Abstract: A trunk line identification system includes a line switch part connected to each of a plurality of channels of a transmission line, channel processing parts for processing signals received from and transmitted to the transmission line via the line switch part, where the channel processing parts are provided with respect to each of the channels and include first and second parts, and a switching system for switching signals. When relaying a signal received by a first channel processing part from the transmission line via the line switch part to the transmission line via the switching system and a second channel processing part, the first part of the second channel processing part transmits an identification signal to the first channel processing part via the switching system, and the second part of the first channel processing part judges whether or not a line being used is a trunk line based on whether or not the identification signal is detected.

Abstract: An improved data communication system in which a block polling of a plurality of terminal blocks is first carried out, thereafter, a terminal polling of terminals in the terminal block(s) from which a response to the block polling has been issued is carried out. The data communication system includes a center office, a cable and a plurality of terminals which can be grouped in a plurality of terminal blocks. The center office includes a MODEM, a block polling unit, a response monitor unit for detecting a data transfer request signal from the terminal blocks during the block polling, and a terminal polling unit which sequentially effects a polling of terminals in the terminal block from which the request signal has been issued.