lOoG THE BELL SYSTEM TECHNICAL JOURNAL, SEPTEMBER 1954 



nature of the assembly resulted in a telephone repeater so low in cost 

 and so simple in application and installation that it has found extensive 

 use primarily in the local telephone plant. 



In the period from 1948 to the present time, over 50,000 series-type 

 negative impedance repeaters were manufactured and incorporated in 

 the Bell System telephone plant. These repeaters have been used largely 

 on intraexchange trunks and on trunks extending from the exchange 

 areas to near-by smaller towns. Such installations have been very efTec- 

 tive in improving the transmission on short haul calls and in many cases 

 have also reduced trunk costs by permitting the use of smaller and 

 cheaper conductors. They are usually operated at gains that reduce the 

 nonrepeatered trunk loss by more than half. 



Negative impedance repeaters are especially suited to exchange trunk 

 use, not only because of their simplicity and ease of maintenance, but 

 also because unlike earlier types of repeaters, they preserve the dc 

 continuity of the circuits on which they are installed. This latter fea- 

 ture means that they do not interfere appreciably with the signaling 

 methods ordinarily used on such trunks. Also, they have the added 

 advantage that, in the event of tube failure, the circuit still functions 

 but with its loss substantially raised until the defective tube has been 

 replaced. 



APPLICATION OF SERIES-TYPE NEGATIVE IMPEDANCE REPEATERS 



In a multioffice exchange area, trunks can generally be grouped in 

 three classes. Largest in number are those known as interoffice trunks 

 which extend directly from one local operating center to another local 

 center in the same exchange operating area. In some cases direct trunks 

 between two centers can not be justified and each office has trunks to a 

 central location known as a tandem center where a through connection 

 is made when required. This second class of trunks is known as "tandem" 

 trunks. Frecjuently trunks of this type are provided to supplement the 

 direct trunks and thus give the added advantage of alternate routing. 

 A third type of trunk extends between the local office and the toll office 

 and therefore always forms part of a toll connection to the local office. 

 These are known as toll connecting trunks. In single office areas the toll 

 connecting trunks to neighboring small towns are known as "tributary" 

 trunks. 



In some large multioffice exchange areas there are also trunks between 

 the toll office and tandem centers which may l)e some distance from the 

 toll office and through which local offices reach the toll office for long 

 distance calls. These trunks are also called tandem trunks but are more 



