a band control system for a digital subscriber line network in which a first apparatus and a second apparatus situated at a subscriber station and a center , respectively , are interconnected by a cable for interchanging at least a digital data signal with each other . the system may cause one of the first apparatus or the second apparatus to monitor receipt of signals from the other of the first apparatus or the second apparatus ; send , based on a result of monitoring , a band variation command to the other apparatus for causing the other apparatus to vary a band by using a frequency band not used for signal transfer ; cause the other apparatus to receive the band variation command ; and vary the band in accordance with the band variation command .