Radio Network Controller
The Radio Network Controller (RNC) is a governing element in the UMTS radio access network (UTRAN) and is responsible for controlling the Node Bs that are connected to it. The RNC carries out radio resource management, some of the mobility management functions and is the point where encryption is done before user data is sent to and from the mobile. The RNC connects to the Circuit Switched Core Network through Media Gateway (MGW) and to the SGSN (Serving GPRS Support Node) in the Packet Switched Core Network. Interfaces. The logical connections between the network elements are known  as interfaces. The interface between the RNC and the Circuit Switched Core Network (CS-CN) is called Iu-CS and between the RNC and the Packet Switched Core Network is called Iu-PS. Other interfaces include Iub (between the RNC and the Node B) and Iur (between RNCs in the same network). Iu interfaces carry user traffic (such as voice or data) as well as control information (see ), and Iur interface is mainly needed for soft handovers involving 2 RNCs though not required as the absence of Iur will cause these handovers to become hard handovers. Until 3gpp R4, all  the interfaces in the UTRAN are implemented using ATM only, except the Uu (air) interface which uses WCDMA technology. Starting R5, IP bearers can be used over Ethernet instead. Physically, these interfaces can be carried over SDH over optical fiber, E1 (sometimes referred to as PDH) - over a copper wire or microwave radio. Several E1s can be bundled to form an IMA Group. Since the interfaces are "logical", many interfaces can be multiplexed onto the same transmission line. The actual implementation depends on the network topology; examples are chain, distant star, mesh and loop configurations. Protocols. Iub, Iu and Iur  protocols all carry both user data and signalling (that is, control plane). RNC roles. In a relationship to a UE (in a soft handover situation) an RNC can play two different roles. These are: However, as far as the NodeB is concerned, the RNC may play a third role: It is important to know that one RNC can assume more than one role at any time. An RNC also controls the power of a NodeB.