RNC RLC Buffer Congestion Management
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Solution Overview
Problem
In cellular access systems, the Transport Network (TN) often becomes a bottleneck during congestion situations, leading to inefficiencies in traffic handling between Radio Base Stations and User Equipments (UEs) through the Radio Network Controller (RNC).
Innovation Solution
The RNC is equipped with a Radio Link Control (RLC) function to detect congestion in the Transport Network and discard Service Data Units (SDUs) in receive buffers at either the UE or RNC, using commands like Move Receiver Window and Explicit Congestion Notification to manage congestion by dropping SDUs and transmitting ACKs.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Reliability
If the RNC continues to transmit RLC SDUs during congestion, then the buffer occupancy increases, but the congestion situation worsens and retransmissions increase
Solution Approach 1:
The RNC performs preliminary actions by detecting congestion conditions and proactively discarding RLC SDUs from transmit buffers before the buffers become completely full. This preliminary discarding prevents further congestion escalation and reduces the need for retransmissions, thereby maintaining reliability while managing throughput during congestion events
Solution Approach 2:
The invention applies discarding by selectively dropping RLC SDUs in the RNC transmit buffer when congestion is detected. The discarded SDUs are not retransmitted, which prevents further congestion while the system recovers. This approach trades off some data delivery for overall network stability and prevents the congestion spiral that would occur with continuous transmission
2Productivity
If the RNC discards RLC SDUs during congestion, then the congestion is mitigated, but data loss occurs in the receive buffer
Solution Approach 1:
The invention applies local quality by making the discarding action location-specific - RLC SDUs are discarded only in the RNC transmit buffer when congestion is detected in specific directions (uplink or downlink). The discarding is not global but localized to the affected buffer and direction, preserving data integrity in non-congested paths while managing congestion in affected paths
Solution Approach 2:
The system changes the parameter of buffer occupancy by actively reducing it through selective discarding of RLC SDUs when congestion thresholds are reached. This parameter change (reducing buffer occupancy) directly addresses the congestion condition while the system monitors and adjusts discarding behavior based on ongoing congestion status
3Device complexity
If the RNC detects congestion using traditional methods, then the detection is simple, but the response time is delayed and congestion handling is inefficient
Solution Approach 1:
The invention implements feedback mechanisms where the RNC continuously monitors buffer occupancy levels and transmission conditions, and uses this feedback to dynamically adjust discarding decisions. The system responds to real-time congestion indicators by modifying its transmission and discarding behavior, enabling timely congestion handling while maintaining a relatively simple detection framework
Data Source
AI summary
A Radio Network Controller, an RNC (105), for an HSPA enabled cellular access system. The RNC is arranged to be connected to a Radio Base Station (110) by a Transport Network (120) through which the RNC (105) sends and receives traffic to and from UEs (115). The RNC (105) comprises a function (210) for Radio Link Control, RLC, for the UEs, and is arranged to detect congestion in the Transport Network (120) to or from each UE. The RNC (105) is arranged to, when detecting congestion in the Transport Network (120) to or from a UE (115), use its RLC function (210) to discard an RLC Service Data Unit, SDU, in a receive buffer, the receive buffer being in the UE (115) if the congestion is in the direction to the UE (115) and in the RNC (105) if the congestion is in the direction from the UE (115).