Reference / Paper · 1921
Automatic Telephony: A Comprehensive Treatise on Automatic and Semi-Automatic Systems (2nd ed.)
Classic US text on Strowger/AE and other automatic systems, with circuits.
Reference / Paper · 1921
Classic US text on Strowger/AE and other automatic systems, with circuits.
AUTOMATIC TELEPHONY 7\fe Qraw-JJillBook (sx 7m PUBLISHERS OF BOOKS FOIL/ Coal Age * Electrical World Electric Railway Journal v Engineering News-Record American Machinist v lngenieria Internacional Engineerings Mining Journal v Power Chemical & Metallurgical ^Engineering Electrical Merchandising AUTOMATIC TELEPHONY A COMPREHENSIVE TREATISE ON AUTOMATIC AND SEMI-AUTOMATIC SYSTEMS AR MEMB SSEY SMITH, E.E. FORMERLY PROFESSOR OF TELEPHONE rNEERING, PURDUE UNIVERSITY ., AND WILSON LEE CAMPBELL, E.E. FELLOW A. I. E. E. Second Edition McGRAW-HILL BOOK COMPANY, Inc. NEW YORK: 370 SEVENTH AVENUE LONDON: 6 & 8 BOUVERIE ST., E. C. 4 1921 Copyright, 1914, 1921, by the McGraw-Hill Book Company, Inc. THK JIAH.K PKTCSS V o B K PA. fogmeeriiig Lbctiy TK C5 Oo MR. ALEXANDER ELLSWORTH KEITH J__^ WHO BY HIS GREAT RESOURCEFULNESS, UNLIMITED ENTHUSIASM, 3 h3 £tj S3 ry* STEADY DETERMINATION AND KEEN FORESIGHT HAS BEEN FOR TWENTY-SEVEN YEARS THE LEADER IN THE DEVELOPMENT OF AUTOMATIC TELEPHONY, THIS VOLUME IS RESPECTFULLY DEDICATED. cs 48S231 PREFACE TO SECOND EDITION Prior to the year 1908, the use of automatic and semi-automatic telephone apparatus was confined almost entirely to the "Independent" telephone companies of the United States of America. Since that year, its adoption, first, by the Governments of various Canadian cities and provinces, then by the Governments of Germany, Austria and France, and, latterly, by the Governments of England, Italy, Australia and New Zealand; and its introduction into Denmark, the Argentine Republic, Cuba, the Hawaiian Islands and the lands of the far East, has created a world-wide interest in this system and a general demand among telephone engineers for a more extensive knowledge of it. An endeavor to meet this demand has resulted in the preparation of this volume, which is the first to be devoted exclusively to automatic and semi-automatic telephony, or which has attempted to treat the art in the manner which its importance requires. Even within the pages of this book the authors have found it impracticable to narrate the full details of the practice of all the manufacturers. They therefore have described fully, typical circuits and apparatus of the Strowger type, with brief outlines of other important systems. The authors feel that the principles and methods brought out in these chapters are sufficiently applicable to other makes of equipment to supply the wants of students of the general art. Radical changes have been made in revising this book from the first edition. As far as possible, only live matter has been retained, live subjects introduced, and old things re-cast into such form as will be more readily understood. The writers do not believe in coddling those who are too lazy to work. Much of the thorough knowledge which people need is to be obtained only by patient, persistent, consistent study. But there is no need of hiding those fundamental principles which are simple behind the customary mass of physical material which is used in putting those principles into practice. For this reason most of the electrical diagrams have been rearranged from the stand-point of the requirements of teaching. The authors wish to express their thanks to Mr. Fred L. Baer, of Chicago, for his assistance on traffic and trunking, to Mr. P. V. Christensen, of Copenhagen, Denmark, for information concerning his traffic distributer system, to Mr. E. L. Grauel, of Rochester, New York, for information on the practical uses of the private automatic exchange, to Dr. Frank B. Jewett, chief engineer of the Western Electric Company, New York City, for information concerning the system of that company, viii PREFACE TO SECOND EDITION and to Mr. Harry E. Hershey, of Chicago, for his assistance in the preparation of technical matter and copy. They also wish to acknowledge the assistance and co-operation of a number of the members of the engineering staff of the Automatic Electric Company, and thus publicly to express to them their appreciation and thanks. The Authors. Chicago, III., January, 1921. PREFACE TO FIRST EDITION Prior to the year 1908, the use of automatic and semi-automatic telephone apparatus was confined almost entirely to the "Independent" telephone companies of the United States of America. Since that year, its adoption, first, by the Governments of various Canadian cities and provinces, then by the Governments of Germany, Austria and France, and, latterly, by the Governments of England, Italy, Australia and New Zealand; and its introduction into Denmark, the Argentine Republic, Cuba, the Hawaiian Islands and the lands of the far East, has created a world-wide interest