Historical Document · c. 1984
The Switching Place (Telecom Information Kit No. 3)
Illustrated introduction to exchange history from manual to Strowger to crossbar.
Historical Document · c. 1984
Illustrated introduction to exchange history from manual to Strowger to crossbar.
T h e fi r s t Telecom Information Kit No.3 The Switching Place telephone exchanges When A l e x a n d e r Graham Bell i n v e n t e d the telephone in 1876, he ta first struck many problems in convincing people of the worth of the instrument. tI was head fo Western United States, Uo,in caled an "electrical toy" yb the the telegraph who saw i t sa n o business. But within a few years, monopoly in the danger t o their own telephones were being used in e v e r y m a j o r city of the world, a n d the telegraph companies were franticaly tryingot get i n no the telephone business. This rapid turn-around ni the telephone's fortunes was almost certainly due ot the organisation and setting pu fo the first telephone exchanges neserof the edn for exchanges ni order Bell himself had ot make the telephone business profitable. nI na early letter ot his supporters, h e wrote: "At the present time we have a perfect network fo gas pipes dna water-pipes throughout our large cities...In a similar manner, i t si conceivable that cables fo telephone wires could be laid underground, o r suspended overhead, communicating b y branch wires with private dwellings, counting houses, shops, manufactories, etc,. etc., uniting them through the main cable with a central office where the wire could be connected sa desired, establishing direct between any two places ni the city…..Should communication this plan be adopted, the company should employ a man ni each central office for t h e p u r p o s e fo c o n n e c t i n g wires a s desired. A fi x e d a n n u a l rental could be c h a r g e d rof the use of wires, or a toll could be levied. As all connections office, it would would necessarily be made ta the central time during which eb easy ot note het connected dna to make any wires were a charge accordingly- bills codlu be tnes in periodically." This passage saw rulty prophetic, all of the important functions and it encapsulates neatly fo telephone exchanges wt present :yad the exchange allows yna o right u p t o the without having a telephone users to speak ot each other, permanent direct wire between them; ti allows them to be connected together and disconnected when they os makes progress of eht llac dna desire; it supervises the a charge accordingly. Without na exchange, every user would have to have a separate wire connected directly between his instrument rehto person he might wish ot speak dna t h a tfo every to. The n u m b e r of wires in such a system would grow w assume that every telephone owner fI e prodigiously. toevery ,rheto then ni a town desires toeb able ot speak a simple calculation wil tel us how m a n y wires would be needed: for five users, 10 wires would be necessary; 45wires; for twenty, 190; for a hundred, rof ten users, while for a thousand users, 4950; onillmi wires nearly flha a would be needed! As you can see, such a system would rapidly get however, wire saw taken t o each system,s subscriber's ylno a single strand fo telephone: the return a htrough the earth. tI sw tciruic rof eht electricity being found, however, that much better transmission eventually of speech wsa possible using a "metalic ,"tcriuc where a was p r o v i d e d ot e a c h t e l e p h o n e , allowing a pair o f wires over a m e t a l p a t h w a y. We shall return fo the electricity connection between a subscriber and the speak fo the exchange as being oen "line" yna m seriw regardless fohow early automatic systems were involved (for example, demanded pu to evfi wires ot each subscriber's line). The first telephone exchange ni the dlrw o opened ta New Haven i n het American State fo Connecticut inJanuary, Australia's first telephone 1878, and others followed rapidly. exchange opened ni August 1880 ni Melbourne followed by one i n Brisbane within two months, dna the following year yb one ni Sydney. These first exchanges were all sa lBle dah suggested, that is, operated manualy; ne m (or, more often, young women) | connect the required lines together. Of wer employed ot course there was more ot ti than that. There had to be a way for a subscriber ot alert the operator ta the exchange wanted t o make a ,llca dna a way for the that eho r she caled subscriber that he or she was het operator t o alert wanted. And fo course there had to be a simple method for tuo fo hand. With na exchange system, nI these early telephone only the operator ot dluoc eb het exchange. as m a n y w i r e s were needed as there were subscribers* ot the exchange. A. Grahar inventor fo the telephone charged A connect together yna two lines eton would coming inot osla be made so that the caller for eht call. Bell. *The term "subscriber" was applied