Radio Network Controller

Radio Network Controller

The Radio Network Controller is a governing element in the UMTS radio access network 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 and to the SGSN in the Packet Switched Core Network.
The logical connections between the network elements are known as interfaces. The interface between the RNC and the Circuit Switched Core Network is called lu-CS and between the RNC and the Packet Switched Core Network is called lu-PS. Other interfaces include lub and lur.
lu interfaces carry user traffic as well as control information, and lur interface is mainly needed for soft handovers involving 2 RNCs through not required as the absence of lur will cause these handovers to become hard handovers.
Until 3gpp 4G,all the interfaces in the UTRAN are implemented using ATM only,except the Uu 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-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 lub,lu and lur protocols all carry both user data and signalling. Signalling protocol responsible for the control of the Node B by the RNC is called NBAP.


NBAP is subdivided into Common and Dedicated NBAP, where Common NBAP controls overall Node B functionality and Dedicated NBAP controls separate cells or sectors of the Node B. NBAP is carried over lub. In order for NBAP to handle common and dedicated procedures,it is divided into : Node B Control Port which handles common NBAP procedures and Communication control Port which handles dedicated NBAP procedures. Control plane protocol for the transport layer is called ALCAP.Basic functionality of ALCAP is multiplexing of different users onto one AAL2 transmission path using channel IDs.ALCAP is carred over lub and lu-CS interfaces.Signalling protocol responsible for communication between RNC and the Core Network is called RANAP, and is carried over network is called RANAP, and is carried over lu interface.Signalling protocol responsible for communications between RNCs is called RNSAP and is carried on the lur interface.
RNC Roles, In a relationship to a UE and RNC can play two different roles. These are :

  1. D-RNC : Drift RNC
  2. S-RNC : Serving RNC

    However, as far as the NodeB is concerned, the RNC may play a third role:

   - C-RNC  :  Controlling RNC it is important to know that one RNC can assume more that one role at any time. An RNC also controls the power of a NodeB.
   Universal Mobile Telecommunications System Operations and Maintenance Centre External links Specifications NBAP Specifications RANAP Specifications ,RRC Specifications.

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