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Incoming Trunk Circuits 



Several types of incoming trunk circuits arc providcnl to meet the 

 various switchboard conditions where a switchboard and concentrating 

 equipment are located in the same building or to meet the interoffice 

 trunk loop conditions where the switchboard and concentrating otjuip- 

 ment are in separate locations. In all cases the incoming trunk circuit 

 receives signals from the switchboard, directly or indirectly and trans- 

 mits them to the outgoing trunk circuit and to the controller. The in- 

 coming trunk circuit also receives signals from the outgoing ti'unk circuit 

 and transmits them to the switchboard. The incoming trunk circuits, 

 where necessary, also convert the two-wire transmission circuit from the 

 switchboard to a four-wire transmission circuit to meet the intertoU 

 trunk facilities. Fig. 2 shows a typical incoming trunk circuit. 



Trunk Selection Switches 



The trunk selection switches are the elements which, under the direc- 

 tion of the controller, connect the incoming trunks, requesting service, 

 to the outgoing trunks. Crossbar switches having 20 verticals and 10 

 horizontals, providing 200 crosspoints, are used. A crosspoint is the 

 intersection of a vertical and a horizontal element of the switch. The 

 incoming trunks are connected to the verticals and the outgoing trunks 

 to the horizontals of the switch. By the switch operation any of the 20 

 incoming trunks may thus be connected to any of the 10 outgoing trunks. 



A single switch will accommodate 20 incoming and 10 outgoing trunks. 

 To increase the number of incoming trunks a similar switch is added for 

 each additional 20 trunks with the verticals connected to the new in- 

 coming trunks and the horizontals connected to the corresponding 

 horizontals of the first switch. To increase the number of outgoing 

 trunks switches are added with the horizontals connected to the new 

 group of outgoing trunks and the verticals connected to the correspond- 

 ing verticals associated with other groups of incoming trunks. The switch 

 arrangement may be envisioned as one large switch having as many 

 verticals as there are incoming trvmks and as many horizontals as 

 there are outgoing trunks. 



Outgoing Trunk Circuit 



Only one type of outgoing trunk circuit is icciuired, as shown in Fig. 3. 

 All outgoing trunks from a particulai' concentrating eciuipment terminate 

 in the same distant toll center. The outgoing trunk circuit receives 



