PDCP Buffer Management for 5G Out-of-Order Packets
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Solution Overview
Problem
In 5G/NR dual connectivity scenarios, the significant difference in data speed and latency between 4G/LTE and 5G/NR connections leads to out-of-order packets, causing buffer congestion and overflow at the PDCP layer, which impacts upper layer performance and user experience.
Innovation Solution
Implementing a buffer occupancy counter that triggers the provision of buffered data to the upper layer when a threshold is met, in addition to or as an alternative to the t-reordering timer, to manage the PDCP buffer and reduce dropped traffic and required buffer size.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Productivity
If data is received on both 4G/LTE and 5G/NR connections simultaneously, then data speed and spectral efficiency are improved, but out-of-order packets occur due to different latency and speed characteristics
Solution Approach 1:
The system performs preliminary actions by buffering incoming packets in the PDCP buffer before final delivery to upper layers. This allows the system to receive and temporarily store packets from both 4G/LTE and 5G/NR connections, reordering them if necessary before delivery, thus preventing out-of-order delivery while maintaining high data speeds from dual connectivity
Solution Approach 2:
The PDCP (Packet Data Convergence Protocol) layer acts as an intermediary between the radio layer and upper protocol layers. It mediates the packet flow from multiple connections, performing reordering and buffer management to ensure packets are delivered in the correct order to TCP, thus resolving the packet order issue while maintaining productivity benefits
2Reliability
If a large PDCP buffer is allocated to handle out-of-order packets, then packet loss is reduced, but buffer congestion and overflow occur impacting upper layer performance
Solution Approach 1:
The buffer size is made dynamic rather than static. The system continuously monitors buffer occupancy and adjusts the buffer size adaptively based on current network conditions, traffic patterns, and reordering needs. This allows the buffer to expand when needed to prevent packet loss and contract to avoid congestion and overflow, resolving the contradiction between reliability and device complexity
Solution Approach 2:
The system implements feedback mechanisms by monitoring buffer occupancy levels and using this information to dynamically adjust buffer management parameters. When the buffer approaches dangerous levels, the system responds by adjusting buffer size or triggering packet delivery to upper layers, preventing overflow while maintaining reliability. This closed-loop control resolves the contradiction between having sufficient buffer for reliability and avoiding buffer congestion
3Reliability
If the t-reordering timer is set to accommodate HARQ and RLC retransmission delays, then packet loss rate is maintained low, but buffer congestion occurs before timer expiration
Solution Approach 1:
The t-reordering timer value is made dynamic rather than fixed. The system adjusts the timer duration based on current network conditions, including observed retransmission delays, traffic patterns, and buffer occupancy levels. This allows the timer to be extended when retransmission delays are high (maintaining reliability) and reduced when buffer congestion risk is present (improving productivity), resolving the contradiction between these two objectives
Solution Approach 2:
The system periodically evaluates and adjusts the t-reordering timer based on monitored network conditions and buffer status. Rather than using a static timer value, the system implements periodic reassessment and adjustment of the timer parameter, allowing it to adapt to changing conditions and balance reliability requirements with buffer throughput considerations
4Adaptability or versatility
If 18-bit sequence numbers are used in PDCP to enable reordering, then packet reordering capability is improved, but large reorder buffers are required increasing memory requirements
Solution Approach 1:
The effective buffer size requirement is made dynamic through adaptive buffer management. While 18-bit sequence numbers provide extensive reordering capability, the system dynamically adjusts the active buffer usage based on actual traffic patterns, packet arrival rates, and observed out-of-order scenarios. This allows the system to maintain full reordering capability when needed while reducing effective buffer size during normal operation, resolving the contradiction between adaptability and memory requirements
Data Source
AI summary
A user equipment (UE) can receive a first data stream and a second data stream; store data units of the second data stream, as stored data units, in a buffer while a retransmission operation is performed for the first data stream; determine that a threshold is satisfied with regard to the buffer, wherein the threshold is associated with a counter that is maintained based on the storing of the data units; and provide the stored data units based on determining that the threshold is satisfied.