Manual / Guide · 1958
AE Training Bulletin 802: Fundamentals of Apparatus and Trunking
How linefinders, selectors and connectors are trunked together.
Manual / Guide · 1958
How linefinders, selectors and connectors are trunked together.
FUNDAMENTALS OF APPARATUS AND TRUNK O R I G I N A T O R S O F T H E D I A L T E L E P H O N E This is one of the helpful booklets in the AUTOMATIC ELECTRIC TRAINING SERIES on STROWGER AUTOMATIC TELEPHONE SYSTEMS Electrical Principles of Telephony Mechanical Principles of Telephony Fundamentals of Apparatus and Trunking The Plunger Lineswitch and Associated Master -Switch Rotary Lineswitch The Connector The Selector Pulse Repeaters Trunking Power and Supervisory Equipment Party -Line Connectors and Trunk-Hunting Connectors Reverting-Call Methods The Test and Verification Switch Train Toll Switch Train Switching Selector -Repeater Private Automatic Exchanges with PABX Appendix Community Automatic Exchanges Manual Switchboards Linefinder Switches May we send you others pertaining-to equipment in your exchange ? CONTENTS PART I . TRUNKING Page .......................... ............................. 1. INTRODUCTION 1 2 . TRUNKING 1 P A R T I1 3 . INTRA-OFFICE TRUNKING . 100 LINE O F F I C E . . . . . . . . . . . . . . . . . . . . . . . . . 3.1 Connector . . . . . . . . . . . . . . . . . . . . . . . . . . . 3.2 Linefinder . . . . . . . . . . . . . . . . . . . . . . . . . . . 2 2 3 ........... 3 3 . 4 Method of o p e r a t i o n . . . . . . . . . . . . . . . . . . . . . . 3 4 . 1000 LINE O F F I C E . . . . . . . . . . . . . . . . . . . . . . . . 3 4.1 S e l e c t o r . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4 4.1.1 R e v e r s a l s . . . . . . . . . . . . . . . . . . . . . . . . 5 4.1.2 S l i p s . . . . . . . . . . . . . . . . . . . . . . . . . . 5 4.1.3 G r a d i n g . . . . . . . . . . . . . . . . . . . . . . . . . 6 3.3 L i n e equipment. g r o u p r e l a y s and d i s t r i b u t o r 4.2 Method of o p e r a t i o n . . . . . . . . . . S o . . . . . . . . . . 6 . 10. 000 LINE O F F I C E . . . . . . . . . . . . . . . . . . . . . . . . 6 6 . SUBSCRIBER GROUPING . . . . . . . . . . . . . . . . . . . . . . 6 7 . PARTY LINES . . . . . . . . . . . . . . . . . . . . . . . . . . . 8 8 . SWITCH TRAINS . . . . . . . . . . . . . . . . . . . . . . . . . . 8 5 . INTER - O F F I C E TRUNKING P A R T 111 9 . IMPULSE R E P E A T E R . . . . . . . . . . . . . . . . . . . . . . . 10 . INCOMING SELECTOR . . . . . . . . . . . . . . . . . . . . . . . . 1 0 9 . AUTOMATIC T O MANUAL . . . . . . . . . . . . . . . . . . . . . 10 11 12 . AUTOMATIC T O AUTOMATIC . . . . . . . . . . . . . . . . . . . 1 0 13 . P-A-XANDP-A-B-X . . . . . . . . . . . . . . . . . . . . . . . 10 FUNDAMENTALS. OF APPARATUS AND TRUNKING PART I. 1. INTRODUCTION This bulletin deals p r i m a r i 1y with the fundamentals of trunking. Facts concerning apparatus a r e included to give a better understanding of trunking. 2. TRUNKING "Trunking" may be defined a s the interconnection of equipment both within an office and between offices. A single trunk provides a path between switching equipment for one call. Therefore, the amount of traffic that can flow through an office, and the efficiency of the office to handle the traffic flow depends greatly upon the way the office is trunked. Strowger automatic switching equip.ment is, in general, mechanically a n d -electrically standardized, while the way in which the equipment is connected o r trunked is very flexible. TRUNKING will not be in contact with more than one pair a t a time. These a r e known a s nonbridging contacts. A control bank is made up of one hundred terminals o r contacts arranged in ten levels with ten terminals per level. Each of these terminals represents a control lead. The contacts of the control bank a r e constructed s o the wipers will engage the succeeding s e t of terminals prior to leaving the present set, and a r e known a s bridging contacts. Figure 1 shows a control bank (upper) and a line bank (lower) with the associated wipers. In cases where a switch has an extra control (EC) lead the control bank of the switch has two hundred terminals arranged in the same manner a s a line bank. A trunk within an office will usually consist of either three conductors (i-, -, C) o r four conductors (+, -, C, EC). The plus (+) and minus (-) leads make up the transmission path, and the control (C) lead provides for control of the switches in the circuit. The