data traffic at a standard Gb-interface. In the other direction, the cellular core signals the UNC at its interfaces, the UNC converts these signals into packets, and the UNC sends the packets to the mobile station over the IP network. The entire process is transparent to both the mobile station and the cellular core. From the mobile station's perspective, the wireless network is simply an additional radio resource. IP-specific functionality is abstracted from the higher level service and control logic. The core cellular network, on the other hand, interacts with the UNC as if it was a conventional base station. At a high level, the UMA network functions as a single cell within the larger cellular network.
The UNC participates in a process of authentication whereby the mobile stationis granted access to the core cellular network. As a part of this process, the mobile station contacts the UNC at an address on the IP network. The UNC contacted may or may not be the UNC that serves the geographic area in which the mobile station is located. A mobile station may contact multiple UNCs before locating the serving UNC and becoming registered as part of the UMA network. A security gateway at the serving UNC employs a challenge-response mechanism to authenticate the mobile station. As part of this process, the mobile station exchanges encrypted communications with the core cellular network. The encrypted communications contain information derived from a subscriber identity module (SIM) located within the mobile station. When authentication is successfully completed, the serving UNC provides