ontrols multiple Node Bs. The number of Node Bs connected to one RNC varies depending on the implementation and deployment. 3.2 Generic Protocol Model for UTRAN Interfaces 3.2 Generic Protocol Model for UTRAN Interfaces Figure 3.2 presents the general UTRAN interface protocol model. It horizontally separates all the UTRAN related functionalities from the underlying terrestrial transport technology into two layers: Radio Network Layer (RNL) and Transport Network Layer (TNL), respectively. All UTRAN related issues are visible only in the Radio Network Layer. The Radio Network Layer comprises the protocols that are especially designed for the UMTS system, dealing with the management and use of Radio Access Bearers (RABs) across the UTRAN. It is independent from the underlying transport technology. The Transport Network Layer represents standard transport technology that is selected to be used for UTRAN but without any UTRAN-specific changes. It is responsible for the management and use of transport bearers, including the signaling, establishment, maintenance and release of transport bearers. Radio Control Plane User Plane Network Layer Application Protocol Stream(s) Transport Transport Network Transport Network Transport Network Network Plane Control Plane Plane Layer ALCAP(s) Signalling Signalling Bearer(s) Bearer(s) Bearer(s) Physical Layer Figure 3.2: General Protocol Model for UTRAN interfaces [3GP02c] In addition to horizontal layers, the UTRAN protocol structure is vertically divided into two planes: Control Plane and User Plane. The Control Plane consists of Application Protocols and Signaling Bearers. The Application Protocols take care of all signaling functionality in the UTRAN required for setting up and maintaining Radio Access Bearers and Radio Links, which include setting up dedicated signaling and user connections and exchanging other UTRAN-specific control information. The Signaling Bearers are used for transporting the application protocol messages. The involved application protocols in the control 