PDCP Control PDU Design for Sequence Number Gap Reordering
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Solution Overview
Problem
The issue of sequence number (SN) gaps in the Packet Data Convergence Protocol (PDCP) layer of 3GPP standards, particularly exacerbated by PDU set discards, leads to reordering delays at the receiver side, especially in scenarios involving multiple PDCP SDUs and split bearers, impacting application-level throughput and user experience.
Innovation Solution
Implementing a new PDCP control PDU that reports SN gaps by indicating the first discarded PDCP SN and optionally providing a bitmap or count values of subsequent discarded SDUs, allowing the receiver to promptly adjust its state and reset reordering timers.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Reliability
If PDCP SDUs are discarded before successful transmission, then transmission reliability is improved by removing undeliverable data, but sequence number gaps are created causing reordering delays at the receiver
Solution Approach 1:
The transmitter proactively generates and sends a PDCP control PDU with SN gap report information before the receiver experiences reordering delays. This preliminary notification allows the receiver to adjust its reordering timer and process discarded SDUs in advance, eliminating the time loss that would otherwise occur during reordering operations.
Solution Approach 2:
The PDCP control PDU carries feedback information about discarded SDUs and their sequence numbers back to the receiver. This feedback mechanism enables the receiver to understand which SDUs are discarded and adjust its reordering behavior accordingly, resolving the contradiction between reliable discarding and timely reordering.
2Productivity
If multiple PDCP SDUs are discarded in PDU set discard scenarios, then transmission efficiency is improved by clearing failed data sets, but large sequence number gaps are created exacerbating reordering delays
Solution Approach 1:
When PDU set discard occurs, the transmitter immediately generates a PDCP control PDU containing SN gap report information about all discarded SDUs in the set. This preliminary action prevents the receiver from experiencing extended reordering delays that would result from waiting for individual SDU acknowledgments or timer expirations.
Solution Approach 2:
The invention combines information about multiple discarded SDUs into a single PDCP control PDU using bitmap or count-based representations. This merging approach maintains transmission efficiency by clearing the entire PDU set while providing comprehensive notification to the receiver, preventing reordering delays without requiring separate control PDUs for each discarded SDU.
3Adaptability or versatility
If split bearer traffic is dispatched to multiple RLC legs, then network resource utilization is improved by distributing traffic load, but sequence number gaps occur due to imbalanced traffic dispatch
Solution Approach 1:
The PDCP control PDU with SN gap report provides feedback to the receiver about which SDUs were discarded due to imbalanced traffic dispatch on split bearers. This feedback allows the receiver to adjust its reordering timer and process discarded SDUs appropriately, maintaining the adaptability benefits of split bearer while mitigating reordering delays caused by traffic imbalance.
Data Source
AI summary
Various solutions for packet data convergence protocol (PDCP) enhancements on control protocol data unit (PDU) for sequence number (SN) gap report are described. A transmitting apparatus may obtain one or more PDCP service data units (SDUs) from an upper layer. The transmitting apparatus may detect that the one or more PDCP SDUs, each associated with a PDCP SN, are discarded before successfully transmitted to a receiving apparatus. Then, the transmitting apparatus may transmit a PDCP control PDU to the receiving apparatus. The PDCP control PDU may indicate a PDCP SN gap report associated with the discarded PDCP SDUs.