1980 Bell System Noise Survey of the Loop Plant

This paper presents the principal findings of measurements taken in 1980 of voltages and current induced in Bell System loops by commercial power lines. These findings give designers of new, solid state, electronic switching and terminal equipment a detailed characterization of power induced signals. The last Bell System noise survey was in 1964, and since then power system load and telephone loop length have increased. Use of shielded cable has grown, and the number of poorly balanced party lines has decreased. The net result is that the loop plant contribution to main-station levels of induced longitudinal and metallic voltages indicate only a slight increase at the 90th percentile of 1 to 2 dB since 1964 — an increase too small to be considered statistically significant. When the central office and loop plant contributions are combined, main station metallic noise shows a decrease at the 90th percentile of 6 dB since 1964. Because of diurnal variations, distributions of peak noise levels on a loop over a 24-hour period are 4 to 6 dB higher than corresponding distributions based on one-time measurements made during business hours. This survey provides the first characterization of induced longitudinal current based on measured data. Finally, as a result of the stratified, two-stage sampling scheme used for the survey, differences among urban, suburban, and rural environments have been identified, and the unique behavior of long rural loops has been highlighted.