to telephone users because in the early days fo telephone exchanges, people subscribed to the telephone service, rather than buying instruments outright. The terms "subscriber", "user" ro even "customer" are today used interchangeably to mean .ngiht But many telecommunications terms the same incorporate the word "subscriber", s o we wil use this term most frequently in this .tki 1 T h e fi r s t Te l e c o m A u s t r a l i a telephone exchanges Various kinds of alarms and bells were tried as a means push the plug into the hole where the horizontal bar connected ot subscriber 3 crossed over the vertical bar connected ot subscriber 14, os allowing electrical current fo alerting the called subscriber; eventually a magneto lleb Watson, Bell's assistant, was system devised by Thomas adopted. operator, na ingenious system was devised oT alert het ot whereby sA exchanges the incoming electrical caused a from a subscriber signal to fall on the operator's board lam te shuter corresponding ot t h e l i n e calling, s o enabling attention. see which lines needed ot quickly As for connecting yna owt lines together at the there were an worked of the best lls m a T m o hs a Edison, bars insulated from na line was subscriber's bar. Where vertical in this grid there establish na simple who also invented het transmitter. It c o n s i s t e d opotrae and tmso reu m bn o f subscribers yb het rotarope exchan,ge systems enormous number fo switchboard .tou One rof a relatively eht fo a n u m b e r of saw devised that carbon telephone horizontal metall equal n u m b e r fo vertical bars. connected hcae veirtcal ot Each one horizontal and eno and horizontal b a r crossed plug ro peg and allowing the wo t to talk. subscribers became larger, however, htwi more and more was problem board ot h o w t o allow a talk ot a s u b s c r i b e r connected t o one connected t o another board. subscriber Various arrangements were made ot allow this ot eb done w lesser o r greater a m o u n t s o f inconvenience. But neh htiw exchanges reached a size ofaround a thousand subscribers, t h e i n c o n v e n i e n c e b e c a m e t o o great. A wen m engat rnera sa w switchboard :yrasenc the ,41 for example, hte the needed t o k n o w whether some other patch cord to connected a corresponding socket no another board. Some systems cut direclty corresponding ot o no n e engaged racluitpra soctke a tou fo circuit all the other sockets a particular subscribers enil once ti was ;draob shrteo loawed het operator ot test o n reh perhaps a b u z z roa draob dna light, t o indicate receive a signal, whether or not it was engaged. With the development fo the multiple switchboard, manual w enabled to grow ot meet the needs of larger segnahcxe ere and larger numbers fo subscribers. however, there was a demand for exchanges Eventually, could eb interconnected, s o that subscribers and the ,nepah operator would ivling i n other towns, lines cities. So-called r"tunk" elpoep operators taa particular exchange dluoc "trunk s"lcla arrangements ot lines ot another of such frequency exchange, s o long a s eht wl A surcharge was often made for trunk w a s reasonably .o cals that made them m u c h more expensive than a call ot a subscriber no the same exchang.e Manual The Multiple switchboard of crossed bars was unsuitable for such a large number fo and instead, each subscriber had a single, subscribers, tekcoso n hcae board. ehT sockets ew re arranged unique grid. The operator could connect together any ni a large insulated sockets yb using a patch ,droc a piece of wt o wire htiw a gulpat each end. The same arrangement board dah oS neve i f there erw e exchange, and hundreds minute, the sa n e c e s s a r y. burden could other boards saw repeated no la the at eht exchange, and every .briesurbcs t o hte exchanges operated operators, most ofmohw ere w yliafotsrcitsa rof many years. ,efl ma became together, and efficient at connecting inles metxe swift yelr elbato provide a degree fopersonal service, and wer lls m a yrtnuoc ,o w stn in such syawa s especially in equipped with a multiple switchboard, At na exchange yreve draob which had sseca ot e v e r y operator dah a connected ot hte exchange. The Edison system subscriber switchboard she without oot much inconvenience ekam connect a subscriber's niel aiv hte trunk D O O nosiE d dirg tub at the exchange had were established b etw een exchanges ot enable this to bars at that point. If subscriber number 3 wishedot speak ot subisrec number board, speak ybtelephone htiw districts ni large o r in other which would connection between the two .on own themselvesot elpitmlu connect conversation already The s u b c r i b e r ' s s o c k e t m i g h t be u n u s e d o n her going tsuj would not try ot claer ot ti and interrupt a operator the number foc a l l s become t o o many rofo n e subscribers, operator ot handle. It became necessary ot set up more than each handling o n