A method for transferring time-synchronous data, particularly voice and video data, over a network, particularly the Internet, between at least two terminals, where between the terminals a connection is established using a SIP server and where the SIP protocol is used for establishing the connection, is—with respect to high Quality of Service for transferring time-synchronous data and with a technically simple and cost effective design—developed in a way that the SIP server analyzes the connection and/or the terminals or the like and that based on the analysis it determines an ideal bandwidth for optimizing the transfer of time-synchronous data.