extra control (EC) lead, when present, provides for extra control, restrictions, certain types of ringing, etc. It should be clearlyunderstood that a subscriber's loop o r an inter-office trunk, consists of only two conductors (+ and -), and that the control (C) and extra control (EC) leads never leave the office. Trunks a r e connected to all Strowger switches either a t the switch jacks (i.e., to the mechanism) o r the banks. There a r e two types of banks associated with every Strowger switch, a line bank and a control bank. A line bank is made up of two hundred terminals o r contacts, arranged in ten levels, with ten pairs on a level. E a c h pair of terminals represents the plus (t) and the minus (-) of a trunk o r line. The contacts of the line bank a r e constructed s o that the wipers Figure 1. Control bmzk m d line bank with associated wipers. BART 11. INTRA-OFFICE T R U N K I N G I GROUP RELAYS LINEFINDER 8 DISTRIBUTOR LlNE . EOUIP TELEPHONE 72 TELEPHONE 4 4 72 LlNE -EOUIP 44 . I .......... .......... .......... .32.. ...... .......... .......... .. .......... .......... .......... CONNECTOR I .......... .......... .......... .7<........ .......... .......... .......... .......... Figure 2. Schemalic diagvam of 100 line offzce. To better understand intra-office trunking, we shall f i r s t discuss a small one hundred line office. Next we will expand to a one thousand line office and then to a ten thousand line office. In each case the equipment will be discussed a s it is introduced into the offices. 3 . 100 LINE OFFICE ; No matter what the size of the office, in order that each subscriber may both originate and receive calls, his line must be terminated in two places within the office. To receive calls the subscriber's line must terminate on a s e t of connector terminals, and to originate calls, his line must terminate on a s e t of linefinder ter'minals . Since the linefinder is a non-numerical switch and will not respond to the pulses of a subs c r i b e r ' s dial, i t must be connected directly to a numerical switch that will respond to dial pulses. In the case of a one hundied line office, each linefinder is trunked back to back (switch jack to switch jack) with a connector. Through thistypeofanarrangement,commonly known a s a linefinder-connector-link, only one call may take place a t a time. Therefore, the number of calls that mag take place simultaneously, depends directly on the numb e r of line finder -connector -links available. Figure 2 illustrates a one hundred line office, showing the termination points for two subs c r i b e r s , the linefinder -connector -link and other apparatus involved, and the trunking between the apparatus. Only one linefinder connector-link is shown for illustration, but let us say it represents ten such links. pulses. A call e n t e r s a connector through the connector mechanism, and is connected to one of one hundred lines, via the bank terminals, depending on the two digits dialed into the connector. The connector bank terminals a r e numbered a s shown in figure 3 . The number assigned to each position is the number that must be dialed to bring the wipers to that position. In other words, to reach position 53, the wipers must step up to the fifth level and rotate to the third position. Each position in figure 3 r e p r e s e n t s a s e t of terminals (+, -, C) o r (+, -, C, EC) depending on the particular circuit involved. F o r our discussion we will consider each position a s three t e r minals (+, -, C). ) @ @ @@ @ @@ @@ @ @@@@@@@@@@) @ @ @ @ @ @ @ @ @ @ @@@@@ @@@@ @ @@@@@@@@@a @ @ @ @ @ @ @ @ @ @ @ @ @ @ @ @ @ @ @ @ @ @ @ @ @ @ @ @ @ @ @@@@@@@@@)@ 1 3.1 Connector. The basic Strowger switch is @ @ @ @ @ @ @ @ @ @ the connector, and i t functions to complete a call to the desired party in response td dial Figure 3. Bank-contact position numbering. B > CONNECTOR I C O N ~ ~ E C T O 2R CONNECTOR 10 TERMINAL 0 LOCK Figure 4. Connector bank multiple. e 8 t The group r e l a y s and distributor assign an idle linefinder to s e a r c h for the line and a t the s a m e time prevent the remaining linefinders from searching. After the calling line is found by the assigned linefinder, the group relays and distributor drop out of the connection, and prepare to assign another idle linefinder to find the next line desiring service. As was previously stated, we d e s i r e to have ten connectors serve the one hundred lines. These ten switches shall be arranged on a shelf having the corresponding positions in all the banks multipled together and then to a bank terminal block. The bank multiple is shown in figure 4. All of the connectors on the shelf have a c c e s s to any of the one hundred lines. Of course if a connector's wipers a r e connected to a particular line, no other connector on the shelf may engage the s a m e line. F o r example, i f connector one on a shelf is engaged with line "35", none of the remaining connectors on the shelf may engage line "35". 