UNCLASSIFIED Development of Automatic Telegraph Switehing Systems BY R. D. PARKER Uncla.ssijied There are two general types of automat~ !elegraph switd1£ng systtms. In one, eUctrieal circuits are e.st.ablished to pr1Jvidt comnmnicatWn patht between lelegra'[Jh silJti-Ons in customers' o.flic~ and are broken wMn t.ltt need for communication is over. In the other, direct tllectrical circuits between stations are not established, but coded informatiott in messages automatically routes them through telegraph networks to their dBStino,tions. This paper de.scribes briefly same of the systems now in use, and oldliTUs some of the requirements whic.h had to be met to provide a sali-Ojadory seniice. INTRODUCTION A switch as used in electrical engineering is defined in dictionaries as a device for making, breaking, or changing the connections in an electric circuit. When a switching operation is performed, circuits are changed from one connection to another and devices are cut in and out of circuits. The idea is to transfer or shift these electrical circuits and devices around. A telephone et:>atral office or exchange is an excellent example of the above, in that e\ectrical voke-tran.smission circuits are shifted about to establish commuoication paths between parties, and are broken when conversations are over. Thus, a telephone exchange area such as the District of Columbia is operated as a large ele(trical switching system. Telegraph circuits are established, broken, and shifted around like telephone circuits, and when telegraph circuits t0 subscribern' premises ate connected and disconnected on reque;ts from subscribers a telegraph exchange system is created. In telegraphy, however, another concept of switching tias grown up. This is because telegraph mes.sa~e.:;. as contrasted with telephone mes.sages, can easily be stored, held in transit, and re-transmitted sometime after reception. The pieces o[ paper or other message recordings are the things shifted or switched around. A. telegraph office where messages on blanks are shifted from an in­ coming circuit to one or more of several outgoing· circuibi has been called a "relay" office, in that received messages- are later transmitted Vi their destinations, but, such an office is also in fact a message­ switching center. A "relay" office does indeed suggest a relay race, 73 UNCLASSIFIED A.ppr-c•ved for t:iv ~~S.i\ o IJ1-08-2008 FOIA. Case# 51551 UNCLASSIFIED AUTOMATIC SWITCHING SYSTEMS R. D. PARKER UNCLASSIFIED in which a baton is handed at a poiri.t of exchange to the next runner; the introduction of tbe telephone in the latter part of the 1870's a rather but this comparison fails to take into account the switching which is extensive telegraph switching system was in operation in New York performed in t..elegTaphy. Ii in a relay race each baton carried a coded City. In this system, Morse circuits from a central exchange radiated message which had to be decoded at the exchange point to determine to customer's offices. Each customer hired a Morse opera.tor, who which runner of a group would carry a particular baton !or the next handled messages and requested the central-e:rcha~ operator to make lap and batons were passed accordingly the analogy would be more circuit connections to other customers. The advent of the telephone, complete. This imagined reshuffling of batons is message switching. saving the expense of the Morse operators, quickly put an end to this In recent years a new technical development has permitted telegraph type of telegraph switching. messages to be routed or directed aulomaticall'I/ from their point of origin The actual introduction of a printing-telegraph switching or exchange to their destination by means of coded electrical impulses which are in public service had to wait for the development of a satisfactory start­ a sense part of the message but precede the nonnal address, the text, stop type o( telegraph printer, teletypewriter, teleprinter or, teletype and the signature. An arrangement following this method ()f working, machine, as it is variously called. It is re&!lonable to ask what is meant with a center where messages are stored, analyzed, and retransmitted, by "start-stop" and why it is impartant in planning an exchange system. becomes, therefore, an automatic message-switching system. The St.art-stop means a method of operating printing-telegraph ma­ circu·rt arrangements are fixed and the messages are switched._ This chines in which the code group needed for selecting a charaeter ls operation may be electromechanical or electromagnetic, with mes­ preceded by an impulse, always; of the same kind, which indicates to a sages stored by perforated tape, on magnetic t.apes and drums or some receiver that a code group representing a character will follow immedi­ other manner, but direct electrical paths between sending stations and ately. This simple impulse is the "start". Furthermore the oode addressees are never