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BELL SYSTEM TECHNICAL JOURNAL 



of the call and ^iven the number of the subscriber by whom it is made. 

 From the subscriber's standpoint calls are made with this equipment in 

 practically the same manner as when ordinary telegraph facilities are 

 employed. 



Switchboards 



The switchboards used in teletypewriter exchange service contain 

 facilities for interconnecting subscriber lines, connecting them with toll 

 circuits, or interconnecting toll circuits as required, together with the 

 necessary means for establishing and supervising the connections. 

 They are described in considerable detail in the previously referred to 

 paper on switchboards and signaling facilities. ^ As indicated in the 

 discussion of subscriber lines, the transmission circuit through the 

 switchboard is essentially a differential duplex telegraph repeater. 

 One such repeater is connected between the cords of each pair. This 

 repeater is so designed that it introduces very little distortion in the 

 connection. The coefiticient of the switchboard cord circuit is 0.3. 

 Figure 8 is a schematic diagram showing the principle of the trans- 

 mission circuit. 



-VW 



VA- 



Fig. 8 — Principle of switchboard transmission circuit. 

 Operator's teletypewriter inserted at X when required. 



Toll Circuits 



The toll circuits of the teletypewriter exchange network are of the 

 standard types that are in general use for telegraph transmission. 

 These include voice-frequency carrier telegraph systems on cable 

 circuits ^ or on channels of carrier telephone circuits on open-wire 

 lines, high-frequency carrier telegraph systems on open wires, ^ metallic 

 systems on cables,^ and two-path polar and differential-duplex 

 grounded telegraph circuits.^ An idea of the relative capabilities of 

 these types of facilities may be obtained from Table I which shows 

 the coefificients of a single section of each type. 