3.4 Method of operation. So that we maybetter understand how all the equipment functions and the trunking that is necessary to complete a call, let us t r a c e a call through a one hundred line office. In figure 2, when the subs c r i b e r on line 44 r a i s e s his handset to make a call, his line equipment signals the group relays that a line wants service. The group relays and distributor then assign a n idle linefinder to find the line. The wipers of the assigned linefinder step vertical and rotary to connect line 44 through the linefinder to i t s associated connector. At this point the group relays and distributor drop dut of the connection. When the subscriber dials the the connector steps to digits "7" and "2" the seventh level and rotates to the second contact. If line 72 is not busy, the connector switches through and telephone 72 is rung. It should be noted that in this one hundred line office, due to the s m a l l amount of apparatus involved, the trunking involved is a l s o small. 3.2 Linefinder. There a r e two types of linefinders, the one hundred line and the two hundred line. The one hundred line .Linefinder has one hundred s e t s of terminals, through which calls may enter, while the two hundred line linefinder has two hundred s e t s of t e r minals, through which calls may enter. In each type there is only one point of exit, the switch jacks. In the one hundred line office with which we a r e concerned a t the present, the one hundred line linefinders a r e used. The group of linefinders that s e r v e the same group of lines a r e mounted together on a shelf with their banks in multiple. Information on the bank numbering, multipling of banks and arrangement of linefinders is found in bulletin 801, $29, and in bulletin 821. The linefinder's main function is to find a line which is desiring service. Associated with the linefinders a r e the individual line equipments and the group relays and distributor. 3.3 Line equipment, group r e l a y s and distributor. Each line has i t s own line equipment. It most generally consists of two relays that control the traffic on the line. The line equipment's functions a r e to call for a linefinder when the handset of the telephone on the line is lifted, to provide markings s o the linefinder can find the proper line, and to mark the connector terminals of it's line busy. A s e t of group r e l a y s and distributor a r e associated with each group of linefinders. 4. 1000 LINE OFFICE - A one thousand line office is made up of ten connector groups o r shelves and five two hundred line linefinder shelves. In o r d e r that all of the one thousand subscribers may call each other, a selector must be inserted between the linefinders and the connectors. This is illustrated in figure 5. As before each linefinder must be associated directly with a numerical switch, and in this case it is a selector. Therefore, instead of having a linefinder -connector -link as in the one hundred line office, there is a linefinderselector -link. All of the selectors which a r e tied back to back with the linefinders a r e Figure 5. Schematic diagram of 1000 line offzce. known a s f i r s t selectors. C o r r e s p o n d i n g levels of all the f i r s t selectors a r e multipled together, and then trunked to the connector groups (only three of the possible ten a r e shown). The level from which each connector group i s trunked determines the number of the connector group. 