US Patent 7106834 Digital subscriber line diagnostic system

A communication device transmits very low frequency signals in order to help diagnose the cause of a communication problem in a DSL communication system. The communication problem may be, for example, the inability of a remote data transceiver unit (DTU-R) to successfully train-up with a central data transceiver unit (DTU-C). The very low frequency signals may be used to communicate the status or settings of the remote transceiver unit to the DTU-C, to download settings parameters or executable code to the DTU-R, or to coordinate the transmission of testing signals.