Development of a directional coupler to be used on the two-wire sections of dial-up telephone lines

This project aims to develop a directional coupler for use on dial-up telephone lines. The need for such a coupler originated with the acceptance of the CCITT's (International Telegraph and Telephone Consultative Committee) V.32 bis recommendation regarding the operation of high-speed modems. This standard allows modems to communicate on the telephone network both simultaneously as well as within the same frequency band. The modems involved use a mechanism called echo cancellation to overcome this problem (with echo cancellation the modem subtracts that which it transmits from the signal on the line to get the signal it wishes to receive). However, a third party surveying the (two-wire sections of the) line is unable to interpret the signals on the line as there appears to be only one garbled signal. This paper develops the theoretical basis necessary for a directional coupler that will separate this garbled signal into its constituent signals and thus enable interpretation of such signals by a third party.