4.1 Selector. A selector operates vertically in response to dial pulses, but once i t has CONNECTORS I N " 6 0 0 n GROUP reached a level i t rotates a u t o m a t i c a 11y, hunting a trunk to an idle switch, in this case a connector. It searches from left to right and will switch through to the first idle connector that i s found. If no idle connectors a r e found in the connector group the selector will return busy tone to the calling party. Figure 6 illustrates how trunks lead from a selector bank to the connectors. Selector banks, which a r e similar to those associated with the connector, a r e multipled in units of ten, and each such unit with its associated selectors is termed a "shelf". A straight bank multiple for one level i s illustrated (only three selectors a r e shown) in figure 7. It should be noted that a terminal s t r i p is associated with the bank multiple of each shelf. These terminal s t r i p s aremounted on a distributing terminal assembly (DTA) where it is convenient to multiple a large group of selectors on each level and trunk to succeeding switches, 7 SELECTOR IllD TERMINAL 5. C O N N E C T O R S IN ' 2 0 0 " GROUP 40 . 0 0 0 . , 0 . 0 . 1 Figure 6. Trunks leading from a selector bank to connectors. Figure 7. Straight selector bank multiple for one level. Trunks terminate on terminal strip. ! TO " 8 0 0 " CONNECTOR 1 2 3 4 5 6 7 8 GROUP 3 1 0 TO '800' SHELF '1 ' 2 3 CONNECTOR 4 5 6 7 8 SHELF GROUP 9 1 0 \ REVERSAL Figure 8. Straight multiple between the terminal strips of eight shelves on one level. b t b Figure 9. Multiple between the terminal strips of eight shelves with a reversal in first 9 trunks. Let u s use the 800 group a s an example. There a r e ten connectors in the group and therefore there a r e ten trunks needed. These ten trunks can be made available to all the s e l e c t o r s by multipling a l l the terminal s t r i p s of the eighth level, a s illustrated in figure 8. Since all of the selectors have a c c e s s to the same trunks and in the same o r d e r , the f i r s t connector, which is associated with the f i r s t trunk, is used quite often in comparison to the connector associated with the tenth trunk. 4.1 -1 Reversals. To help balance the connector usage within a shelf, a r e v e r s a l may be inserted a t the mid point of the selector group having a c c e s s to this particular connector shelf. In figure 9 you will note that a l l the trunk3 with the exception of the l a s t have been reversed. The tenth trunk is excluded from the r e v e r s a l in o r d e r to meter how many times selectors must s e a r c h over a l l ten contacts on the eighth level attempting to find a n idle trunk. Because of the r e v e r s a l the upper four shelves of the selectors s e a r c h over the trunks in a 1 , 2, 3, 4, 5, 6, 7, 8, 9, 10, sequence, while the lower four shelves s e a r c h o v e r the contacts in a 9, 8, 7, 6, 5, 4, 3 , 2 , 1 , 1 0 , s e q u e n c e . 4.1.2 Slips. Another method used to help equalize the load to succeeding switches in a small office is the slip. A slip may be found in the bank multiple of a selector shelf that terminates on a terminal block o r a distributing terminal assembly (DTA). The slip changes the sequence in which the trunks to the succeeding switches appear to individual selectors within a shelf. Normally only the f i r s t five contacts in each bank a r e included in the slip, while the remaining five contacts a r e multipled straight. This type of slip is illustrated in figure 10, and is known a s a "forward slip of one". In figure 10 only seven of the ten selector banks for one level a r e shown. Notice that trunk number one is the f i r s t choice of selector one; trunk two, the f i r s t choice of selector two; trunk three, the f i r s t choice of selector three; trunk four, the f i r s t choice of selector four; and trunk Figure 10. Selector bank multiple for one level having a "slip" of one in the first five contacts. five, the f i r s t choice of selector five. Starting with selector s i x the same trunk sequence is repeated. 4.1.3 Grading. The method of making available to a particular level of a selector group more than ten trunks is termed grading. The traffic load to the succeeding switches determines the n u n b e r of trunks necessary. Each selector shelf on one level, of course, still has access to only ten trunks, but a portion of the ten trunks may differ between shelves. As an example, the fifth selector level in figure 5 required fifteen trunks. The eight shelves could be graded a s shown in figure 11 to provide fifteen trunks. With this arrangement, each shelf has a c c e s s to only ten trunks, however shelves A, B, C, and D, each have a c c e s s to trunks 1, 2, 3, 4, 5, 6, 7, 8, 9, 10, in that o r d e r , while shelves E, F, G, and H, each have a c c e s s to trunks 11, 1 2 , 13, 14, 15, 9, 8, 7, 6, 10, in that order. Once again the l a s t contacts of each shelf a r e rnlultipled straight to provide for metering. The other four common trunks a r e reversed a t the mid point of the selector group to further add to the e q u a l i z a t i o n of usage of the succeeding switches. 