Radio Access Network user plane congestion

Description

RAN user plane congestion is a condition in which the radio access network (RAN) cannot adequately handle the user plane traffic demand, resulting in performance degradation. It is defined as the situation where the demand for RAN resources to transfer user data exceeds the available RAN capacity to deliver the user data for a significant period of time, typically in the order of a few seconds or longer. This congestion can manifest as packet drops, increased latency, and reduced throughput, which may or may not be noticeable to the end user depending on the severity and the application requirements.

The causes of RAN user plane congestion are multifaceted. It can result from a surge in user demand, such as during peak hours or at special events, or from insufficient RAN capacity due to limited spectrum, hardware constraints, or poor radio conditions. Congestion can occur in various parts of the RAN, including the air interface, backhaul, or baseband processing. The 3GPP has defined mechanisms to detect, report, and mitigate congestion, involving both the RAN and the core network.

Detection of RAN user plane congestion typically involves the RAN monitoring its resource usage and comparing it against thresholds. When congestion is detected, the RAN can signal this to the core network, which can then take actions such as applying congestion mitigation policies. These policies may include traffic shaping, prioritization, or offloading to WLAN. The core network elements involved include the PCRF (Policy and Charging Rules Function) and the PCEF (Policy and Charging Enforcement Function), which can enforce policies based on congestion reports. In 5G, similar mechanisms exist with the PCF and SMF.

Mitigation strategies for RAN user plane congestion include radio resource management techniques, such as scheduling and admission control, as well as core network-based approaches like congestion-aware policy control. For example, the network can reduce the bit rate of non-critical applications, delay less time-sensitive traffic, or steer traffic to alternative access networks like WLAN. 3GPP specifications such as TS 22.101, TS 22.805, TS 23.060, TS 23.203, TS 23.401, TS 23.705, TS 29.212, TS 29.213, and TS 29.217 provide the requirements and procedures for handling congestion.

RAN user plane congestion is a key consideration in network planning and optimization. Operators strive to minimize congestion through adequate capacity planning, efficient radio resource management, and the deployment of additional spectrum or small cells. The standardized mechanisms for congestion detection and mitigation enable a coordinated response across the network, ensuring that critical services maintain quality while less critical traffic is managed appropriately. Understanding RAN user plane congestion is essential for engineers designing and operating mobile networks to deliver satisfactory user experience under varying load conditions.

Purpose & Motivation

RAN user plane congestion management was introduced to address the growing challenge of maintaining quality of service in mobile networks as data traffic volumes increased. Before standardized mechanisms, networks lacked a coordinated way to detect and respond to congestion in the RAN, leading to unpredictable performance and user dissatisfaction. The purpose of defining RAN user plane congestion is to provide a clear, measurable condition that can trigger mitigation actions across both RAN and core network domains.

The motivation stems from the need to ensure that when congestion occurs, the network can take appropriate actions to protect critical services and manage user expectations. For example, during congestion, the network might prioritize voice over data, throttle heavy users, or offload traffic to WLAN. Standardized congestion handling allows operators to implement policies consistently and enables interoperability between equipment from different vendors.

Furthermore, the definition evolved to specify a 'significant period of time in the order of few seconds or longer' to distinguish transient fluctuations from sustained congestion that warrants intervention. This refinement helps in avoiding unnecessary reactions to momentary spikes. The concept is integral to policy and charging control frameworks, enabling dynamic policy adjustments based on RAN conditions. Overall, RAN user plane congestion management solves the problem of resource scarcity by enabling intelligent, policy-driven responses that balance network efficiency and user experience.