in this system and a general demand among telephone engineers for a more extensive knowledge of it. An endeavor to meet this demand has resulted in the preparation of this volume, which is the first to be devoted exclusively to automatic and semi-automatic telephony, or which has attempted to treat the art in the manner which its importance requires. Even within the pages of this book the authors have found it impracticable to narrate the full details of the practice of all the manufacturers. They therefore have described fully, typical circuits and apparatus of each of the more important or instructive types on the market, but have found it necessary to confine their discussion of such subjects as " Traffic," "Development Studies," "Central Office Building Design," "Long Distance Line Equipment," etc., to the practice of some one manufacturer Wherever this has been found necessary, the practice of the Automatic Electric Company has been followed, and the authors feel that the principles and methods brought out in these chapters are sufficiently applicable to other makes of equipment to supply the wants of students of the general art. The authors wish to express their thanks to Mr. P. V. Christensen, of Copenhagen, Denmark, for information concerning his traffic distributer system; to Mr. David S. Hulfish, of the Canadian Machine Telephone Company, Brantford, Canada, for information concerning the Lorimer system; to Dr. A. Rapp, Berlin, Germany, for information on the system of the Siemens-Halske Company; and to Mr. Gerard Swope, New York, for his assistance in the preparation of the description of the system of the Western Electric Company. They also wish to acknowledge the assistance and co-operation of a number of the members of the engineering staff of the Automatic Electric Company, and thus publicly to express to them their appreciation and thanks. The Authors. Chicago, III., October, 1914. ix CONTENTS 1. Page Introduction 1 Trunking 3. Automatic Electric Company's Apparatus 4. Trunking, Its Physical Arrangements and Variations 5. Subscriber's Station Equipment, Automatic Electric Company 6. Private Automatic Exchange 7. Measured Service Equipment 8. Automatic Traffic Distributer Equipment 9. Automatic Sub-offices in Connection with Manual Central Offices 10. The System of the Western Electric Company 11. Automanual System 12. Long Distance and Suburban Equipment 13. Rural Automatic Telephones 3 2. . . 38 107 . 151 164 192 204 226 236 253 277 309 14. Cut-overs and Interconnections of Manual and Automatic Offices 15. Power Plant, Supervisory and Testing Equipment 345 16. Traffic 381 17. Development Studies 404 Index 425 . 325 AUTOMATIC TELEPHONY CHAPTER I INTRODUCTION The telephone was invented in 1876 and exhibited at the Centennial Exhibition at Philadelphia. The first automatic telephone switching apparatus, that of Connolly and McTighe, was invented not long after and a patent applied for in Thus the idea of remote controlled electrical switches for con1879. necting telephone lines is almost as old as the telephone itself. The Strowger automatic system originated about 1887, and its first patent was applied for in 1889. The commercial development began with the La Porte, Indiana, exchange in 1893, and has continued steadily ever since. The conditions of labor became more difficult with every passing The world war rendered the situation acute. These conditions year. companies in America publicly to adopt the automatic on a comprehensive scale. Foreign governments also started similar measures of relief for a condition which was almost intolerable to led the largest operating the public. Today (1921) the automatic switching of telephone lines is the foremost subject in telephony. "Automatic" telephony is the term applied to the use of machinery to perform those parts of the work of switching which were formerly done by hand. See definitions of "automatic" in Webster's New International Dictionary (1915), Standard Dictionary (1913), and Worcester's Dictionary (1897). The requirements of a telephone subscriber must be stated in terms which are independent of apparatus and methods. Much error will be avoided if we divest our minds of the conditions imposed by any one means of rendering service. The subscriber requires, without unnecessary delay, without undue stress on himself, to be connected to the telephone which he desires (and to no other), to talk to the called subscriber with ease and without interruption or eavesdropping, and to have this service available continuously. Stated concisely, the requirements are: 1 AUTOMATIC TELEPHONY 2 1. 2. 3. Speed of connection and disconnection. Ease of obtaining connection and disconnection. Accuracy of connection. 4. Voice transmission. 5. Secrecy. 6. Continuous service. The manual switching of telephone lines involves the following inherent features: 1. One or more human intermediaries between the subscribers. This leads to: 2. Translation of calling subscriber's desires from sound into ideas; 3. 