In this thesis, a gateway solution for internetworking between LAN and PSDN is investigated. The proposed gateway model consists of two parts; the first part handling the subnetwork specific function and the second part filling gaps and differences in each subnetwork's protocols. The first part of the thesis presents a systematic approach to identify and parametrize the services and protocols operating in each subnetwork. The result of the first part then acts as an input designing the mapping function. The second part of the thesis analyzes the different subnetwork's characteristics and shows the performance characteristics in the gateway where two subnetworks with large different traffic are connected to. This is accomplished by using Simulation Language for Alternative Modeling (SLAM). This analysis formed the base for the design guidelines of the mapping task. The third part of the thesis discusses in detail the implementation of the mapping function into a mapping task in a real gateway. The mapping task is implemented in 'C' language and the initial design is done by Program Description Language(PDL).