A bidirectional transmission path is typically coupled to unidirectional receive and transmit paths to effect amplification in repeaters or the like. A determination of whether the bidirectional path includes either loaded or nonloaded type 2-wire cable is made by inserting a test signal having a predetermined...http://www.google.com.tw/patents/US4368362?utm_source=gb-gplus-share專利 US4368362 - Automatic telephone loaded/nonloaded facility type identification circuit
