PDU Set Sequence Number

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.