established. group is followed by an impulse opposite in kind to the start, which Thus telegraphy, unlike telephony, has two types of switching brings the receiver to a rest eoiiditlon in preparation for the next st.art systems., one, circuit switching and the other, message switching, each impulse. This final impulse in the complete sequence for each charac­ with its own problems. Accordingly, it is desirable to consider them ter is the "stop". Sending and receiving machines do not operate at under sepaTate sections: Section I, Circuit Switching; and Section If, exactly the same speed, and without the start-stop feature a receiver Automatic Message Switching. would quickly gain or lose enough relative to a sender to cause errors in the received copy. A fresh start at each charac~r keeps the machines in good relationship with each other and prevents these errors. Most SECTION I-CIRCUIT SWITCHING printing-telegraph machines in use today operate on this principle. Such machines operating at about the same speed in words per minute CElft'.RAL and having like code character combinations may be freely inter­ There are two well established system.it operating today in which conneet.ed by electrical circuits for a typed form of communication printing-telegrapb circuits are switcbed to provide direct communication service. Thus, with the "start-stop" code a printing-telegraph circuit­ paths between subscriber st.ations: one in Americ.a, known as Tele­ switching system became feasible. typewriter Exchange Service or TWX, and the other in Europe, started as a national service but now rapidly expanding on an international basis, and known as TELEX 1• TWX was initiated in the early 1930's BELL SYSTEM TWX SERVICE and TELEX followed wlth the first insta11ations "m England. The Bell System established a public nation-wide printing telegraph Shortly after World War I, when the writer started the development switching service in 1931, following a long development and trial of printing-telegraph switching systems for the American Telephone and program. It was realized that little or no local business could be Telegraph Company, he was informed by General J. J. Carty, who was expected, and because of the necessity for recording and later billing then head of its Devefopment and Research Department, that before customers for long dist.ance calls., manually operated toll switchboards were especially designed for the service. AJ.I communications between toil-operators in dist.ant cities, and between operators and subscriben, ' TEX is the abbreviation adopted by the Radio Corporation of America for Its overaeM Teleprinter Exchange Service and ~ciat.ed TELEX station~ in Europe and were made by means of printing-telegraph machines. The typed other a~as. records proved t.o be of assistance t() operaton in spite of the sJow speed UNCLASSIFIED 74 75 UNCLASSIFIED UNCLASSIFIED AUTOMATIC SWITCHING SYSTEMS R. D. PARKER UNCLASSIFIED of the machines-60 words per minute-as compared with speech, and performed under the control of impulses transmitted over the com­ the time required to establish toll-connections was found to be essential­ munication circuits. An example is the use or a "concentrator", ly the same as that for manuaL toll-telephone service. placed in a local area where a number of TWX subscribers are grouped. A simple intercity TWX connection is iilust.rated in Fig. 1. St.a­ The concentrator is arranged to associate these subscribers automat­ tion A in New York is electrically connected to St:.ation B in San ic.ally with trunks to a ~n TWX switchboard at some diet.ant point. Francisco so that coded signals for typing and machine control pu°rposes may be passed back and forth to give a typed-record form of com­ munication service. A call is initiated by a subscriber's turning on the power switch of the t.eletypewriter station set. This signaJs the switchboard operator, who requests the number of the ca\Jed party by operating the keyboard of a machine which is an integral part of the switchboard. The party originating the call types this information, the operator selects a trunk to the distant city and passes the called number by typing to the incoming operator, who connects the trunk to the called st.ation line and rings the station bell. Toll tickets are made out for billing purposes as with telephone service. Disconnect signals are flashed to the operators by theactofturningoffthepower-switch at either subscriber's station; Fra'ncisco , I New York ' TRUNKS ---;' - ---1- -t =- =3 ro OTHER CITIES ----j. f---+- •~ WITCI!- r--- BO•l!e a -1----------------------1-­' -~. "~ ~-----------------r-• ·~ • 1~1111.,•L L-------------------~ T!~-L Fii· 2-Teletypewrlter E1cb1.nae Serv\ce--Interclty Connection -WE>IT L-------------------J. i Ualn~. AutomaHc Conceuuatora L---------•---------J. ·- ~ I_---- - -- .,._ - ---- ______ !_ ~ A connection between two TWX subscribers, involving the use of I ' concentrators, is illustrated in Fig.