2 THE BELL SYSTEM TECHNICAL JOURNAL, JANUARY 1951 



tion of requirements, made possible by limiting the system to 200 miles 

 (instead of the usual transcontinental 4000 miles), has been an important 

 factor in helping to meet the low-cost objective. Elimination of the need for 

 two cables permits use of a large part of the five million miles of toll and 

 exchange circuits in Associated Company voice frequency cables for carrier 

 operation. 



An important aim of type N carrier is directed at the exchange plant 

 where, even though the mileage is less, the number of circuits for exceeds 

 that in toll use. Here the benefits of a high grade carrier facility are numerous. 

 Exchange Plant makes use of a large percentage of small-gauge, high- 

 capacitance cables, heavily loaded to reduce the net loss. Economically it 

 is difiicult to apply carrier or voice repeaters to these relatively short- 

 length circuits. Many circuits to suburban points are now routed over toll 

 trunks, because of the high loss of the exchange type circuits. Type N can 

 be used to afford a low-loss, high-grade exchange circuit which can be 

 switched in the manner usual for tandem and similar circuits. Low-loss 

 trunks employing type N will be of great benefit in the ever increasing 

 distances in the suburban areas between satellite points and their outlets. 

 Direct, high-grade trunk groups, always at a premium and first selection of 

 automatic switching equipment, can be increased in number. 



Another important objective, in addition to the provision of a stable, 

 low-loss, high-velocity talking circuit, is that of providing built-in signaling 

 arrangements suitable for dialing and supervision. Such a system is urgently 

 needed to meet the rapidly expanding demands of toll line dialing, as well 

 as for exchange circuits. Such a signaling channel has been made available 

 at a frequency just above and directly associated with the voice channel 

 which it serves. 



The emphasis placed on economy in the development of the N system 

 has not resulted in a marked lowering of standards of performance. On the 

 contrary, the N system, within its range of operation, sets new standards 

 in many respects, notable among which is stability of net loss. The ob- 

 jectives have been met rather by a combination of new approaches and new 

 circuit elements, backed up by closely coordinated cooperative effort in 

 cost reduction of components, assemblies, and finally, of the complete 

 system. 



Among the many features making possible such a development as Nl 

 Carrier, certain are outstanding. The most important of these is the com- 

 pandor, a device for compressing and expanding the volume range of 

 sj)eech, thereby affording an improvement in the amount of noise and cross- 

 talk which can be tolerated. The effects of the compandor are far reaching, 

 both in the line and in the terminals. The need for expensive line treatment, 