4.2 Method of operation. In a one thousand line office a three digit directory number is required to reach a subscriber instead of a two digit number a s used in a one hundred line office. The f i r s t digit selects the hundreds group and the remaining two, the proper line in the group. F o r example, let u s say figure 5, r e p r e s e n t s five shelves of linefinders, each shelf having sixteen switches. These eighty linefinders a r e trunked to eighty f i r s t selectors T O " 5 0 0 ~C O N N E C T O R I . SHELF GROUP < (8 shelves). The f i r s t selectors' banks a r e multipled straight on levels one to three and s i x to ten a t the DTA, while levels five and six a r e graded on the DTA. Levels one to three and six to ten each lead to a connector group containing ten switches. Only two of these connector groups a r e illustrated. Levels four and five each lead to fifteen switch connector groups. Only one of these groups is illustrated. Even though there a r e one hundred and ten connectors serving the one thousand lines, only eighty calls may take place simultaneously, because there a r e only eighty linefinders and eighty f i r s t selectors. The number of calls that may take place a t one time depends on the switch rank with the smallest number of switches. 5. 10,000 LINE OFFICE In expanding a one thousand line office to a ten thousand line office there must be another rank of selectors introduced. I n s t e a d of accessing connector groups directly from levels of the f i r s t selectors, we must a c c e s s a second rank of selectors. The levels of the second selectors then access the connector groups. Now in order to reach a party, four digits must be dialed i n s t e a d of three. Figure 12 represents a ten thousand line office illustrating equipment and trunking to provide for three thousand of the possible ten thousand lines. To serve the three thousand lines illustrated, i t is necessary to have fifteen linefinder shelves. If there a r e twenty linefinders per shelf o r a total of three hundred linefinders, the office then requires three hundred f i r s t selectors to be trunked back to back with the linefinders. Also assume there a r e one hundred selectors in each of the three second selector groups, and in each connector shelf, ten connectors. In actual practice the number of switches provided in each shelf o r group in the office would depend on the traffic through the particular shelf o r group. 6. SUBSCRIBER GROUPING The major grouping of subscribers is the exchange. The exchange is further divided into various offices, and within each office the subscribers a r e grouped by directory number. Usually the geographic location of the subscriber determines his exchange and office within the exchange. While his location normally has nothing to do with the remainder of digits in his directory number. At present, directory numbers in larger a r e a s Figure 11. Grading arrangement giving eight shelves access to fvteen trunks from one level. 6 and those a r e a s which desire to be a part of the direct distance dialing system, consist of Figure 12. Schematic diagram of 10,000 line office. seven digits, that is two letters 'and five numbers. The two letters of the directory number a r e the first two l e t t e r s of the exchange name. Thus a telephone number might be written a s Fillrnore 5-7463, and dialed a s FI 5-7463. Since the l e t t e r s on the dial I represent numbers, note figure 13, the resulting dial pulses when the number i.s dialed would be 3-4-5-7-4-6-3. Notice in figure 13 that no letters are associated with the numb e r s one o r zero. Theoretically 10,000,000 telephone lines may be represented by a seven digit numbering system. Starting with the called subscriber, the final two digits of the directory number locate his line within a hundreds group. The preceding digit selects the hundreds group desired, while the fourth digit of the directory number selects the proper thousands group. The third or "office" digit selects the proper ten thousand group. The desired one hundred thousand group is selected by the second digit and the desired one million group i s Figure 13. Telephone dial. selected by the f i r s t digit. It should be noted that each rank is a multiple of ten, o r within each