The terrestrial network supporting fax and modem modes – the Public Switched Telephone Network (PSTN) – is still predominantly analogue in nature as far as the local loop (subscriber connection) is concerned. The local loop connects the end user (subscriber) with the global PSTN via a hierarchy of exchanges (central offices). The exchange provides -48 VDC line voltage used for signaling and dialing and a 90-120 VAC ringing voltage. The subscriber loop uses a balanced two-wire pair for both transmission and reception. In the telephone set and in the exchange line card a special device called a hybrid separates the two signal directions. Leased subscriber lines can be ordered as four wire lines having a dedicated pair for each signal direction.
WARNING: DO NOT CONNECT ANY OF THE DECODER OR PC SOUND CARD INPUTS DIRECTLY TO A TELEPHONE LINE! Doing so is illegal and will seriously damage your PC. Use a suitable line adapter or a recorded signal.