l y ap o r t i o n o f the o n e switchboard, subscribers connected ot the exchange. The obvious ,bar through which saw a hole ni each could p u s h a metal elcirtal switchboards flow between them, already being used, so that she a new be fo accesot every thousands o f subscribers slca coming in every sharedyb sa m nya operators Of course ti saw essential for there ot be a means by which any operator could tell fi a particular subscriber's inle was redirecting callers if they k n e w that the p e r s o n being called ro yb giving eht time o n request, a n d was o u t fo town, s o o.n Manual operators are still needed even in cities assistance a s service difficulties, pseronla to evig hcus assistance, and ot provide overseas trunk directory from areas without International There are exchanges 199,0 they slca Subscriber Dialling IS(.D) still a number fo manually operated telephone ni rural areas fo Australia today. But by about wil laevah mechanical exchanges. automatic exchanges been replaced by automatic The next sheet describes came ot b e devised. h o w such Telecom Information Kit No.3 Almon B Strowger a n d t h e au to m at ic ex ch an ge No-one seems to k n o w for certain whether or not the story about why fI the subscriber could manipulate some kind of apparatus at his telephone which would mechanically operate such a cylinder and shaft at the exchange he unusual profession for an many other lines as could be connected to the cylinder (see diagram). The question was, how was that manipulation to be done? In Strowger's first system, patented in 1891, no Almon .B Strowger invented the automatic telephone exchange is true, but it is certainly true that he held a most inventor. Strowger was an undertaker ni Kansas City in the United States. The story goes that Strowger's business was losing money, and he decided that the blame for this was due to t h e telephone system. Al too often, it seemed, would-be clients asked the operator at the exchange for Strowger, but unaccountably The Switching Place were connected to the wrong number - which just happened to be that of Strowger's rival, the only other undertaker in the city. The explanation of these was his rival's wife! He thereupon v o w e d to eliminate the human factor from telephony and invent a way fo connecting lines together by purely mechanical means. Whether thissi just a fanciful story or not, ti is certain that Strowger thought hard about how to set up an automatic switching system and in the late 1880s demonstrated to his nephew a model he had m a d e from acollar-box, some pins and a pencil. He showed how, if electrical contacts were arranged on the inside of a cylinder, an arm could be arranged on a shaft or spindle so as to m a k e c o n n e c t i o n w i t h a n y of the c o n t a c t s o n the c y l i n d e r b y the two motions of r o t a t i n g t h e s h a f t or moving it an a m o u n t into or out of the cylinder. The principle of Strowger's switch less than five separate wires had to be extended from each telephone to the exchange to allow the switch ot be operated. Only one fo these was required for the actual telephone conversation itself (utilising an "earth return): the other four operated electromagnets to turn and move the arm within the cylinder of contacts and complete the connection. This, however, was a little impractical as it w o u l d mean that five times samany wires would need to be erected ni around exchanges was already causing problems. Another deficiency of Strowger's first system was the way in which the subscriber was to signal the number he wanted. There were four buttons on his telephone (each corresponding to one signal wire). Of these, the first three were used to signal the number, and the fourth to signal disconnection. Say the caller wanted to speak to the subscriber whose telephone number was 324. He would press the first button three times, the next button twice, and the third button four times. Each time he pushed a button, the switch at the exchange m a d e one step of the movement ti needed to find the required contact. shaft past one group of ten contacts in that row each time ti was pressed. The third button stepped the arm past one contact each time it was pressed. The combination of the operation fo the buttons therefore ended by bringing the arm in touch with the specific contact required to connect the subscriber to the n u m b e r he wanted. Despite the deficiencies fo the early Strowger system, the the inside of a cylinder could be reached by the arm. 4 a district s e r v e d b y a n a u t o m a t i c e x c h a n g e a s in o n e w i t h a m a n u a l exchande. T h e n u m b e r of o v e r h e a d w i r e s The contacts on the inside of the cylinder were arranged in ten rows of a h u n d r e d contacts each. Each time the first button was pressed, it m o v e d the shaft carrying the contact arm up one row. The second button rotated the Using