4. Translation of these ideas into action; Common spoken language between subscriber and operator; 5. Crowding many operators together in a place difficult to ventilate; 6. Fluctuations in service due to personality and health of operator and the amount of traffic; 7. Immediate impairment of service by strikes, epidemics, etc.; 8. Co-operation of operator with subscriber in securing his desires; largely negatived by the machine-like conditions of the work. Automatic switching involves the following inherent features: 1. Nothing but automatic switches between the subscribers. This leads to: 2. The definite control of the automatic switches by the subscriber (by electrical impulses) 3. The accurate response of the switches to the control, over lines of varying properties; 4. Uniform service, almost independent of traffic, public health, and personnel; 5. Absence of human co-operation, except when desired and requested. Automatic telephony may be divided for study into (1) trunking*, (2) mechanism, and (3) electrical circuits. These will be treated separately, with such cross-references as are helpful. CHAPTER II TRUNKING The methods of trunking which are about to be described are pecuadapted to automatic switching, and are among the chief factors The general plan of an automatic in the success of automatic systems. liarly exchange divides the subscribers' lines into groups and sub-groups. A 10,000-line exchange is composed of ten groups, each having 1000 lines. Generally each 1000-line group is subdivided into ten smaller groups of 100 lines each. Two variations from this plan should be noted, namely, a final subdivision into 50-line groups (practised by the late American Automatic Telephone Company) and into 200-line groups (installed by These variations will be the Western Electric Company in Europe). referred to more fully later and the present discussion will be confined For the purpose of selecting the line to the plan more generally used. of any subscriber the 100 lines of any unit are usually considered as made up of ten groups each having ten lines. The groups and sub-groups need not follow the decimal system, but it is more convenient to have them do so, since the numbers in common use are thus expressed and may be used as call numbers without change. When the decimal system of grouping is used, the digits of the call number indicate the location of the line to which it belongs. Thus, No. 4375 indicates line No. 75 in the third hundred of the fourth thousand. The selection of a line proceeds by successive choice. First the thousand is selected, then a certain hundred in that thousand, and finally the ten and unit in the selected hundred. We will first describe a 100-line system and build it up to the larger sizes. — One-hundred-line System. One hundred telephones may be given by equipping each line with a connector switch, which has the ability, under the control of the subscriber, to connect with any of the hundred lines. In Fig. 1, let the calling telephone be connected by two Associated with these wipers is a wires to a pair of springs or wipers. bank of contacts, each of which is a small flat brass piece. They are arranged in pairs (100 pairs) one for each subscriber's line. The subscriber at the calling telephone can move the line wipers of his connector switch so that they will come into contact with any pair of the hundred. The numbering of each pair of the bank contacts is based upon the number of steps required to lift and rotate the wipers to them. It requires only one step to lift the wipers to the first or lowest level, hence service 3 AUTOMATIC TELEPHONY 4 all numbers in that level begin with "1." After having been lifted to the first level, four steps are required to rotate the wipers to contact 4, so that this will be termed "14." Similarly, to connect with telephone 47 requires four vertical steps and seven rotary steps. 02 03 04 06 05 08 07 09 . 90 !ao !?o >o >o >0 '20 12 II 13 Ife E^Ti PI © o Calling Telephone Fig. 17 18 10 19 *l 1. — Essential principle o o No. 14 No. 47 of connector switch. The order of numbers shown for the bottom level is characteristic of It starts with "11" and runs in regular order to "19," which is followed, not by "20" but by "10." This is because the digit "0" means "ten," so that the number "10" signifies one vertical step all the levels. and ten rotary steps. Bank Multiple Connecto:' I witches Normal L ines '. Telephones M No. 00 Line Wipers 'Lines Line Wipers Line Wipers * Connector 93 Fig. 2. — Principle of 100-line exchange. A complete 100-line automatic exchange requires only 100 connector switches, interconnected so that that of any other subscriber. any subscriber can extend his line to (Fig. 2.) Three telephones are here used T RUN KING 5 to show the relations of the entire hundred. Imagine that the single line running from