350 THE BELL SYSTEM TECHNICAL JOURNAL, MARCH 1957 



1. INTRODUCTION 



Although carrier has been used successfully to provide trunks in the 

 Bell System for more than 35 years, it has not been economically feasible, 

 up to the present time, to apply carrier telephone techniques extensively 

 to the rural telephone plant. The technical and economic problems as- 

 sociated with providing telephone service to customers in rural areas has 

 long been one of the most difficult problems facing the telephone indus- 

 try. The widely scattered locations of customers in rural areas have led 

 to a large number of rural telephone routes with only a few customer 

 lines per route. This has precluded the use of large cables on any one 

 route, which would be economically attractive in urban areas. The ex- 

 tensive use of carrier has not been feasible because the distances from 

 the rural customers to the Central Ofhce are in the 5- to 20-mile range 

 in which carrier has not been generally economical in the past. 



The two lines of attack which were taken on this problem were to 

 reduce the cost of telephone plant through less expensive small cables 

 and open-wire plant,^ and to provide an economically attractive carrier 

 system designed to meet the particular needs of rural telephone service. 



This paper discusses the broad objectives for a rural customer carrier 

 system, the major parameters of the Pi system which was developed to 

 meet those objectives, and its circuit, equipment, and power arrange- 

 ments. It also covers the engineering and maintenance methods to be 

 used by the Bell System Operating Companies to install and operate 

 the system. 2 



2. BROAD OBJECTIVES FOR PI CARRIER SYSTEM 



The broad objectives for the Type PI carrier system resulted from 

 the stringent economic limits imposed on the system to enable it to prove 

 in over conventional rural plant of the latest and most economical de- 

 sign, from the requirements of rural telephone transmission and signaling, 

 and from Bell System experience gained with earlier carrier systems for 

 customer and trunk use. The low cost objective for this system also im- 

 plied the need to achieve an appropriate balance among an economic 

 first cost of equipment, low in-place cost due to simplified engineering 

 and installation practices, and accompanying low annual costs due in 

 part to simplified system maintenance. 



To achieve an economic carrier system for rural telephone use, the dc 

 power requirements of the terminals and repeaters had to be kept low. 



' Lester Hochgraf and R. G. Watling, Telephone Lines for Rural Subscriber 

 Service, A.I.E.E. Communication and Electronics, No. 18, p. 171, Ma}', 1955. 



2 These aspects of the PI system are covered in more detail in four papers on 

 "The PI Carrier System." A.I.E.E. Communication and Electronics, No. 24, 

 pp. 188, 191, 195, 205, May, 1956. 



