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BELL SYSTEM TECHNICAL JOURNAL 



of trunks handled by the various tandem operators can be varied from 

 hour to hour by means of keys located between each group of 10 cords. 

 Release of the tandem trunk by the originating operator gives a 

 disconnect signal, simultaneously at both the tandem and " B " boards, 

 and the tandem and "B" operators then take down the connection. 

 The tandem trunk may be reselected for a new call even before the 

 tandem operator has taken down the cord on the previous call. 



o— 



o— —o 



-/ 



TANDEM COMPLETING 



I TRUNKS TO "B" 

 POSITIONS IN 

 MANUAL AND 

 DIAL OFFICES 



INCOMING TRUNKS 



CALL INDICATOR 



— NOTES — 



I- IDLE TRUNK INDICATING IS PROVIDED 

 ON THE TANDEM COMPLETING TRUNKS, 

 THE INDICATING LAMP SOCKETS BE- 

 ING COMBINED WITH THE DESIGNATION 

 STRIPS. 



2- ONE-WAY OR TWO-WAY TRUNKS MAY BE 

 PROVIDED BETWEEN MANUAL OFFICES 

 AND THE TANDEM OFFICE. 



3- ONE-WAY TRUNKS, ONLY, ARE USED BE- 

 TWEEN DIAL OFFICES AND THE 

 TANDEM OFFICE. 



Fig. 1- 



-Manual automatic listening straightforward and call indicator 

 tandem arrangements. 



Figure 1 shows, schematically, the circuit and equipment arrange- 

 ments in the manual straightforward tandem system. Figure 2 is 

 a photograph of the manual straightforward tandem switchboard 

 which serves Detroit, Michigan, and surrounding communities, while 

 Fig. 3 indicates the scope of the Detroit Tandem System. 



Where the volume of traffic to be switched is too small to warrant a 

 tandem switchboard, tandem operation frequently is obtained by 

 routing the traffic over straightforward trunks terminating on manual 

 "B" positions at a convenient local office and providing trunks from 

 these positions to other central offices. The operators at these manual 

 "B" positions, therefore, combine the functions of tandem and "B" 

 operators. 



Call Indicator Tandems 



When manual offices are converted to dial, it is necessary to provide 

 means for completing calls from dial subscribers to all offices, including 

 manual offices, within their local dialing area. The usual arrangement 