rank there a r e ten choices. Figure 14 represents the dial, the switch ranks, and switch levels involved in reaching a desired directory number. Essentially, the process of reaching a desired number is one of elimination. To better u n d e r s t a n d the (ST 2ND 3RO 4TH 5TH SEL SEL SEL SEL SEL CONN CONN CONTACTS Figure 14. TIzeovetic 10,000 line system. process let u s take the number F I 5-7463 and t r a c e i t through the figure. When ithe "I?" is dialed the f i r s t selector steps to the third level, thereby e 1i m i n a t i n g the nine other possible levels o r a total of 9,000,000 lines. In dialing the "I" into the one million group (second selectors) the selector steps up to the fourth level and once again eliminates the remaining nine levels, o r another 900,000 lines. Another 90,000 of the remaining 100,000 lines a r e eliminated when the third digit, a five, is dialed into the one hundred thousand group (third selectors) leaving 10,000 lines. Now by dialing the fourth digit, a seven, into the fourth selector we e 1i m i n a t e another 9,000 lines. The lines a r e reduced j.n numb e r to one hundred when the fifth digit, a four, is dialed into the fifth selector. When the sixth digit, a six, is d i a l e d into the connector ninety of the remaining one hundred lines a r e eliminated, leaving only the ten-on level six. Nine of these a r e eliminated when the seventh and final digit, a three, is dialed, leaving only the desired line. 7. PARTY LINES Thus f a r we have discussed only :systems with one subscriber per line. In many cases a subscriber does not require a line to himself, for his traffic load is nod g r e a t enough to warrant it. To s e r v e sevel-a1 such s u b s c r i b e r s living in approximately the same locality a party line is arranged, anti i t may s e r v e from two to ten parties. P a r t y line s e r v i c e is divided into two classifications, terminal per station and terminal per line. On a terminal p e r station party line, each subscriber on the line is represented in the office by a s e t of connector terminals. If there a r e four parties on a line each connector group w i l l s e r v e one lhundred s u b s c r i b e r s , while only twentyzfive lines a r e required. A terminal per station four party, party line is illustrated in figure 15. On a terminal per line party line, the line is r e p r e s e n t e d in the office by one s e t of connector terminals, even though there a r e a number of parties on the line. This type of party line r e q u i r e s a special type of party line connector which is discussed in bulletin 813. SUBSCRI B E R STATIONS 51 52 53 54 51 52 5 3 5 4 3 5 5 6 5 7 5 8 5-9 5 0 CONNECTOR TERMINALS Figure 15. Terminal per station, four party, phrrty line. SUBSCRIBER STATIONS LINE e m m m m m ..,i 51 52 53 5 4 5 5 5 6 5 7 5 8 5 9 5 0 CONNECTOR TERMINALS Figure 16. Terminal per line, four party, party line. It is also necessary to dial three digits into this connector, two to find the c o r r e c t s e t of terminals and one to ring the c o r r e c t subscriber on the line. If there a r e four parties on a line a s illustrated in figure 16, then such a connector group can s e r v e four hundred subscribers on one hundred lines. To s e r v e a given number of subscribers, terminal per station party lines require more shelves of switches, while the terminal per line party lines require m o r e switches per shelf. Whenever a subscriber on a party line dials the directory number of another party on the s a m e line, he receives busy tone, because he is already occupying the line. In o r d e r that one party may signal and talk to another on the s a m e line, special equipment, a reverting call switch, must be provided in the central office. Reverting call methods a r e discussed in detail in bulletin 8 14. 