diagram of the set top box according to the present invention the set top box of fig2 includes a network interface 200 a controller 202 an input unit 204 a codec 206 an up sampler 208 and a summer 210 the controller 202 checks an ip address and a port number for joining a base channel through the epg information received from the ip network when power is applied to the set top box and accesses the base channel through the network interface 200 using the checked ip address and the port number the controller 202 receives the multiplex packet over the base channel accessed by the network interface 200 and interprets the pat pmt program association table program map table information contained in the multiplex packet received over the accessed base channel to perceive and store the down broadcasting pids allocated for the down broadcast data the controller 202 obtains the selected channel information from the epg when a user selects and requests a certain channel through the input unit 204 the selected channel information contains the ip address or and the port number set for transmitting the difference packet the controller 202 extracts the down packet having the down broadcasting pid corresponding to the selected channel information from the multiplex packet received over the base channel and provides it to the codec 206 the codec 206 can be implemented using a software layer method and decodes the down packet provided from the controller 202 and outputs it as the down broadcast data the up sampler 208 up samples the down broadcast data output from the codec 206 thereby converting it into the broadcast data of a prescribed size and outputs it to the digital television through the summer 210 the.