uture. 3 UMTS Terrestrial Radio Access Network (UTRAN) This chapter gives a more detailed and extensive introduction of the UMTS terrestrial radio access network (UTRAN) including its basic network elements, interfaces, transport channels, protocol stacks, and key management functions. Along with the evolution of UMTS, the UTRAN is under development aiming for an efficient network architecture, much higher transport bandwidths, improved transport network cost-efficiency, and enhanced support for the packet data services. This chapter starts with a detailed introduction of the UTRAN from UMTS Rel99 and then proceeds further towards evolved UTRAN networks along the UMTS evolution path. 3.1 UTRAN Architecture and Interfaces The UTRAN has been introduced in chapter 2.2. Its architecture is highlighted in Figure 3.1. UTRAN consists of one ore more Radio Network Subsystems (RNS) each containing a Radio Network Controller (RNC) and a group of Node Bs. Node B Node B Core Network Node B Node B UTRAN Figure 3.1: UMTS terrestrial radio access network: nodes and interfaces X. Li, Radio Access Network Dimensioning for 3G UMTS, DOI 10.1007/978-3-8348-8111-33 C Vieweg+Teubner Verlag | Springer Fachmedien Wiesbaden GmbH 2011 3 UMTS Terrestrial Radio Access Network (UTRAN) As already introduced in chapter 2.2, there are two internal interfaces defined within the UTRAN: The lub interface is used for communication between the Node B and the RNC. Each Node B is connected with its controlling RNC via an lub interface. The Iur interface is used for communication between different RNCs. It is new to UMTS as compared to GSM, and is typically (but not exclusively) used to support mobiles that are in soft handover to Node Bs controlled by different RNCs without going through the Core Network (CN). In addition the UTRAN defines two external interfaces towards the CN and towards the UE respectively: The Uu interface realizes the radio connection between the UE and the Node B. It is the radio interface between the UTRAN and the UE, based on 