TANDEM OPERATION IN THE BELL SYSTEM 381 



ringdown signaling type. More recently, tandem arrangements have 

 been employed in toll offices in connection with the toll lines used for 

 long distance calls, all of which, with a few exceptions, are handled 

 over ringdown signaling circuits. 



It is the purpose of this article to describe the operating and service 

 features of the different types of tandem arrangements employed in 

 the Bell System and to indicate the extent to which they are used. 

 Tandem equipment having trunks incoming from other tandem equip- 

 ment, is a subtandcm; some equipments operate both as tandems and 

 subtandems. 



A consideration of tandem operation may logically begin with the 

 switching requirements of a single local exchange area. It follows 

 from our definition that a tandem connection involves the cooper- 

 ation of at least three different offices for its completion. So long 

 as all switching operations are confined within a single office there 

 is, therefore, no occasion for tandem connections. Neither is there 

 any occasion for them when the number and relative locations of the 

 various offices, call them A, B, and C, etc., within the exchange area 

 are such that it is still practicable to handle interoffice calls over direct 

 trunks. With increase in area and number of offices, a point is obvi- 

 ously reached, however, where it is no longer practicable to go, for 

 example, from office A to office U directly, U being located in a remote 

 division of the exchange, although it will still be feasible to go directly 

 between offices A, B, and C, and between offices U, V, and W. Given 

 such an extended exchange area, it will be found to contain some inter- 

 mediate geographical position at which a tandem office can be profit- 

 ably located with trunks extending to all the local offices and with 

 switching facilities such that calls from A, B, and C to U, V, and W 

 will be routed to it and will be completed by the interconnection of 

 trunks between the tandem office and these various outlying offices. 

 Such a tandem office would of course be a local tandem office. 



Passing from the problem presented by the handling of local traffic 

 within an exchange area, and reserving discussion for later paragraphs, 

 it may be stated as evident that numerous other situations arise within 

 the telephone plant for one or another type of tandem operation. 

 These it is convenient to classify as follows: 



I. Manual tandems, at which connections are made manually by plug 

 and jack operation. These include — 



a) Manual straightforward tandems, for completing connections 



from manual trunks to manual trunks. 



b) Call indicator tandems, for completing connections from dial 



trunks to manual trunks. 



