TELETYPEWRITER EXCHANGE SYSTEM 505 



chiefly by the press and brokers and was operated on a Morse tele- 

 graph basis, generally using composite or simplex line facilities. 

 Later, with the introduction of the modern teletypewriter, the carrier 

 telegraph, and other improvements, together with the growth of 

 American business and the demand for rapid and accurate written 

 communications, this private line business expanded rapidly and 

 service was furnished not only to the press and brokers but also to 

 other financial institutions, manufacturers, government bureaus, police 

 departments, and a wide variety of retailers and distributors of goods. 

 This business has become nationwide. Many of these private line 

 systems are provided with switching facilities for use by the customer 

 in each system, although the supervisory arrangements are rather 

 elementary. 



In addition to the private line telegraph service and the arrange- 

 ments which had been developed and applied to that service, the 

 many developments in the telephone field formed an important con- 

 tribution to the teletypewriter exchange service. It is obvious that 

 in providing TWX service, which is a point-to-point service with 

 connections set up and taken down on the subscriber's order, use can 

 be made of many traffic and service practices used in the telephone 

 service. Furthermore, certain telephone apparatus such as switching 

 relays, cords, plugs, etc., can be employed to advantage. 



With this background, when it was decided to furnish a nationwide 

 teletypewriter exchange service to the public, Bell System engineers 

 had the problem of determining what general plan of design to adopt. 

 There were two alternatives: (1) to provide a service using the tele- 

 phone plant and existing telephone switchboards, or (2) to provide 

 separate switchboards for use with the telegraph plant. The im- 

 portant advantages of the first plan are: 



(o) The switchboards and signaling arrangements designed for and in use in the 

 telephone plant could be employed. 



(b) The same operating groups handling the telephone service would handle this 

 service. Inasmuch as telephone service is on a 24-hour basis throughout the country, 

 the TWX service could be furnished on the same basis with a relatively low operating 

 cost. 



The disadvantages of the first plan are: 



(a) Because the teletypewriter operates on a d-c. basis it would be necessary to 

 provide an oscillator and associated apparatus at the station to generate an audio- 

 frequencv alternating current for modulation by the signals sent by the teletypewriter, 

 and a rectifier to convert the a-c. pulses received from the distant station to d-c. 

 pulses for operation of the receiving mechanism of the station teletypewriter. 

 Furthermore, it would be necessary to furnish a telephone instrument at the station 

 to permit the subscriber to communicate with the operator unless a teletypewriter 

 or other type of recording instrument were provided at each operating position. 



(b) Relativelv expensive telephone lines known as inter-toll trunks would be 

 required between central offices. If the cheaper telegraph channels were used as 