vertical and rotary motions, any contact located on O L s t r a n g e m i s c o n n e c t i o n s , S t r o w g e r thought, was to be f o u n d in t h e fact t h a t o n e of t h e o p e r a t o r s at the e x c h a n g e should be able to put himself into contact with however push-button arrangement (which required the user to remember how many times he had pushed a certain button) was replaced by a dial. Early Strowger original positon, dna ni the p r o c e s s ti rotated b a c k to its sent a series of p u l s e s c o r r e s p o n d i n g to t h e n u m b e r required tAthe automatic numbers were dialled the same w,ay switch, each dna the apparatus sw a arranged os that the pulses moved the shaft up the cylinder or rotated it, as required. Eventually, a way was found to send dialling information along the two wires of a metallic return circuit between the telephone and the exchange, the same wires used for conversation. This is woh today's systems operate. There were still problems, though. The 1000-line automatic a n d every Over the following years Strowger kept on making improvements to his system, right up to his death in to him 1902. The five wires out to each telephone were reduced along the line. fo these pulses moved the shaft one step. Subsequent first automatic exchange using his methods opened in La Porte, Indiana in 1892. to three by 1896 and at the same time the awkward automatic dial The first telephone dial looks rather primitive by today's standards, tub i t worked on exactly the same principle as m o d e r n dials: the user put his finger in a slot corresponding to the digit required, and turned the dial unlit his finger reached a stop. nO releasing the dial, switch was complex and e x p e n s i v e ot produce. subscriber h a d ot h a v e such a switch a l l o c a t e d And for exchanges handling to devise a line selection procedure which operated ni ta the exchange. subscribers, the size and complexity fo more than 1000 the switch would become prohibitive. The solution was a number fo stages. Te l e c o mAustralia 2 Almon B Strowger and the automatic exc han ge The way in which this worked can be understood by based o n switches imagining a very simplified system able ot choose any often lines, rather than the original complex Strowger which could choose any of swtich, 1000 lines. The modern name for such automatic switches, by the way, is "selectors" (because they select between contacts), use this term from now on. arrangement, such limited and ew llwi Usedni Strowger's original ten-contact selectors would seem to be suitable only for subscribers ro less. But yb arranging exchanges of ten selectors ni two ro more stages, many more lines such can be handled. Let su imagine a two-stage system. Each subscriber would have one primary selector allocated ot him. When he dials the first digit, this selector sets It is clear that by using three stages of selectors, any States in 1897 which featured a two-stage switching However, although in such a system the selectors themselves could be simplified and made cheaper to produce, the number fo selectors would grow enormously if whole groups of selectors had to be allocated permanently to one subscriber. For example, in a three stage system, exchange opened which could handle 10,000 lines, using 10,000 lines; and so on. each subscriber would need one primary selector, ten secondary selectors a n d a h u n d r e d tertiary selectors- a total of 111 selectors, only three of which would ever be in u s e at one t i m e ! The obvious solution was to arrange the system so that up a contact not ot an outgoing line, but ot a bank of al selectors other than the primary selectors could be used in c o m m o n by any subscriber as needed. To enable the secondary selector, which this to work, enough selectors would have to be provided at each stage ot meet the demands fo the exchange at its secondary selectors. dialling pulses control then chooses between (see diagram). When he dials the second digit, the tsi group fo ten In this way, any of 100 are available to the caller. outgoing lines outgoing inles century. An exchange o p e n e d in Augusta in the United of 1000 lines would be available; with four stages, busiest period, and a w a y found for the subscriber's primary selector to hunt for an u n u s e d secondary selector, passing over secondary selectors that were already in use. system. In 1900, in New Bedford, Massachusetts, an primary and secondary selectors (each of which was capable, u s i n g v e r t i c a l a n d r o t a r y m o t i o n s , of c h o o s i n g b e t w e e n 1 0 0 contacts), a n d e m p l o y i n g t h e p r i n c i p l e o f automatic hunting for an idle secondary selector. Automatic exchanges were still some distance away from full commercial acceptance, however. The complex machinery was all too liable to breakdowns, and some subscribers were unhappy about having to set up