Controlling Radio Network Controller

Description

The Controlling Radio Network Controller (CRNC) is a fundamental logical entity within the UMTS Terrestrial Radio Access Network (UTRAN) architecture, specifically defined as a functional role of the Radio Network Controller (RNC). From a physical perspective, the RNC is the network element, but logically, it can operate in different roles relative to a User Equipment (UE) connection: Controlling RNC (CRNC), Serving RNC (SRNC), and Drift RNC (DRNC). The CRNC role is defined for a specific set of Node Bs (base stations) and the cells they operate. For each cell in UTRAN, there is exactly one RNC acting as its CRNC. The CRNC is responsible for the overall control of the logical resources (like channelization codes and scrambling codes) owned by its Node Bs. It performs critical Layer 2 and Layer 3 functions for the Common Control Channels (CCCH) in those cells.

Architecturally, the CRNC connects to its controlled Node Bs via the Iub interface. Through this interface, it manages the Node B's operation, including cell configuration, radio resource allocation, and measurement reporting. The CRNC is the sole point for Operation and Maintenance (O&M) interaction for its Node Bs. It handles the Radio Resource Control (RRC) connection establishment for UEs accessing the network via its cells. When a UE first establishes a connection, the RNC controlling the cell where the Random Access Channel (RACH) is received becomes the CRNC for that initial access and subsequently the Serving RNC (SRNC) for that UE's connection.

The core operational responsibilities of the CRNC include Radio Resource Management (RRM) for its cells. This encompasses admission control, where the CRNC evaluates if there are sufficient radio and transport network resources to admit a new radio access bearer request without degrading existing connections. It also performs congestion control and manages the allocation of radio resources like power and channelization codes. For mobility, the CRNC handles intra-Node B and inter-Node B (but intra-CRNC) handovers, where the UE moves between cells that are all controlled by the same CRNC. It executes these handovers by coordinating the setup and release of radio links on the involved Node Bs.

A key distinction is between the CRNC and the SRNC. While the CRNC is tied to the control of physical Node Bs and cells, the SRNC is a role tied to a specific UE's connection. The SRNC is responsible for the complete RRC connection with the UE, including ciphering, integrity protection, and the termination of the Iu interface towards the Core Network. In many cases, especially when a UE is not in soft handover with cells from a different RNC, the SRNC and the CRNC for the active cell are the same physical RNC. However, when a UE is in soft handover with a cell controlled by a different RNC (a Drift RNC or DRNC), the SRNC and CRNC roles are separated. The CRNC of the drift cell (the DRNC) manages the radio resources of its cell for that UE connection but forwards the data to/from the SRNC via the Iur interface.

Purpose & Motivation

The CRNC concept was introduced in 3GPP Release 99 to address the need for centralized, intelligent control of radio resources in the new Wideband Code Division Multiple Access (WCDMA)-based UMTS network. Unlike 2G GSM, where the Base Station Controller (BSC) had a relatively simpler time-slot based management, WCDMA's code-based, interference-limited air interface required more sophisticated and dynamic resource management. The CRNC role was architected to provide this centralized control point for a cluster of Node Bs, enabling efficient utilization of scarce radio resources like orthogonal variable spreading factor (OVSF) codes and managing the cell's transmit power to control interference.

This centralized control model solved several problems inherent in a distributed radio network. It allowed for coordinated radio resource management across multiple cells, enabling features like soft handover where a UE is simultaneously connected to multiple cells. The CRNC, as the single controller for a set of cells, could make optimal handover and load balancing decisions. It also provided a single point for network management and configuration, simplifying the operation of the radio access network. By separating the logical roles (CRNC, SRNC, DRNC), the UTRAN architecture achieved flexibility, allowing a UE's connection to be anchored at one RNC (SRNC) while utilizing radio resources from cells controlled by other RNCs, which was essential for seamless mobility across large networks managed by multiple RNCs.

Key Features

• Controls logical radio resources (codes, power) for a defined set of Node Bs and cells
• Performs admission control and congestion control for its controlled cells
• Manages the Iub interface towards its subordinate Node Bs for operation and maintenance
• Handles Layer 2/Layer 3 functions for Common Control Channels (CCCH) in its cells
• Executes intra-CRNC handovers (mobility between cells under its control)
• Serves as the termination point for the NBAP (Node B Application Part) protocol from its Node Bs

Evolution Across Releases

Defining Specifications