Patent ID: 6914981

Claim:
A communication device comprising: cut-off frequency switching means for switching a cut-off frequency for communication data at a communication line; and confirming means for confirming whether or not a communication partner has said cut-off frequency switching means, in advance of sending and receiving of communication data, wherein said cut-off frequency switching means is structured to carry out switching of a D.C. cut-off capacitor, wherein said confirming means is performed while said cut-off frequency means has switched the D.C. cut-off capacitor to be disengaged from the communication line, and wherein said cut-off frequency means maintails the D.C. cut-off capacitor to be disengaged when it is determined that the communication line is part of a multiplex communication system that includes a telephone network line and a data network line, irrespective as to whether or not the communication partner has said cut-off frequency switching means.