each telephone to its line wipers consists of two wires, and that each little circle in the connector bank represents two or more conEach telephone has its own connector switch, the wipers of which tacts. are represented by an arrow. Besides being connected to the line wipers of its own connector, each For example, the a contact pair in every bank. line is multiplied to Condensed Symbol Symbol Condensed Symbol —————————— —————————— •— zzzzzzzzzz o C» . ' Wiper Fig. 3. "Bank" — Symbol of major switch. from telephone No. 14 runs to the wipers of connector No. 14, and No. 14 in every connector switch in the hundred. Those wires which join the bank contacts together are termed the "bank wiring," and those wires which connect the bank multiple to the subscribers' lines are called "normal lines." Upon the connector switch devolve certain duties, among which may be mentioned the control of the wipers by the subscriber, a busy test, means for ringing, and means for releasing. If the system be designed line also to contact pair for common battery talking, the connector must furnish direct current to the lines. SYMBOL w ^jjpujjjj^u One Leve/ 4 Etc.-hol05w. Inclusive ''•Jacks Incoming J Trunks l- Fig. 4. Symbols. — Connectors multipled. —From time to time symbols peculiar to automatic tele- The connector-switch symbol (Fig. 1) is reduced to the two symbols shown in Fig. 3. The multipled connectors phony will be introduced. of Fig. 2 are reduced to the symbol of Fig. 4. AUTOMATIC TELEPHONY 6 A "major switch" is one which performs a major act in switching. A "minor switch" is one which has purely auxiliary functions. Non-numerical Switches. — When each subscriber has one connector, there is considerable waste of apparatus, because not very many of them are in use at the same time. If a residence telephone originates ten calls per day (each lasting two minutes) and receives the same number of calls, these 20 calls occupy the telephone 40 minutes per day, which is a liberal allowance for this class This is only 2.8 per cent of the day. A calculation for a business telephone, allowing 40 calls (20 in and 20 out) at one and one- of service. half minutes each, shows an actual occupancy of the apparatus of scarcely more than 4 per cent of the time. Yet the telephone company must maintain all the apparatus which is individual to the line and pay interest and depreciation on it 24 hours per day in order that it may be used 3 or 4 per cent of the time. It is therefore standard practice to interpose non-numerical switches between the subscriber lines and a reduced number of connectors (usually ten per hundred lines). A non-numerical switch is one which will enable the subscriber (without his knowledge) to get the use of an idle connector switch. — Lineswitches. A lineswitch is a non-numerical switch, attached to the subscriber line, which can connect the line to a trunk leading to an / o Fig. 5. — Symbol idle connector (or other switch in the larger systems) A rotary lineswitch (Fig. 5) is one whose subscriber line comes to the wipers and whose bank contains the trunk terminals. of rotary lineswitch It usually nas one m °t 10n (in one plane). 1 , • /• i \ A Keith lineswitch, plunger type, Fig. 6, accomplishes the same result with both the subscriber line terminals and the trunk terminals built into a bank, and employs a moveable plunger There is a master switch to select idle trunks group of lineswitches. (See page 45.) Finder Switches. A finder switch, Fig. 7, is a non-numerical switch, attached to a trunk leading to a connector (or other switch in the larger systems). On its banks are subscriber lines, multipled also to other finder banks in the same group. The finder of an idle connector can move its wipers into contact with a calling line. It may have one or two motions, rotary only, or vertical and rotary. One-hundred-line System. The effect of lineswitches on a 100-line system of lineswitches of either type is to reduce the connectors from one hundred to an average of ten. Figure 8 shows such an exchange with to cause the connection. for each — — Each subscriber line is multipled to its own set of contacts on the bank of each connector; it also runs to the wipers of its rotary lineswitches. T RUN KING own rotary lineswitch. 