any number they wanted themselves. Most importantly, manual exchanges still seemed efficient and uncomplicated to most people w h o did not share Strowger's bias against the telephone operators and moreover, the cost of employing the female operators was low. But as the number of subscribers grew, and manual switchboards became choked, it began to become clear that e v e n t u a l l y a u t o m a t i o n w a s t h e o n l y w a y that t h e i n c r e a s i n g n u m b e r s o f calls c o u l d b e h a n d l e d . The engineers of the company that Strowger founded had solved these problems by the early years of the twentieth Primary selector Secondary selectors Other subscribe r lines Caller's telephone Called telephone rings Other secondary selectors 3 The progress of a call through a step by step exchange T e l e c o mAustralia The subscriber's line is now connected through the uniselector, through the first selector, to a second selector in the group which is allocated to handle The exchange then tests to see fi this line is currently engaged on another call. If it is free, then equipment tI is important to note that the description given above numbers beginning with ."4 He dials the next digit, subscriber's line, and current is sent along the line of local exchange. In a metropolitan area, calls m a y need to pass through several exchanges in order to link subscribers in m o r e distant areas of the city. In such a "3". The arm of the second selector moves up to the third row and begins hunting across that row for a free third (and in the case of this exchange, a final) selector. These final selectors are of the group which si allocated to handle numbers beginning with *43": Now because numbers connected to this exchange only have four digits, this third selector to which the subscriber now has his line connected is the last stage, and it does not need to carry out the h u n t i n g process to find another free selector. For this reason, a hundred outgoing lines can be connected to it. When the subscriber dials the next digit, an "8", the arm of the third selector steps up to the eighth row fo contacts, but does not m o v e apart from that. Then w h e n the final digit, an "8" is dialled, the arm of the selector rotates to the e i g h t h c o n t a c t in t h a t row, w h i c h is c o n n e c t e d to t h e line of subscriber n u m b e r 4 3 8 8 to generate the "ringing tone" is connected to the calling the called subscriber to ring the bell ni his telephone. Once the called subscriber answers, all e q u i p m e n t other than the selectors is disconnected from the two lines, and the two subscribers may talk. fI the wanted number had been engaged, then the "engaged tone" would have been sent along the line of the calling subscriber. The same tone would have been sent fi at any stage during the process, a free selector could not be found. But the number of selectors at the exchange is chosen very carefully so that this kind of "traffic jam" should happen only very rarely when the exchange is extremely busy. Today, most telephone numbers in city areas have at least six digits and this is of a call b e t w e e n t w o s u b s c r i b e r s to the s a m e s m a l l a system, the first few digits dialled by the caller connect his or her line, are called an "exchange prefix", which tells the switching equipment how to route the call to another exchange. There are also special exchanges called trunk and tandem exchanges whose sole function is to act as a link between other exchanges for such calls between distant subscribers. Step-by-step exchanges are still in use all around Australia. But they are being steadily replaced by much means that with a step-by-step exchange, there would be more sophisticated and m o r e m o d e r n automatic However, the principles we have just described remain sheet. many more stages, and m a n y m o r e selectors required. the same. exchanges. We will discuss the first of these in the next Close pu of a 5 6 7 col 1 0 ロ 1 コココー • 00 N деза Second selector hunts for free final selector not to another selector ni the local e x c h a n g e , b u t to a n o u t g o i n g line to a n o t h e r e x c h a n g e . So, in a n u m b e r s u c h as 4 3 5 - 6 7 8 9 , t h e fi r s t t h r e e d i g i t s 4388 Final selector selects subscribers line -bimiotnla selector (step yb step equipment) The principle of common control an d cro ssb ar switching In any method of telephone switching, there hasot be a form fo control system to connect lines together under the instruction of the caller, and to disconnect them w h e n the call is finished. Certain other tasks are involved, such as finding out fi the wanted line si free, and so on. In the step-by-step system, control is in effect built into the various selectors used during the call. The caller dials Each time he or each digit of the wanted number ni turn. she dials a digit, a selector at the