TANDEM OPERATION IN THE BELL SYSTEM 397 



relatively small, their provision may not be justilied, and other means 

 must be jirovided for oomi)lelint; connections to dial offices, whether 

 reached over direct or tandem trunks. F'or tandem oj^eration under 

 these conditions, stcp-b>'-step " B " operators' positions, directly associ- 

 ated with the tandem equipment, usually are provided. The incoming 

 trunks are of the straightforward type, and terminate on incoming 

 selectors controlled by means of key sets on the "B" operators' posi- 

 tions and associated senders. An idle "B" operator is connected 

 automatically to the incoming trunk through call distributing equip- 

 ment and, upon receiving the order and setting up the required digits on 

 her key set, her position is automatically disconnected. The final dis- 

 connection of the tandem and local office selectors is under the control 

 of the originating operator. Except for the selectors on which the in- 

 coming trunks terminate, all of the selectors are used in common, 

 whether controlled from the "B" positions or by pulses received over 

 dial trunks. 



The step-by-step "B" board arrangement sometimes is used as a 

 sub-tandem in connection with manual straightforw^ard tandem opera- 

 tion in a large nearby city, since it provides a convenient means for 

 completing connections to subscribers connected to dial offices within 

 the tandem area. 



Figure 11 shows schematically the step-by-step tandem arrangement, 

 including "B" board. Figure 12 illustrates the operators' positions. 



Fig. 12 — Step-by-step tandem — "B" operators' positions. 



