 38.415 vj10 | PDU Session User Plane Protocol | Rel-19 |