TANDEM OPERATION IN THE BELL SYSTEM 383 



TABLE I 



Tandem Systems at Bell System Toll Centers 



(These Constitute Most of the Tandem Systems in Use) 



Type of Tandem No. in Use 



Manual Straightforward Tandems 13 



Call Indicator Tandems 5 



Toll Office Tandems 27 



Straightforward Toll Line Tandems 3 



Toll Switching Trunk Tandems 2 



Panel Sender Tandems — Total 6 



— With Operators' Positions 5 



Panel Office Selector Tandems 34 



Step-By-Step Tandems— Total 61* 



— With "B" Board Operators' Positions 5 



— With Intermediate Dialing or Key Pulsing Operation 4 

 Trunk Concentrating Tandems — No. of Cities 9t 



* 26 of these have trunks incoming from other tandems. 

 t With 148 groups of trunk concentrating switches. 



single-ended cords on the tandem positions, and, in all but the smallest 

 boards, the trunks are connected automatically to the tandem operator 

 in rotating sequence, a flashing supervisory lamp associated with the 

 trunk indicating to the tandem operator the trunk to which she is con- 

 nected. When the tandem operator is in this fashion connected to a 

 trunk which an originating operator has selected and over which she 

 wishes to have a call completed, both operators receive momentary 

 tone signals indicating this fact and the originating operator passes 

 the name of the central office desired. The tandem completing trunks 

 appear in the outgoing trunk multipleat the tandem board, usually 

 with idle trunk indicating lamps, and the tandem operator extends the 

 connection from her position by simply plugging the incoming trunk 

 into an idle trunk to the office desired. Plugging into the trunk auto- 

 matically signals the "B" operator at the called office. The tandem 

 operator's telephone set may be released from the incoming trunk, 

 either by means of a release key provided at her position for that 

 purpose or by the act of plugging into the outgoing trunk. The release 

 key enables the tandem operator to receive a call while establishing 

 the connection on a previous call. 



In order to distribute the load and assure the minimum of delay at 

 the tandem board, the various groups of incoming trunks are sub- 

 grouped and the subgroups terminated on dififerent tandem positions. 

 In addition, on the larger trunk groups, arrangements are provided so 

 that if the operator upon whose position a subgroup is located is busy, 

 when one or more operators upon whose positions other subgroups 

 terminate are idle, this is indicated to the originating operator in order 

 that she may select a trunk to an idle tandem operator. The number 



