This report covers the principal physical and transmission characteristics of the Bell System customer loop plant. Items covered include a statistical characterization of physical composition, measured and calculated transmission characteristics, and measured noise and crosstalk performance. A survey conducted in 1964 provided the data base for this report and comparisons of data obtained from a similar survey in 1960 illustrate that, in many respects the composition of loop plant changes only slowly with time. Consequently, the 1964 survey results are believed to be representative of today's plant. The types of analyses presented in this paper are of increasing interest to certain Bell System customers because of the increasing number and types of services provided over local telephone facilities.
The problem of protecting apparatus against lightning surges from connected transmission facilities has become more complex with the use of solid state devices in apparatus design. Consideration of the protection requirements for such apparatus has indicated that existing information concerning the incidence and characteristics of lightning surges is insufficient to develop optimum protection measures. A recently completed field investigation provides additional information in this specific area. The results of this field investigation and supplemental laboratory surge tests indicate that, in well‐shielded underground cable pairs, electrical surges do not exceed approximately 90 volts peak, and that transistorized apparatus capable of withstanding such surge amplitudes needs no further protection. In aerial and buried cable, however, transistorized apparatus requires protection up to the full sparkover potential of 3‐mil protector gaps, i.e., to about 600 volts peak. A firm basis for testing and evaluating transistorized apparatus from the lightning surge voltage standpoint is presented.
Although long rural routes comprise less than 2 percent of the total Bell System loop plant, they pose serious transmission and economic problems. Planning for adequate transmission and lowest costs is facilitated by a new voice-frequency electronics plus cable design procedure that provides for an 8-dB EML and a 15-20 percent cost saving over conventional resistance design. Additional cost savings of the same order, together with improvements in noise and loss, are expected in the future through the use of distributed digital multiplexing. The rural-route data base gathered to assure accuracy in economic and service comparisons, together with the studies leading to both the near-term voice frequency plan and the ultimate multiplexing proposals, are described.
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