7 The individual apparatus in the central office and a set of contacts on each connector bank. The connector mechanisms and wipers are common to all they may be used by any subscriber. consists of a lineswitch — Line? Swfhchos> —o £-^> _J ^Normal C&b/e Fig. 6. — Symbol of plunger lineswitch. Line Finder V V V Fig. 7. Trunk Trunk Trunk — Symbol of finder switch. The normal cable, starting from the multipled banks of the connectors, runs to the plunger side of the lineswitches, each line being attached at This cable may be very short, for the connectors are often mounted on the back side of the that point to the subscriber's line to which it belongs. same frame which carries the lineswitches. When a subscriber takes his receiver from the hook, his lineswitch operates and connects his line to a trunk and therefore to a connector A U TO MA TI C TELEPHON Y 8 The subscriber then operates his dial, lifting and rotating the wipers of the connector to the desired contacts. The connection now switch. extends through the normal cable to the lineswitch of the called line and thence to the called telephone. The lineswitch of the called tele- phone is merely a meeting point the line does not go through it. The same exchange equipped with Keith lineswitches is shown in Fig. 9. : lOOTelephones Fig. 8. —One-hundred-line system with rotary lineswitches. — The 1000-line system is built up out of ten The selection is based on first choosing the unit or Thousand-line System. 100-line groups. Line Switches group in which the called line is located and then picking out the particular line in that group. (See Fig. 10.) The telephone lines come into the central office to lineswitch wipers and connector banks as before. The lineswitch banks have trunks 8 Line Switches T RUN KING 9 Though only the first, second, and eighth hundreds are represented, the others are easily imagined. The first function of the selector (to select the level) must be comThe second (to hunt an pletely under the control of the subscriber. idle trunk) should be completely self-acting, for it is of no interest to the subscriber which trunk or connector he gets. Let us suppose that subscriber No. 261 desires a connection with telephone No. 805. The calling subscriber first removes his receiver from the hook. This enables the lineswitch to connect his line to an 10- Selectors per Group 10 -Connectors, per Group One Level of Switch Banks Symbol Meaning .Bank Wiper Bank Bank Wiper Fig. 10. — Thousand-line system. He then pulls "8" on his calling device, in response to which the selector lifts its wipers to the eighth level. Then, in a very brief time, the selector rotates its wipers until they touch the contacts This extends the of an idle trunk line (leading to an idle connector). The subscriber calling line to a connector in the eighth hundred group. then dials "0" which lifts the connector wipers to the "0" or tenth level. He now dials "5" which causes the connector wipers to rotate If the line is busy, the to the fifth set of contacts on the "0" level. connector will refuse to complete the connection, and will send back a busy tone to the calling telephone. If the line is free, the connector will idle selector. complete the connection, ring the bell of the called telephone (No. 805) and permit conversation. When the subscribers hang up their receivers, the selector and connector return to normal, and the lineswitch frees the trunk for use by others. A I r()MA TIC TELKPIIO.X Y j 10 The wires which multiple together the banks of adjacent switches Thus we have "lineswitch bank wires," "selector bank wires," and "connector bank wires." The wires which multiple together different groups of selector banks are called "selector bank multiple." The wires which lead from a switch to a different kind of switch ahead Thus we have "lineswitch trunks," from lineswitch are called " trunks." banks to selector jacks (leading to wipers), and "selector trunks" from are termed collectively "bank wiring." selector banks to jacks of connectors. Wires from connector banks to subscriber lines are called "normals." SYSTEMS LARGER THAN ONE THOUSAND LINES It is useful to view an automatic exchange as a network of trunks which connect switching junctions. It makes easier the explanation of the larger systems. In its simplest form we may consider a 100-line exchange (Fig. 11) as a single switching junction C between a number of telephone lines, T. The actual junction is the multipled banks of the connectors of Fig. 2. We will call +, Ss a "junction" because it is a point where the various subscriber lin Fig. 11. ire connected to each other. — Hundred-line exchange (one junction). — 12. Hundred-line exchange with lineswitches (two junctions). Fig. A 100-line exchange with lineswitches (Fig. 12) is composed of The banks of lines TT and the subscriber junctions for the the connectors C are the Fig. The link which (Compare lineswitches L. 8.) wires leading to the two switching junctions with suitable connecting links. carries thj calls to the lineswitches is there are subscriber lines TT. composed of as many circuits as The link which ties the lineswitches to the connectors has in it as many circuits and connectors as are necessary Each part of this link must be thought of as includto carry the traffic. ing the lineswitch trunk and its connector switch clear to the tips of its wipers. The lineswitch is a switching junction which resembles a reducer in It merely reduces a large number of circuits to a smaller number of circuits. a pipe line. T RUN KING The 1000-line exchange of Fig. 11 13 has three kinds of