SWITCHING PLAN FOR TELEPHONE TOLL SERVICE 



443 



more of such circuits are connected together for a long built-up toll 

 connection, the complete circuit will operate at as nearly as practicable 

 its minimum working net equivalent. While under these conditions 

 the permissible values of the pads associated with the terminal re- 

 peaters naturally vary in individual cases, it has been found possible 

 to work out for general use a series of values which should give satis- 

 factory results. These are indicated in Fig. 11. It will be noted 

 that these values are such that a circuit switched at both ends to 

 other toll circuits is operated at either .5 db or 1 db less than its 

 minimum working net equivalent, this deficit being made up by a 

 corresponding margin at the ends of the circuit. For example, by 

 reference to Fig. 11, it will be noted that whereas the design values of 

 the three intermediate links of a five-link connection equate to 11 db, 

 these links will contribute a total loss of only 9 db. On the other 



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 OFFICE 



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Fig. 10 — Illustration of typical transmission data of terminal repeater-;-switching 

 pad method of operation. Fig. 1^ — -Circuits between switching pad offices in terminal 

 condition. Fig. 2- — Circuits of Fig. 1 interconnected at switching pad offices. 

 Fig. 3 — Connection between non-pad offices switched at pad office. 



hand, the end links will contribute a total of 8 db, whereas their 

 design values equate to only 6 db. The 2 db marginal deficiency in 

 the intermediate links is compensated for by the 2 db marginal surplus 

 in the end links. When intermediate links are used as end links in 

 built-up connections, the switching pads at the terminating ends 

 restore the necessary positive margins. 



The design of the very long intermediate circuits, such as some of 

 those connecting two regional centers, requires special consideration 

 and treatment to meet the transmission requirements specified. By 

 making use of a fundamental feature of four-wire circuits equipped 

 with echo suppressors and by employing circuits with the highest 

 velocities of propagation for this purpose, these circuits may be 

 designed in practically all cases to contribute not more than the desired 



