THE TYPE PI CARRIER SYSTEM 351 



This was especially important since previous Bell System experience 

 indicated that where commercial power is used to supply the system, 

 some form of reserve must be provided, and where commercial power is 

 not available, the use of primary batteries places a premium on mini- 

 mizing the power i-equired. 



From these considerations two additional major objectives were de- 

 rived : low manufacturing costs for the components and assembled equip- 

 ment, and the use of transistors to minimize power supply drains. In 

 addition, flexibility was needed in the proposed carrier system because 

 of the difficulty of accurately forecasting the demand for rural service. 



These objectives have been met in the design of the Type Pi rural 

 customer telephone system. It is a fully-transistorized system con- 

 sisting of independent two-way carrier channels applicable in increments 

 of one to four at a time in the frequency band above the regular voice 

 frequency circuit. Each channel uses a terminal at the central office and 

 at a remote point with intermediate repeaters as necessary. Between 

 terminals, the system is equivalent to a rural voice frequency line with no 

 changes required in the central office or rural customer equipment. Be- 

 yond the outlying terminal, distribution is by voice frequency wire on 

 a single or multiparty basis. The system can be applied to existing and 

 new lines utilizing combinations of fine gauge exchange cable and copper 

 or steel open- wire. Systems can be used on each of several pairs on a 

 given pole line, the number depending on the line characteristics. 



3. MAJOR PARAMETERS OF PI CARRIER SYSTEM 



This section summarizes the important features incorporated in the 

 PI carrier system and the reasons governing their choice. The system 

 has a number of features in common with Bell System toll carrier sys- 

 tems, but it also differs in several important aspects because of specific 

 rural requirements. One aspect is the signaling, which requires different 

 arrangements at the two ends of the circuit because of the widely' differ- 

 ent signals carried in the two directions. Another is that the remote ter- 

 minals of the individual channels are usually distributed along the line 

 rather than grouped at a common location. 



3.1 Transmission Plan 



It is difficult to divorce the considerations leading to the choice of 

 carrier frequency range from those affecting the choice of modulation 

 in the carrier system. Studies of growth on rural lines indicated that a 

 system giving three or four channels (customer circuits) on one pair of 

 wires, in addition to the physical circuit, should be sufficient if systems 

 could be applied to each of several pairs on a given open-wire line. 



