1240 THE BELL SYSTEM TECHNICAL JOURNAL, OCTOBER 1951 



circuits were very extensively used for short-haul repeatered and non- 

 repeatered facilities. 



A large curtailment in the demand for loading on new long cable circuits 

 inmiediately followed the commercial exploitation of the Type K cable- 

 carrier system, which started during the middle 1930's. The drastic nature 

 of this impact was subsequently increased by the standardization of a still 

 more economical (K2) cable carrier system,^^ and by the post-war extensive 

 installation of coaxial cable systems. The very recent development of a 

 relatively inexpensive short-haul carrier system (Type N), which uses two 

 pairs in the same cable for its opposite-direction paths, promises an ad- 

 ditional substantial reduction in the need for new loaded toll cable facilities, 

 even for short distances. However, it seems probable that the demand for 

 new loading may continue indefinitely on a low-level basis for more or less 

 special short-haul situations where carrier telephony may be more expensive. 



During the past two decades or so, loading cost-reduction has been 

 carried so far that the prospects of further substantial cost-reductions are 

 not now in sight. It seems improbable that any further design cost-reduction 

 could be large enough to reverse the present general trend towards a large 

 dependence upon carrier telephony for new short-haul toll cable facilities. 



Exchange Area Loading 



During the period covered by the present review, telephone transmission 

 over exchange area cables has been entirely on a voice-frequency basis. 

 Moreover, the use of vacuum-tube repeaters in conjunction with loading 

 (or on non-loaded cables) has been statistically insignificant in comparison 

 with the very extensive use of loading. In consequence, exchange area 

 loading does not have to share with developments in repeaters and in 

 carrier systems the great credit which it has earned with respect to the 

 improvement of exchange area transmission performance and the reduction 

 of plant cost. 



The simple pattern in the evolution of exchange area loading practices, 

 relative to those for toll cable loading, is of course basically due to the 

 shortness of the circuits and the relatively uncomplicated service-require- 

 ments. 



In certain important respects, the improvements achieved by the nearly 

 continuous development work are generally similar in the two types of 

 loading, notably: (1) the improvement in transmission quality obtained by 

 increasing the transmission band-width, and (2) the successive facility-cost 

 reductions resulting from the successive developments of lower-cost loading 

 apparatus. These plant-cost reduction activities were carried out to a 

 greater degree in the exchange area loading. It is especially noteworthy 



