TRANSMISSION SYSTEM FOR TELETYPEWRITER 547 



TABLE I 

 Transmission Coefficients for 60-Speed Teletypewriter Exchange Circuits 



Maximum 

 Section Length 

 Coefficient Normally 



Type of Circuit per section ♦ Used, Miles 



D-C. grounded system on open wire 2.5 to 4 300 



D-C. metallic system on cable circuits 2 to 3 150 



High frequency carrier system on open wire .... 2.6 1,150 



Voice frequency carrier system on cable oj open 



wire circuits 2.0 to 2.2 3,500 



From the coefficients given in the table and the earlier discussion 

 of the teletypewriter switching plan, it is apparent that the carrier 

 systems, voice-frequency or high-frequency, where available, are 

 most suitable for the longer backbone toll circuits of the nationwide 

 network. For the short circuits of from 100 to 200 miles where cable 

 plant is available, the metallic telegraph circuits on cable are exten- 

 sively used, while for the scattering circuits of similar length, and most 

 of the shorter toll circuits, use is made of two-path polar and differ- 

 ential duplex facilities. In some instances where single section 

 facilities of the required grade are not available between two centers, 

 regenerative repeaters permanently associated with multi-section 

 circuits are used to provide satisfactory over-all circuits. Also in 

 certain instances where circuits are not required for through switching, 

 multi-section circuits without regenerative repeaters are sometimes 

 provided and classified "for terminal purposes only." 



All the components of the network — teletypewriters and their 

 associated subscriber lines, transmission circuits in the switchboards, 

 and toll circuits interconnecting the switchboards — are designed to 

 give a satisfactory over-all transmission performance with a minimum 

 cost for the plant as a whole. Results obtained in service indicate that 

 the system is meeting a commercial need and that its performance is 

 satisfactory, but developments are continually under way to effect 

 further improvements in service and economies in operation as experi- 

 ence is gained with the system. 



References 



1. "Switchboard and Signaling Facilities of the Teletypewriter Exchange System," 

 A. D. Knowlton, G. A. Locke, and F. J. Singer, this issue of the Bell System 

 Technical Journal, and Electrical Engineering, v. 55, September 1936, pp. 1015- 



25. 



* The term "section"ashereuseddesignatesthepart of a telegraph circuit between 

 2 telegraph repeaters or a section of a telegraph circuit without any intermediate 

 telegraph repeaters. For example, a telegraph repeater section operated by the voice- 

 frequency carrier telegraph method is that part of the circuit between carrier telegraph 

 terminal sets, regardless of the number of intermediate telephone repeaters in the 

 carrier circuit. 