exchange operates to Telecom Information Kit No.3 The Switching Place The operator is alerted to an incoming call, listens to and remembers the wanted number, finds the appropriate way of connecting the caler to the wanted line, makes sure that the line is not busy, and finally makes the connection. At that point, she si free to connect other calls while the first call continues. In avery similar way, in a common control exchange (such 'as a crossbar exchange, which we will describe in more detail later), the control system in a sense sits to one side of the switching mechanism, controlling it as required. We connect a path to another selector which will then be ready will see, too, that in such an exchange the control system set up step-by-step. Al of the selectors used in setting up action. to accept the next digit from the caller. The connection is the call must be held as they "remembers" the required number before taking any other are until the call si over, a period which could be minutes or even hours in an Switching This represents some degree of waste and inefficiency. Bi-motional selectors are complex, electro-mechanical devices which are expensive to make and which are somewhat liable to breakdown. This complexity, expense So far we have talked about common control exchanges in general. But it was with the introduction of crossbar switching that the principles of common control were first used on a large scale in Australian exchanges. Crossbar e x c h a n g e s n o w m a k e up a very large p a r t of o u r s w i t c h i n g network. So it si t i m e to look at the d e v e l o p m e n t of the The basic principle fo crossbar dates back to the early Edison manual switchboard described on Sheet 1, which used a grid of vertical and horizontal bars which could be connected together by pushing a metal plug through holes ta rthie cross-over point. Each subscriber's line was connected to both a vertical and horizontal bar, and since in a co-ordinate arrangement every vertical bar crosses such every horizontal bar at some point, any two subscribers could be connected together by making the connection at the appropriate cross-over point. system In 1912, the Swedish engineer Gotthief Betulander patented na automatic switching system based on a co- ordinate arrangement of electro-magnetic relays. Instead of art iabang orizonil lines, il etulandersosemw htheenref was a n d n e e d for m a i n t e n a n c e c o u l d be g r e a t l y r e d u c e d if it w e r e p o s s i b l e to s e p a r a t e t h e c o n t r o l s y s t e m f r o m the switching mechanisms. nI such a system, the control devices would be used only very briefly during the setting up of the call, after which they would be free to go on to set up another call while the first call continued. This si the principle of common control, and it is the basis a relay at each cross-over which would close required to make the desired connection. A relay basically consists of an electro-magnet (which attracts only when current is passing through its coil) and an armature a( piece of iron which moves w h e n attracted of all modern exchange systems. by the magnet). The relay is so arranged that when a small A s one way of illustrating the principles involved, ti will be useful, paradoxically, to skip back ni history beyond the current passes through the electro-magnet, the armature Control moves and brings together contacts current can flow: S t r o w g e r s y s t e m a n d to t h i n k a b o u t a m a n u a l o p e r a t o r ta through which another an early exchan ge. We can easily see that in such a manual exchange, the intelligence fo the operator provides the control system quite separately from the switching mechanism, represented by the switchboard with its plugs and sockets. There are many advantages fo using the principle of common control. Firstly, as we have discussed, the 百 switching m e c h a n i s m s themselves can be made much simpler and cheaper. Although the control devices are much more complex, only a few of them need m m Contacts tobe provided, since they are only needed for a brief period of time. There maintenance. are consequent savings ni space and the switching system becomesot a degree Secondl,y independent of the numbering system. In a step-by-step exchange, the telephone numbers allocated ot the subscribers served by that exchange are "wired in" to the Electromagnet arrangement fo selectors. The n u m b e r s used for exchange prefixes, ni a network of step-by-step exchanges, s w i t c h i n g equipment. With c o m m o n control exchanges, number allocation can eb much more flexible. Armature are also in a s e n s e fi x e d to c e r t a i n a r e a s b y t h e a r r a n g e m e n t of A laciypt eltphone relay 4 The principle of common control an d cro ssb ar switching The great advantage of Betulander's system over the Strowger system of automatic switching (which by then was