The effect of fluctuating contact resistance in analog telephone subscriber loops

The effect of submillisecond changes in a simulated contact resistance (CR) on the signals transmitted by split-band, full-duplex, two-wire, 1200 b/s voiceband (analog) data modems used with three differently configured telephone loops is studied. A fluctuating, defective electrical junction in a real telephone loop is simulated with an FET-resistor parallel circuit that is optically decoupled from the laboratory instruments that are driving and monitoring the fluctuations. Both the transmitter and receiver can record erroneous data when the simulated junction fluctuations are greater than approximately 5×. The data structure, the duration and amplitude of the resistance fluctuation, and the location of the fluctuating resistance in the transmission line all affect transmission quality. It is shown that practical levels of sealing current are insufficient to eliminate errors in analog data and voice transmission due to fluctuating, degraded junctions