main are the Mobile service Switching Center (MSC), Visitor Location Register (VLR) and Gateway MSC (GMSC). The MSC is responsible for the call handling of circuit switched calls, i.e. call setup, routing, inter-system mobility management, etc. The VLR is a database which stores copies of the service profiles and position information of subscribers who are currently being served by the MSC. The MSC/VLR is linked through the Iu-CS interface with the RNC and through the GMSC to the external mobile or fixed telecommunication networks. Packet switched domain consists of Serving GPRS Support Node (SGSN) and Gateway GPRS Support Node (GGSN). The SGSN is in charge of relaying packets from radio networks to the core network. Moreover, it also handles mobility management and session management. While the GGSN serves as a gateway to access outside networks. The SGSN is connected via the Iu-PS with the RNC and via the GGSN with the external packet switched networks. Other network elements in the core network, i.e. Home Location Register (HLR), Equipment Identity Register (EIR), and Authentication Center (AuC) are shared by both packet switched and circuit switched domains. The HLR is a central database that contains detailed relevant information of each subscriber including associated telephone numbers, supplementary services, access priorities and security keys. The EIR is also a database that stores information about the identity of the mobile equipment that prevents calls from stolen, unauthorized or defective mobile stations. The AuC handles the authentication and encryption keys for every subscriber in HLR and VLR. 2.3 Evolution of UMTS This section introduces in detail the path of UMTS evolution, starting from the first UMTS release (Release 99) up to the Long-Term Evolution (LTE) which is at present the latest step being standardized and developed in the 3GPP. For each evolved UMTS radio access network, its main features and key technologies are briefly introduced. 2.3 Evolution of UMTS 2.3.1 UMTS Release 99 Release 