438 BELL SYSTliM TKCIINICAL JOURNAL 



Primary OtUlets 



Primary outlets are switching offices having direct circuits to one or 

 more regional centers and each having direct circuits to all toll 

 centers in the area for which it is the primary outlet. Also, 

 each primary outlet is connected to every other outlet within as 

 large an area as practicable, usually within a State. 



Supplementary Offices 

 Secondary Outlets 



Secondary outlets are switching offices having direct circuits to one 

 or more regional centers and are intended primarily to furnish 

 alternate routes for toll centers for reaching the regional centers, 

 thus providing a greater degree of flexibility in the plant. 



Secondary Switching Points 



Secondary switching points are additional switching offices intended 

 to provide routes which are more direct thus reducing back haul 

 for intra-area business. 



Transmission Considerations of General Toll Switching Plan 



An important part of the development of the plan was the determi- 

 nation of proper transmission requirements such that any toll con- 

 nection established in accordance with the plan would have satisfactory 

 transmission efficiency. 



Before the perfection of telephone repeaters, the provision of satis- 

 factory transmission efficiency depended largely upon limiting the 

 total attenuation loss of the complete circuit. At the present time 

 the perfection of repeaters has practically removed that limitation. 

 For example, the attenuation in a New York-Chicago circuit in cable 

 is such that without the use of repeaters the ratio of input power to 

 output power for speech currents transmitted over the circuit would 

 be 10^^, while by the use of repeaters at the terminals and at 17 inter- 

 mediate points the ratio actually is 10. 



The removal of the limitation formerly set by circuit attenuation 

 makes possible the increase of the efficiency of circuits to the limit 

 determined by some other characteristic of the circuit. There are 

 various things which under different conditions may determine this 

 limit. One is the effect on transmission of echoes, namely, portions 

 of the speech currents reflected back from the distant end of the 

 circuit or from intermediate points. Another is the distortion due to 

 the building up of greater transmission gain at certain frequencies 

 than at others, which effect may result if repeaters introduce too 



