The ATN (Aeronautical Telecommunication Network) is designed to present a global internetworking infrastructure for future aeronautical telecommunication. This report presents research activities on ATN conducted by ENRI (Electronic Navigation Research Institute) from fiscal 1996 to 2000. In these 5 years, we developed an experimental simulation system called EAST (Enri ATN Simulation Test bed). Using EAST, we verified the protocols and air/ground applications of ATN through the international experiments with Eurocontrol and Air Service Australia. The result of the experiments showed that ICS (Internet Communication Service) and ULCS (Upper Layer Communication Service) protocols of EAST are compatible with those of foreign experiment systems, and it also showed that air/ground applications of EAST have connectivity and compatibility with those of them. To evaluate the performance of the air/ground applications, we also conducted an experiment with EAST and pseudo AMSS (Aeronautical Mobile Satellite Service) subnetwork. The outlines and results of those experiments are also described in this report.