8. SWITCH TRAINS Thus f a r we have discussed only the local switch train, but in each automatic office there a r e usually three separate switch trains. In addition to the local switch train, there is a toll switch train and a t e s t switch train. The toll switch train provides a separate path over which a toll call reaches the called subscriber. The switches involved have certain special features, which a r e not provided on local switches. The toll switch t r a i n is controlled by the toll operator and not by the subscribers, a s is the local switch train. Bulletin 816 discusses in detail the toll switch 'train, The test switch train is u s e d by office maintenance men to test lines and equipment throughout the office. It is a l s o used by toll operators to verify toll calls. Local subs c r i b e r s never have a c c e s s to the t e s t switch train. Additional information on the test switch t r a i n can be found in bulletin 815. PART 111. INTER-QFFICiE When considering inter-office trunking, we must keep in mind that each office may be different. We a r e going to consider both trunking between automatic offices and automatic and manual offices. In both of the above c a s e s the offices may be connected 1by either f r e e service (sometimes termed extended a r e a service) trunks o r toll trunks. Figure 1 7 illustrates two offices, one automatic and the other manual, connected by one-way f r e e service trunks. There i s , you will notice, some equipment which we have not yet discussed. Namely the impulse repeater and the incoming selector. 9. IMPULSE REPEATER The impulse r e p e a t e r consists princ:ipally of relays, which repeat dial pulses originating in one office to the switching equipment of another office. The u s e of r e p e a t e r s i n inter-office TRUNKING trunking eliminates the need f o r the control ( C ) l e a d b e t w e e n offices, t h e r e b y limiting the i n t e r - o f f i c e t r u n k to two conductors. T h e i m p u l s e r e p e a t e r d o e s not improve t r a n s m i s s i o n but i t keeps the t r a n s m i s s i o n level from dropping over lc lg distances. The primary function of an impulse repeater is to improve pulsing over long distances, Between automatic offices, both one-way and two-way repeaters can be used. One-way repeaters .allow calls to be made in only one direction over one trunk, while with twoway repeaters, calls can be made in either direction. Another type of repeater is used with one-way trunks between automatic and manual offices. It i s commonly known as an automatic to manual (A-M) repeater. It provides for s i g n a 1i n g the manual operator, supervision, and also allows a two conductor trunk to be used. It is not needed to repeat pulses. Bulletin 8 10 contains more information on impulse repeaters. - OFFICE A AUTOMATIC -- SUBSCRIBERS ) - - MANUAL NUMBERS 611-000 oFFiyo:N ;CCESS-5 \ 1 + OFFICE -B DIRECTORY 111-400 I - h I-WAY I INTER -OFFICE / TRUNKS ERS BOARD Figure 17. Automatic office and manual office connected by one-way trunks, 10. INCOMING SELECTOR Incoming selectors a r e n e c e s s a r y in an automatic office where there a r e inter-office trunks involved. It is usually mechanically and electrically the same a s a local selector; however, an incoming selector is not trunked back to back with a linefinder, but instead, is connected to an inter-office trunk. Each incoming one-way trunk o r two-way trunk is connected to a n i n c o m i n g s e l e c t o r , s o the number of incoming selectors depends on the number of incoming one-way and two-way trunks. The incoming selectors' banks a r e usually multipled with the banks of the local first selectors on levels w h i c h lead to local subscribers. This multiple takes place on the DTA. Levels of local f i r s t selectors, which lead to other offices a r e u s u a l l y n o t multipled to corresponding levells of the incoming selectors. This is illustirated in both figures 1 7 and 18. 11. AUTOMATIC TO MANUAL Figure 17 illustrates an automatic and a manual office inter -connec ted with one -way trunks. You will notice that the trunk: leading from office A to office B is accessed from the fifth level of the f i r s t selector, and goes through an A-M repeater and direct1.y to the manual board. The one-way trunk leading from office B to office A goes directly from the manual board through the repeaters and into an incoming selector. In order for a subscriber in office 13 to call a number in office A, he lifts his handset and gives the desired number to the operator, after she comes on the line. The operator then seizes a trunk to office A, and dials the desired number into the incoming selector and connector thus c o m p l e t i n g the call. A subscriber in office A, who desires to call a number in office B picks up his handset and after his line is found and connected through to a f i r s t selector, dials a "5". The selector finds an idle trunk on 'the fifth level and switches through to the manual board, via the A-M repeater. The operator then completes the call to the desired number with her cord circuit. The number of trunks between offices, of course depends on the amount of traffic. 