Traditionally, and up to this date, Land Mobile Radio (LMR) systems have been interconnected via leased lines and microwave links across circuit-switched networks. With the recent deployment of digital LMR standards such as the Association of Public and Communications Officials (APCO) Project 25 and the Terrestrial Trunked Radio (TETRA), traffic exchange has become more bursty and non-uniform, and as such, less suitable for circuit-switched networks. This thesis proposes a framework for a packet-switched Inter-System Interface (ISI) for LMR systems. Packet-switched networks have the advantage of supporting traffic integration, utilize capacity efficiently, scale easily and seamlessly, and eliminate single points of failure by providing a distributed architecture. Session Initiation Protocol (SIP) signaling messages are defined for setting up and tearing down unit-to-unit calls across the ISI. The Session Description Protocol (SDP) is used to describe how the voice calls are encoded. Voice packets are exchanged between LMR users using the Real-Time Transport Protocol (RTP). Based on the proposed framework, we develop a simulation model to investigate the performance of the ISI when different numbers of LMR users try to establish unit-to-unit calls across the packet-switched ISI. Three packet transport technologies providing Wide Area Network (WAN) connectivity are considered, IP, ATM, and Frame Relay. The results indicate that a packet-switched ISI can take advantage of statistical multiplexing techniques to distribute network resources more efficiently. Quantitative results are obtained for throughput and link utilization. When using an access link providing T1 service, we show that the End-To-End (ETE) delay, and delay variation can be controlled at levels capable of supporting the timely delivery of real-time voice packets. Assuming link utilization is maintained below 100%, the maximum ETE delay experienced in all three packet transport technologies considered is 58 ms and the maximum call setup time is less than 300 ms. An ATM WAN provides the best performance for all time-dependent metrics considered, i.e., ETE delay, delay variation, and call setup time. An IP WAN provides the highest bandwidth efficiency. Selecting the appropriate packet transport technology for the WAN is a tradeoff between the delay that can be tolerated by the voice packets traversing the LMR network and the cost of bandwidth on the access link. / Master of Science