ACE (a short for ASI CNIT Emulator) is a real time simulator of satellite communication systems based on IP packet transmissions. It has been implemented with the financial support of the Italian Space Agency (ASI) by CNIT, a consortium grouping all major technical universities in Italy. ACE is a “virtual satellite” able to operate with real traffic in real time. Using this tool an extensive set of tests can be performed, using both artificial and real traffic, to investigate the satellite performance and the Quality of Service as perceived by the final users. The emulator is based on a cluster of Personal Computers (PC’s), connected through a 100 Mb/s switched LAN. In its current version the system is able to carry up to 80 Mb/s of data traffic per gateway, emulating GEO and MEO satellites.