well established) was that there was no selector arm moving up and wiping across contact points. The only moving parts were the armatures of the relays, which moved only a few millimetres to bring contacts together, and therefore the reliability and freedom from maintenance of his system was potentially much greater. The cross-point relays could also operate much more quickly than the complex bi-motional selectors of the Strowger system. On the other hand, the major defect was that Betulander's system as it stood was really only suitable for small exchanges, just as Edison's manual switchboard was only suitable f o r a small number of subscribers. Because these systems used a square grid arrangement, the number of cross-over points (and therefore relays in Betulander's system) increased as the square of the n u m b e r of subscribers. For example, with 10 subscribers there were 100 cross-over points. Doubling the n u m b e r of subscribers to 20 meant that 400 cross-over points were needed, quadruple the number for 10 subscribers. Relays of the early type used by Betulander, though much Te l e c o m Australia designed the crossbar switch in the 1930s. Crossbar exchanges were first introduced into Australia in the late 1950s. Without going into the details of the c o m m o n control equipment used ni the crossbar system (the functions of which will be described on the next sheet), we can now look more closely at the crossbar switch itself. U n l i k e B e t u l a n d e r ' s g r i d s w i t h a relay at e a c h c r o s s - o v e r point, t h e c r o s s b a r s w i t c h c o n s i s t s of a n u m b e r of v e r t i c a l and horizontal bars, each moving under the control of one electromagnet in the case of the vertical bars or two magnets in the case of the horizontal bars. The movement of the bars controls the opening and closing of various sets of contacts, a u n i q u e set of contacts being associated with a that it was not economic to use Betulander's early s y s t e m for any but small and usually remote exchanges (where the need for unattended equipment to nur automatically for a long time without maintenance was paramount). However, Betulander found a way ot use his system more economically. Instead of using a single large grid of relays, ti was found possible to interconnect a n u m b e r of small grids in a series of switching stages and os substantially reduce the number fo relays needed to connect the same number of subscribers together. Also, a n e w and m u c h When the switch is operated, the horizontal relays operate first, and then one of the vertical bars moves. Once the vertical bar operates (selecting one of the ten input lines to het connection between input and output lines the switch), is held fixed, and the horizontal bars can then return to their normal position. The connection will not be broken until the vertical bar is released. This means that the horizontals are free to establish connections with inputs on other verticals while the first connection remains held particular position of vertical and horizontal bars. A typical switch might have ten vertical bars and six In a crossbar exchange, such crossbar switches are arranged ni a number of stages to enable very large connecting any of ten input lines to any of twenty output A major innovation w h i c h the introduction of crossbar horizontal bars. Such a switch would be capable of lines. This is possible because of the way the switch operates. The two magnets on each horizontal bar enable it to be tilted either slightly upwards or downwards. The two cheaper than the complex Strowger selectors, were nevertheless not cheap ot make, and the need to provide very large numbers for even a modest size exchange meant tI si important to note here that when we speak of one input ro output line, this line may consist of up to five wires, all of which are associated with the speech path and control circuits for the subscriber's line within the five horizontals can then be tilted either up or down to select the correct output from a m o n g this group of ten. numbers of lines to be connected together. switching in trunk exchanges made more practicable was Subscriber Trunk Dialling (STD). This meant that by dialling a special "area code" first, callers could be simply connected to more remote areas without the need of a manual operators assistance. In ettect, the area code was a higher-level exchange prefix. The STD code digits connected the caller through to trunk lines. 