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The radio network controller (or RNC) is a governing element of the UMTS radio access network (UTRAN) and is responsible for controlling the nodes Bs that are connected to it. The RNC performs radio resource management, some of the mobility management functions and is the point where the encryption is performed before the user's data is sent to and from the mobile. The RNC is connected to the central circuit switching network via the media gateway (MGW) and the SGSN (service support node GPRS) in the packet switching network.

Logical connections between network elements are known as interfaces. The interface between the RNC and the circuit-switched network (CS-CN) is called Iu-CS and between the RNC and the switched packet switched network is called Iu-PS. Other interfaces include Iub (between RNC and Node B) and Iur (between RNCs in the same network). The Iu interfaces carry user traffic (such as voice or data) as well as control information (see Protocols), and the Iur interface is mainly required for soft handoffs involving 2 RNCs although it is not necessary since the absence of Iur will cause these handovers to become handovers.

Up to 3gpp R4, all UTRAN interfaces are implemented using only ATM, except the Uu (air) interface that uses WCDMA technology. Starting R5, IP carriers can be used over Ethernet instead. Physically, these interfaces can be carried over SDH over optical fiber, E1 (sometimes called PDH) - over a copper wire or microwave radius. Several E1 can be grouped together to form an IMA Group. Since the interfaces are logical, many interfaces can be multiplexed on the same transmission line. The actual implementation depends on the network topology; examples are string, distant star, mesh, and loop configurations.