ttached cells. It logically corresponds to the Base Station Controller (BSC) in GSM. The Node B is the counterpart of the Base Transceiver Station (BTS) in GSM. It supplies one or several cells. The UTRAN establishes connectivity between the UE and the core network, and is responsible for managing and operating the radio access to the UE, like radio resource management and handover control. The 3GPP standard defines four interfaces related to the UTRAN: Iu, Uu, lub and Iur. The Iu interface is the interface that connects the UTRAN to the Core Network. Dependent on the type of traffic, i.e. packet switched or circuit switched traffic, the Iu is divided into Iu-CS (Iu-Circuit Switched) and Iu-PS (Iu-Packet Switched). The Uu is the radio interface of UMTS based on WCDMA technology, located between the UTRAN and the UE. It realizes the radio connection between the UE and the Node B. The lub and Iur interfaces are used to connect the functional entities inside the UTRAN. The lub interface connects RNC and Node B, while the Iur interface connects two RNCs with each other. A more extensive introduction of UTRAN interfaces is given in chapter 3.1. CN CS Domain Node B 3G MSC/ PSTN, ISDN, PLNM.. Node B HLR/AuC/ Registers Node B X.25, Internet Node B CN PS Domain UTRAN Core Network External Networks Figure 2.1: UMTS Network Structure 2 UMTS Evolution The CN is the backbone of UMTS, connecting the radio access network to other external networks like the Public Switched Telephone Network (PSTN), Internet or other data networks. The UMTS core network evolved from the current 2G networks based on GSM/GPRS, SO that it provides backward compatibility and interoperability between GSM/GPRS and UMTS. In this way, it allows GSM mobile operators to directly proceed to UMTS cellular systems by simply replacing the Radio Access Networks (RAN) without significant changes in the core network architecture. Basically, the CN is divided into circuit switched and packet switched domains. The main functional entities in the circuit switched do