ated. GMSC is the point where the UMTS network is connected to PSTN. It maintains communication with the MSC as calls are transferred from one system to another or to different MSCs within the same system. SMSC: Short Message Service Center (SMSC) is a network element in the GSM network which delivers SMS (short message service) messages. VLR: VLR, a subscriber database, located within the CS domain usually with MSC, supports subscriber mobility. It temporarily holds user's required HLR information when the user is out of its home MSC region and then erased once the user either is registered with either another MSC or another system or inactivity is observed for a specified period. The VLR eliminates the need for the MSC to continually check with user's HLR every time an access is attempted. Packet Switched Elements SGSN: The 3G-SGSN provides both user-plane and control-plane PS related functions. In the control plane, these functions include the GPRS mobility management (GMM) and Session Management (SM) functions. The SGSN also contains a subscriber database, similar to the VLR (Visitor Location Register), which serves as intermediate storage for subscriber data to support subscriber mobility. It provides necessary control and signaling functionality to both UTRAN and GGSN and it may optionally communicate with the MSC with specific interfaces. The procedure for information transfer between the SGSN, the GGSN, the VLR, and the HLR are defined in 3GPP TS 23.016 [5] and 3GPP TS 23.060 [6]. GGSN: GGSN is a network element in the PS domain that serves as a gateway providing connectivity to external packet data networks (PDNs) over the Gi interface. It is connected with SGSN via an IP-based network. It could be considered as a typical router (Home Agent) since it hides the GPRS infrastructure from the external network and for implementing ad- ditional functionality for supporting mobile services. Such additional functionality includes the GPRS Tunneling Protocol (GTP), which manages user mobility by establishing and 