Radio Admission Control

Description

Radio Admission Control (RAC) is a critical decision-making algorithm residing in the network controller of the Radio Access Network (RNC in UMTS, eNB in LTE, gNB in NR). It acts as a gatekeeper for new radio bearer establishment requests, which are typically triggered by a Core Network request for a Radio Access Bearer (RAB) setup or by the UE for a signaling connection. When such a request arrives, the RAC function evaluates whether the radio network has sufficient available resources to support the new bearer while maintaining the agreed Quality of Service (QoS) for all existing, active bearers.

The RAC algorithm operates by comparing the resource requirements of the requested bearer against the current load and capacity of the relevant cell or sector. The key inputs include the QoS parameters of the new request (e.g., guaranteed bit rate, priority level, packet delay budget) and real-time measurements of the current cell load. This load can be measured in various dimensions: uplink/downlink interference levels (crucial in UMTS WCDMA), physical resource block (PRB) utilization in LTE/NR, hardware processing load, or transport network bandwidth availability. The algorithm uses pre-configured thresholds and policies to make its decision. For example, it may block a new high-bit-rate video bearer if the cell's interference is already near a stability limit, but admit a low-priority background bearer.

If the request is admitted, the RAC function reserves the necessary resources (in a logical sense) and allows the subsequent radio bearer setup procedures to proceed. If rejected, the request is denied, and the Core Network or UE is notified, often leading to a "network busy" indication to the user. RAC is closely intertwined with other RRM functions like Congestion Control and Packet Scheduling. While RAC makes the initial admission decision, Congestion Control monitors the cell continuously and can take corrective actions (like degrading some bearers) if the load exceeds safe limits after admission. RAC is therefore a proactive measure to maintain network stability and service quality, which is especially vital for services with strict guarantees like Voice over LTE (VoLTE) or real-time gaming.

Purpose & Motivation

RAC exists to ensure the long-term stability and predictable performance of a cellular network, which is a shared resource with inherently limited capacity. Without admission control, the network could admit more users and sessions than it can physically handle, leading to a "tragedy of the commons" scenario where excessive interference or resource contention degrades the quality for all connected users, potentially causing widespread call drops or unusable data rates.

This problem was particularly acute in interference-limited systems like UMTS WCDMA, where every new connection increases the overall noise floor, affecting everyone else. RAC was introduced as a fundamental RRM function from the earliest 3G standards to prevent such collapse. It allows network operators to enforce capacity planning policies and prioritize certain services or subscribers. For example, an emergency services call or a premium subscriber might be admitted even when the cell is near capacity, while a best-effort data request might be blocked. The evolution from homogeneous voice networks to heterogeneous multi-service, multi-RAT networks has only increased RAC's importance. It is a key enabler for service differentiation and network slicing, as it must understand and enforce the distinct resource requirements of a massive IoT sensor uplink versus an ultra-reliable low-latency communication (URLLC) industrial control link.