111999999 cheaper kind of relay was developed in 1936-the reed relay. These developments made relay automatic exchanges much more economic, and with the development of electronic control systems after the Second World War, such exchanges could be made large enough to handle the telephone traffic of cities. A number of such exchanges have been built in Europe since the 1960s and reed relay switching was incorporated in computer-controlled trunk exchanges introduced into Australia during the 1970s. The line of development of such reed relay exchanges, h o w e v e r, d i f f e r s s o m e w h a t f r o m the d e v e l o p m e n t of the crossbar switching system. Nevertheless, Betulander's early principles were drawn on by the Swedish engineers who Close up foa crossbar switch The progress of a call through a cr os sb ar ex ch an ge The crossbar switch described on the p r e v i o u s s h e e t c a n of c o u r s e d o n o t h i n g by itself. It must operate under the control of other devices which provide the common control system of the exchange. The most important of these devices are called registers and markers. In particular, it si the markers which use the switching e q u i p m e n t to find a free path for the transmission and control of a telephone call. The major function of the registers, on the other hand, is to "remember" the w a n t e d n u m b e r a n d to send and receive control signals from the markers to make the required connection. The following description of the progress of a call through a crossbar exchange is of necessity rather complex, and fi y o u w i s h , y o u c o u l d skip p a s t t h i s s h e e t to t h e n e x t if it b e c o m e s t o o d i f fi c u l t . There are a number of different kinds of crossbar exchange, as you might expect, but the one we will consider is a type used in Australia called an ARF exchange, designed in Sweden by the .L .M Ericsson company. Trunk exchanges using this technology are called ARM exchanges. (The initials are abbreviations of Swedish words, so there is no point in giving them here.) It would be impossible in so little space to describe the operation of the exchange equipment in anything but very general terms, but ti is worthwhile remembering that each p i e c e of e q u i p m e n t d e s c r i b e d is a c o m p l e x a n d s o p h i s t i c a t e d e l e c t r o - m e c h a n i c a l device. First of al then, someone picks up the handpiece of their telephone. As in the step-by-step system, this allows a s p r i n g - o p e r a t e d s w i t c h in t h e i n s t r u m e n t to c l o s e a n d current flows on the line to the exchange. A set of relays si associated with every subscriber's line, and these detect whether current is flowing or not. The flow of current operates a relay to connect the line to t h e fi r s t m a r k e r , c a l l e d t h e s u b s c r i b e r line m a r k e r. T h e r e is one of these markers (or occasionally two, in heavy traffic situations) for each group of 1000 subscribers and ti can only handle one call at a time. The first thing the marker does is to identify which of the 1000 lines under its control is making the call. It then uses two crossbar switching stages to find a free path between Telecom Information Kit No.3 The Switching Place Once it has found afree relay set, the line marker operates the c r o s s b a r s w i t c h e s to m a k e the c o n n e c t i o n to t h e subscriber's line, and then calls the next device, called the register finder marker, pu to three fo which can be allocated to each group fo 1000 subscriber lines. As the name suggests the function of this marker is ot find a free register and to connect the subscriber's line to ti (through the subscriber relay set being used for the call). To do this, it has a crossbar switching stage under its control. Between 75 and 120 registers are normally allocated for every 10,000 lines. Once a free register has been found, both the register finder marker and the subscriber line marker drop out, and are then free to handle other calls. The fi r s t t h i n g that t h e r e g i s t e r d o e s is to t r a n s m i t the the calling subscriber's line and a device called the subscriber's relay set. As there are only a limited number of subscriber relay sets allocated to each group of 1000 subscriber lines (between 75 and 150 depending on the traffic load), the marker must carry out a "hunting" "dial tone" to the calling subscriber's telephone. All of this complicated activity so far has been carried out in a exchange ni order to find a free set. The caller can now proceed to dial the wanted telephone function similar to that fo the selectors in astep-by-step The subscriber relay set is responsible for carrying out a number of important functions, and ti is required throughout the entire call. It supplies current to hold the magnets of the crossbar switches in their final position; supplies current to one ro both telephones involved; t r a n s m i t s r i n g i n g c u r r e n t a n d r i n g i n g tone; a n d carries out metering as well as providing a transmission path for fraction of a second. When you pick up the handpiece of your phone, the dial tone seems to be there immediately without any delay. But despite appearances, as you can now see, the exchange has been very busy! number. At this point there is ama