the present invention discloses a data transmission method for transmitting real - time data from a server to plural terminals via a network having plural local area networks connected to a wide area network . the method includes the steps of grouping plural terminals connected to a same switching hub into a single terminal group , generating a table indicating a rank and an address of each terminal in the terminal group , and transferring the real - time data from a first terminal to a second terminal that is one rank lower than the first terminal in accordance with the ranks indicated in the table .