Controlling Radio Network Controller

Description

The Controlling Radio Network Controller (C-RNC) is a fundamental component within the UMTS Terrestrial Radio Access Network (UTRAN) architecture, defined in 3GPP Release 99 and later. It serves as the controlling entity for one or more Node Bs (the UMTS equivalent of a base station). The C-RNC is responsible for the management and allocation of radio resources within its controlled cells. It terminates the Iub interface towards the Node Bs and the Iur interface towards other RNCs, enabling inter-RNC mobility and resource coordination. Its primary role is to act as the service access point for all services that the UTRAN provides to the Core Network (CN), such as management of connections to User Equipment (UE).

Operationally, the C-RNC executes critical Radio Resource Management (RRM) algorithms. This includes functions like Admission Control, where the C-RNC evaluates whether a new radio access bearer can be established without compromising the quality of existing connections. It also performs Congestion Control to mitigate overload situations, Power Control to manage uplink and downlink transmission power, and Handover Control for managing UE mobility between cells, including soft handovers where a UE is connected to multiple Node Bs simultaneously. The C-RNC manages the macro-diversity combining and splitting for these soft handovers, which is a distinctive feature of WCDMA-based UMTS.

From a protocol perspective, the C-RNC hosts layers of the Radio Network Layer (RNL). It implements the Radio Resource Control (RRC) protocol, which is responsible for the establishment, configuration, maintenance, and release of RRC connections between the UE and the UTRAN. The C-RNC also handles the Radio Link Control (RLC) and Medium Access Control (MAC) layers for user and control plane data. It is the point where ciphering and integrity protection for radio interface communications are typically applied. The C-RNC's functionality is distinct from that of a Serving RNC (S-RNC) or Drift RNC (D-RNC), though a single physical RNC often fulfills multiple logical roles (C-RNC, S-RNC) for a given UE connection.

Purpose & Motivation

The C-RNC was created as part of the UMTS architecture to centralize and intelligently manage the complex radio resource and mobility functions required by WCDMA technology. Previous 2G systems like GSM relied on a Base Station Controller (BSC), but the introduction of CDMA with its soft handover capability and tighter power control requirements necessitated a more sophisticated controller. The C-RNC solves the problem of coordinating multiple Node Bs, managing interference in a CDMA system, and enabling seamless mobility.

Its creation was motivated by the need to separate the complex control plane and resource management functions from the simpler radio transmission/reception tasks of the Node B. This hierarchical division allows for more efficient resource pooling, better scalability, and centralized implementation of advanced RRM algorithms. The C-RNC concept, along with the Iur interface, also addressed the limitation of earlier architectures by enabling direct RNC-to-RNC communication, which is essential for efficient inter-RNC soft and hard handovers without always involving the core network, thereby reducing latency and signaling load.

Key Features

• Terminates the Iub interface and controls one or more Node Bs
• Executes Radio Resource Management (RRM) functions including Admission Control and Congestion Control
• Manages WCDMA-specific soft handover and macro-diversity combining
• Implements Power Control algorithms for uplink and downlink
• Hosts the RRC protocol layer for connection establishment and mobility
• Provides ciphering and integrity protection for radio interface communications

Evolution Across Releases

Defining Specifications