12. AUTOMATIC TO AUTOMATIC Three inter-connected automatic offices a r e illustrated in figure 18. They a r e interconnected by two-way trunks, s o that all subscribers may reach all other subscribers. Office A may access office B by dialing a " 6 " , and access office C by dialing a "5". Office B may access office A by dialing a "4" and access office C by dialing a "5". Office C may access office A by dialing a "4" and access office B by dialing a "6". If there a r e seven trunks between offices A and B, seven trunks between offices A and C, and seven trunks between offices B and C, then there a r e fourteen two-way repeaters and fourteen incoming selectors needed in each office. The subscriber may after dialing the access digit of an office other than his own, dial the directory number of the desired party in the accessed office. 1 3 . P-A-X AND P-A-B-X The Private Automatic Exchange (P-A-X) i s a compact intercommunications system for serving business organizations, institutions, ships, etc. It i s made up of standard Strowger automatic equipment arranged and adapted to meet specified requirements. P-A-X switchboards a r e made in a number of standard sizes and types ranging from a few telephones to any number desired. With the addition of certain equipment a P-A-X may be trunked to a central office, thus allowing calls 'to be made to and from the central office. Such a switchboard is called a Private Automatic Branch Exchange (P-A-B-X). It i s common practice in P-A-B-X installations to have calls from a public exchange intercepted a t an attendant cabinet. The attendant may then complete the call. OFFICE A OFFICE C INTER-OFFICE TRUNKS -4 I N - 1 SUBSCRIBERS OFFICE B LF ""= - 1 - -k - - 1%' SEL )- I -- - A 0 C OIRECTORY N U M B E R S DIRECTORY NUMBERS CoNNNI -\ CONN. 111-400 111-300 DIRECTORY N U M B E R S 711-000 111-300 711-000 OFFICE B ACCESS-6 611-000 O F F I C E A ACCESS-4 O F F I C E C ACCESS'S SIP500 OFFICE A A C C E S S - 4 OFFICE B ACCESS- 6 OFFICE C A C C E S S - 5 Figure 18. Three autonzatic offices inter-connected by two-way trunks. Revised May 1958 Printed in U.S.A. by The Edward Keogh Printing Co. 2000 5-58 O R I G I N A T O R S O F T H E Makers of Telephone, Signaling, and Commurtication Apparatus D I A L T E L E P H O N E . . . Electrical Engineers, Designer$, and Consultants Factory and General Offices: Northlake, Illinois, U. S.A. ASSOCIATED RESEARCH A N D MANUFACTURING COMPANIES - - - - - - Northlake, Illinois, U. S. A. - - - - - - - Brockville, Ontario, Canada - - - - - - - - - - - - - Antwerp, Belgium - - - - - - - - - - - - - - - - - - - - Milan, Italy General Telephone Laboratories, Incorporated Automatic Electric (Canada) Lirnited - - - - Autornatique Electrique, S.A. Automatic Electric, S.A.T.A.P. DISTRIBUTOR IN U.S. A N D POSSESSIONS AUTOMATIC ELECTRIC SALES CORPORATION PJorthlake, Ininois, U. S. A. Sales Ofices i n All Principal Cities GENERAL EXPORT DlSTRlBUTOR AUTOMATIC: ELECTRIC INTERNATIONAL INCORPORATED Northlake, Illinois, U. S. A. REGIONAL DISTRIBUTING COMPANIES A N D RhPRESENPATlVES ARGENTINA, URUGUAY, PARAGUAY, CHILE, AND BOLIVIA D. C. Clegg Sala 61 Rua Conselheiro Crispiniano No. 169 Sao Paulo, Brazil AUSTRALIA Automatic Electric Telepho~iesLilmited 86 Holdsworth Street, Woollahra Sydney, Australia BELGIUM AND LUXEMBOURG Automatique Electrique, S. A. 22 Rue du Verger Antwerp, Belgium BRAZIL Automatic Electric do Brasil, S. A. Sala 61 Rua Conselheiro Crispiniano No. 69 Sao Paulo, Brazil CANADA Automatic Electric Sales (Canada) Limited 185 Bartley Drive Toronto 16, Ontario, Canada CENTRAL AMERICA L. Piti.gliani Apartado Postal 21327 Mexico 'i, D. F., hfexiro COLOMBIA Automatic Electric de Colombia, S.A. Apartado Aereo 3968 Bogota, Colombia EUROPE, NORTH AFRICA, AND NEAR EAST Automatic Electric International, Incorporated P. 0. Box 15 Gefieva Monthrillant Geneva, Switzerland ITALY Automatic Electric S.A.T.A.P. Via Bernina 12 Milan, Italy MEXICO Automatic Electric de Mexico, S.A. Apartado Postal 21327 Mexico 7 , D.F., Mexico NETHERLANDS Automatique Electrique, S.A. Huygenstraat 6 's-Gravenhage, Netherlands P E R U AND ECUADOR J. P. Maclaren Apartado Aereo 3968 Bogota, Colombia VENEZUELA Automatic Electric De Venezuela, CompaAia Anonima Apartado 6362, Est. Caracas, Venez~lela Other Sales Represenltatiwes and Agents Throughout the World . Automatic Electric Company . . A member of the General Telephone System