The present invention provides a communication control method between DSL modem apparatuses mutually connected via a metallic cable. One of the DSL modem apparatuses transmits transmission data using a plurality of sub-carriers simultaneously, while the other DSL modem apparatus decodes the transmission data in the plurality of sub-carriers. The both sides of DSL modem apparatuses exchange data signals by mutually synchronizing and alternatively sharing transmission timeslots and reception timeslots, using the same range of sub-carriers. In addition, the upstream (data transmission from a remote side to a center side) uses the same range of carrier indexes that are used in the downstream (data transmission from the center side to the remote side).