As the access part of a mobile communication network is evolving towards third generation (3G) to support new air interfaces, so is the architecture of the core network. In order to get the maximum return from their investment, service providers want a network that would be adequate for current customer needs, but at the same time be able to provide new, emerging services by simply adding some new capabilities in the form of a hardware and/or software upgrade to their existing equipment. Because the second generation wireless systems are required to support only limited data, such as short messaging services and slow-speed circuit-switched or packet mode data, the current network is principally circuit-switched, but includes an entity called the interworking function to provide the data capabilities. Now, however, the demand for higher data rates is growing at a rapid rate. Because packet-switched networks are inherently more efficient for data services, networks are evolving that combine the more common, ubiquitous circuit-switched fabric with elements of a packet-switched network. One such example is the general packet radio service (GPRS) that can support packet mode data at rates up to about 160 kb/s [9], [10]. In view of the requirements of the 3G systems for both constant and variable bit rate services, the need for such a network appears to be even more compelling than ever before. In fact, because of these 3G requirements and emerging applications (such as conversational voice and video, interactive data, high volume data transfer, and...