switching Each group of subscriber lines TT comes in to its own connector junction C, so that C is able to distribute calls to its own junctions. There is another switching junction, S, which is able to subscribers. distribute traffic to all of the 100-line groups, that is, to all of the Only three C junctions are shown, but ten must be imagined. The junction S is the multipled banks of all the selectors. (Compare The links which connect S to the various junctions C with Fig. 10.) junctions C. are selector trunks, including each connector to its wipers. The junction S receives calls from all the junctions L (lineswitches). Each link between one L and S is composed of lineswitch trunks and selectors to their wipers. — Fig. 13. Thousand-line exchange (three kinds of junctions). Fig. 14. — Thousand-line exchange (re-arrangement) One group of lineswitches and their trunks may serve more than 100 subscriber lines. (Fig. 14.) In this illustration, one lineswitch junction The rotary lineswitch L handles all the traffic originated by 200 lines. with 25 trunks on its bank can handle such cases. of 300 or more lines. It may even take care — Ten Thousand -line System. A 10,000-line exchange is made up of Ten 100-line groups or switchboards make up a 1000Ten of these 1000-line groups make up the entire exchange. line groups. groups of groups. (See Fig. 15.) Only three 100-line boards in each thousand and only three thousands are shown, but the reader will remember that the full size is ten of each. Each systems. own connector junction C as in smaller many connectors as are necessary to dis- 100-line unit has its It consists of as tribute the traffic going to the 100 subscribers TT. Each thousand has its own switching junctions 2-S which distributes each of the 100-line boards. It consists of the bank multiple These second selectors perform the same funcof "second selectors." traffic to tions as the first selectors in the 1000-line system (Fig. 10 and Fig. 13) AUTOMATIC TELEPHONY 12 they trunk to all the hundreds of a given thousand. Usually many second selectors for a 1000-line group as there are connectors in that group. Ten 100-line boards, each having ten connec- in that, there arc as tors, require 100 second selectors, whose banks form the "2-S" for the thousand. Each junction 2-$ must receive traffic from all the thousands in the exchange. Hence on the one side the links (composed of first selector trunks) converge from all the thousands, and on the other side they diverge to the separate 100-line boards within the one thousand. Fig. 15. — Ten-thousand-line exchange (network form). There are as many first-selector junctions (1-S) as there are junctions Each junction (1-*S) consists of the banks of first selectors which (2-*S). are multipled together. It is shall necessary that the traffic coming from the various thousands be distributed so as not to overload any one junction (1-S). To them receives traffic from all the thousands. Thus, the junction marked A (1-S) receives traffic from all the lineswitch The junction B(l-S) receives calls from all the units marked A(L). secure this, each one of junctions B(L). This arrangement is continued throughout all the ten thousands. Each 100-line unit C is shown as having its own group of lines witches This may or may not be the case in practice. Several hundreds may use the same group of lineswitches, especially if they are of the roA given hundred may be served by two or more groups of tary type. L. TRUNKING lineswitches, as heavy. 13 often done with Keith lineswitches, if the traffic is Finally, the lineswitches may be grouped without any relation is to the groups of subscriber lines. The trunking described above is shown in more detail in Fig. 16. The actual apparatus and trunks are indicated instead of the junctions and links which represent them. Three out of the ten different thou- sands are shown, also three out of the ten hundreds in each thousand. All of the lineswitches are in a row at the left. The first selectors and connectors in groups or "sections" of ten each alternate with each other in the second column. The second selectors are at the right, ar- ranged in sections of ten switches each. In the banks of each section of ten connectors we recognize a junction C of the network. These connectors deliver calls to the 100 lines to which they belong. In the banks of the second selectors of a 1000-line group we can All ten sections (100 second recognize a junction 2-S of the network. They are usually lettered A to J selectors) are multipled together. This bank multiple binds together the different hundreds in inclusive. a given thousand. The banks of the first selectors are also multipled together so as to form switching junctions 1-S. One section in each thousand goes into the making up of a given junction 1-S. The ten sections of first selectors in a given thousand are lettered from A to J inclusive. Since there are ten