System for controlling frequency multiplexing modems

A frequency multiplexing control system comprising:

• a first modem unit connected to a transmission line;
• a second modem unit connected to said transmission line, the first and second modem units communicating with each other via a plurality of primary channels;
• a secondary channel provided between the first and second modem units, the secondary channel having a frequency band lower than frequency bands of the primary channels;
• a plurality of first means, respectively provided in the first and second modem units, for detecting an abnormal primary channel and sending to the second and first modem units information concerning the abnormal primary channel via the secondary channel;
• and
• a plurality of second means, respectively provided in the first and second modem units, for disabling the abnormal primary channel in response to said information and for sending a training signal for recovering the abnormal primary channel to the transmission line, via the secondary channel.