The IMT-2000 networks will use ATM technology for broadband multimedia service. It may provide reliable signalling transport because the ATM broadband capabilities could be used to support fast and vastly expanding signalling applications. This paper presents how ATM signalling system could improve the performance of existing common channel signalling system such as No.7. The IMT-2000 networks will interwork with other networks or the IMT-2000 networks. Then, authentication and registration traffic will be issue in IMT-2000 networks. Then, authentication and registration traffic will be issue in IMT-2000 due to interworking. In the case that interworking occurs frequently in authentication event, the delay time of existing common channel signalling No.7 is exponentially increased. However, the delay time of the ATM-based signalling network is smaller than that of CCS No.7. because the ATM-based signalling network has the distributed architecture network based on multicasting technolgy. Especially, compared with CCS No.7, ATM-based signalling network has better performance in the case that interworking occurs heavily in authentication event.