thousands in the exchange, there are ten A sections, ten B sections, etc. In order to distribute the traffic, all the A sections of first selector banks are multipled together to form one junction. This appeared as All the B first-selector banks likewise are multipled A(1-*S) in Fig. 15. to form another junction. It appeared as B(l-S) in Fig. 15. In this way ten bonds are formed which tie together all the ten thousands. As illustrated in Fig. 16, each first-selector section (ten switches) receives its traffic from a lineswitch board which belongs to a given hundred. In a certain sense that particular 100 first selectors, relationship is lines, its lineswitches, and connectors, may be said to belong together. binding only for the connectors. But this The lineswitches and the first selectors may be grouped quite differently. The first selector banks are trunked to the second selectors according to levels and thousands, and according to like sections. The first level of all first selector banks must trunk to second selectors in the first thousand, second level to second selectors in the second thou- sand, third level to the third thousand, etc. The trunks from A first selector banks are trunked to A second selectors in all the thousands. B first selector banks trunk to B second selec- tors in all the thousands. This grouping applies to all the sections. AUTOMATIC TELEPHONY 14 First Selectors L.Sw. I200 £ 2IOO q Connector Fio. 16. — Ten-thousand-line exchange with lineswitches. 100 Trun T RUN KING of 15 The combination of the above two schemes results in the arrangement Consider the A first selectors. From the first level trunks shown. of their banks run ten trunks to the first thousand. A section of second selectors in the From the second level of A first selector banks run ten trunks to the A second selectors in the second thousand. third level trunks to is Likewise the A second selectors in the third thousand. This continued throughout all the levels. Consider the B first selectors. Their first bank level trunks to the B section of second selectors in the first thousand. trunks to B second selectors in the second thousand. leads to B second selectors in the third thousand. The second level The third level This is continued to all levels and thousands. In this way all first selector sections of a given letter trunk to all thousands to second selector sections bearing the same letter. Also each second selector section of a given letter receives traffic from all thousands from first selector sections bearing the same letter. The traffic is effectively mingled. We will trace three calls, illustrating three different conditions in trunking which will be met in actual traffic. They are a call from a number to another number in the same hundred, from a number to another in a different hundred in the same thousand, and between numbers in different thousands. Let No. 2125 call No. 2170. line On taking the receiver from the hook, the switch No. 25 on the "2100" board connects the line to an idle first selector on section A. Pulling "2" on the dial lifts the wipers of that first selector to the second level and automatically picks out an idle trunk leading to section A in the same thousand. The second operation of the one impulse, which lifts the wipers of this second selector to the first level and causes them to seize an idle trunk to a concalling device sends nector in the hundred, known as the "2100" hundred. The last two pulls of the dial, 7 and 0, lift the connector wipers to the seventh level and rotate them to the tenth set of contacts in that level, which is numbered "70." The line now extends through the normal cable to the jacks of lineswitch 70 and out to the called telephone. Let telephone No. 2130 call No. 2241. The call will extend from line- switch 30 on the 2100 board to an idle first selector on the "A" section. Thence it is trunked through the second bank level to a second selector on section A in the same thousand. From here it goes through the second bank level to a connector attached to the "2200" board, and from the forty-first set of contacts, through the normal cable to lineswitch jacks, 41 and out to the telephone No. 2241. If telephone No. 3255 calls No. 1267, the call will be routed as follows: lineswitch 55 on the "3200" board, idle first selector on "B" section, " first level of first selector banks to idle second selector on B" section AUTOMATIC TELEPHONY 16 of first thousand, second level of this second selector to idle connector on " 1200" board, and by the usual manner to the line No. 1267. The Secondary Lineswitch. — The object of the secondary lineswitch to reduce the number of trunks and trunking apparatus by taking advantage of the economies offered by larger groups of trunks. It has is been proved by experience that small trunk groups can not handle as The saving effected by many calls per trunk as large groups can handle. large trunk groups