506 BELL SYSTEM TECHNICAL JOURNAL 



inter-toll trunks it would be necessary to provide frequency converters at each 

 terminal to translate the frequency band required on the subscriber loop to a band 

 suitable for application to the telegraph inter-toll trunks. If the telegraph channels 

 were used between switchboards it would also be necessary to provide the operators 

 with teletypewriters or other means of communication because the telegraph channels 

 do not permit oral communication. 



(c) A number of miscellaneous engineering and plant problems other than those 

 listed in (a) and (i) would be introduced if standard telephone facilities were used to 

 interconnect the stations in the teletypewriter exchange network. 



After due consideration of all these factors it was decided to utilize 

 the telegraph plant and to design and provide the necessary teletype- 

 writer switchboards and inter-office signaling arrangements. By 

 following this plan it has been possible to establish service on a nation- 

 wide basis using switchboards at the larger switching centers and 

 employing modified telegraph private wire testboards at the smaller 

 centers. 



This paper describes the signaling and switching arrangements used 

 in the present system, and particularly the two principal types of 

 switchboards that are in use. The discussion is limited to the most 

 important signaling and switching arrangements, as the transmission 

 features are described in another paper. ^ A description is included of 

 the principal factors entering into the design of the more important 

 circuits used in these switchboards: the subscriber lines, inter-toll 

 trunks, and cords. The subscriber line treatment is divided into 

 three broad classes: local subscribers having either attended-only or 

 unattended service; distant subscribers served over telegraph toll 

 line facilities; and distant subscribers served over telephone facilities. 

 Particular attention is given to the fundamental problem of providing 

 supervisory signals over the telegraph lines used as inter-toll trunks in 

 the inter-office connections. 



Teletypewriter Switchboards 



To reach subscribers in all parts of the country there has been 

 established a network of teletypewriter switching points intercon- 

 nected by telegraph lines. At each of the larger switching points a 

 teletypewriter switchboard is provided, the principal switchboards 

 being the No. 1 Teletypewriter Switchboard having a capacity of 

 3,600 subscriber lines, and the No. 3.4 Teletypewriter Switchboard 

 having a capacity of 1,200 subscriber lines. The former, a general 

 view of which is shown in Fig. 1, is used in large cities such as New 

 York and Chicago, while the latter, a general view of which is shown in 

 Fig. 6, is^used in smaller cities such as Pittsburgh and Kansas City. 



*"A Transmission System for Teletypewriter Exchange Service," R. E. Pierce 

 and E. W. Bemis, this issue of the Bell System Technical Journal, and Electrical 

 Engineering, v. 55, September 1936, pp. 961-70. 