is treated more at length in the chapter on "Traffic," to which the reader is referred. The secondary lineswitch is introduced in two places, (1, local) between subscriber lineswitches and first selectors to reduce the number of selectors required, and (2, outgoing) between selector banks and outgoing trunks to other offices of a multi-office exchange to reduce the number of inter-office trunks, the incoming selectors in the distant office, and the impulse repeaters associated with each trunk in the sending office. The general principle of trunking by primary and secondary lineThe subscribers' lines are attached to the primary lineswitches. The secondary lineswitch boards are shown at switches is shown by Fig. 17. the right. As many as 2500 lines have been put in one large group or division, though on account of the space only 1000 have been shown in the figure. If the traffic from each 100 lines is not too great, only ten trunks will be allowed per 100, as illustrated. The trunks from the banks of the primary lineswitches run to the secondary lineswitches, indicated by the arrows marked "plungers." From the banks of the secondary lineswitches the trunks run to first selectors, ten for each section of secondaries. They are lettered from A to J as usual, and their multipling is done in accordance with the practices already described. It should be observed that not more than one trunk from a primary board runs to a given secondary section. Examining primary board 1, notice that trunk 1 (beginning at the top) runs to secondary section 1, trunk 2 to secondary section 2, etc. This is one of the prime facts of the arrangement, and upon it depends the effectiveness of the large trunk group. By giving each primary board a trunk into each of the ten secondary sections, we render all the first selectors, 100 of them, available to all the subscribers; also, if all the first selectors of a given secondary shelf become bus}' it renders useless not more than one trunk from each primary board or section. This arrangement may be seen in both kinds of lineswitches. If we consider any primary lineswitch, we shall see that it has a choice of ten trunks, any one of which will serve equally well. It has to share this choice with from 49 to 99 other lines. When the subscriber's line, T RUN KING Primary Line SwS. Phingers ,-BanM Fig. 17. 17 Secondary line Sws Plu ngers — Primary and secondary trunking (distributive). A U TOM A TIC TELEPHON Y 18 through its primary lineswitch, has seized one of the ten trunks, it is Herd thus extended to a lineswitch on one of the secondary boards. again it has a choice of ten trunks any one of which will serve equally well. It has to share this choice with a number of other secondary lineswitches. But by the double choice of ten trunks each we secure the ability to seize any one of 100 trunks, each leading to first a selector. If the traffic from 100 subscribers' lines is too great to be carried by ten trunks, it is customary to divide a primary lineswitch board into two parts of 50 lines each, and to give each part from seven to ten trunks. MULTI-OFFICE EXCHANGE The automatic system is easily divided into a number of offices with out increasing the number of digits in the call number. This may be by dividing a 1000-line exchange into two parts. We can put the first, second, third, fourth and fifth hundreds in one office, and the rest of them in another office. For simplicity assume only four hundreds, placing the first and second hundreds in the first office and the illustrated fourth and fifth hundreds in the second office. Fi'rst First Hundred Subs. Lines LineSw. Second Office Office Second Hundred Fourth Hundred Subs. Lines Subs. Lines LineSw LineSw Fig. 18. (See Fig. 18.) — Thousand-line, Fifth Hundred Subs- Lines LineSw. 2-office exchange. We will assume that the traffic division is ideal, i.e., that there is no community factor and therefore half of the originating traffic in each office goes to lines in the same office and the other half to the other office. This is necessary in opening up the subject. In each office the first-selector banks are multipled to each other, but there is no common multiple between the two offices. In the first office, levels 1 and 2 run to local connectors in the corresponding hundreds. Trunks from the fourth and fifth levels run to incoming connectors in the second hundreds. office, terminating in the fourth and In the second office, the reverse is the case. fifth The first and T RUN KING 19 second levels run to the distant office to incoming connectors, while the fourthand fifth levels carry the local traffic to local connectors in the This is one way of handling such a case. fourth and fifth hundreds. The 10,000-line system may also be divided into two offices of 5000 lines each, placing the first, second, third, fourth and fifth thousands in Znd.Sel. Is+.Sel. Connec+or Is+Hund. \ ?nd. Hund. __; Zrd. Hund. BS 4 th.