Reference / Paper · 2025
The step-by-step telephone switching system: Overview
Clear modern explanation of SxS for novices and practitioners.
Reference / Paper · 2025
Clear modern explanation of SxS for novices and practitioners.
The step-by-step telephone switching system: Overview Douglas A. Kerr Issue 4 September 5, 2025 FOREWORD The step-by-step telephone switching system (as it is known in the Bell Telephone System; the “Strowger” system elsewhere) was the earliest “mechanized” telephone switching system to receive broad acceptance, and it remained important for many decades. This is the first of a series of articles on this system, and it gives a detailed overview of the system. First, the history of the system is reviewed. Then the architecture of the system, and the unique switch mechanism it utilizes, are discussed. The overall working of the system in handling calls is discussed in considerable detail, although not at the “circuit” level. Background is given that will be common to the companion articles. The companion articles discuss in greater detail, often at the “circuit” level, the working of major parts of the system. 1 GENERAL 1.1 Companion articles This article is the first of a series. The companion articles describe in detail (including at the circuit level) the different switches used in the step-by-step system. All the articles are indexed on, and available at, my site, The Pumpkin: http://dougkerr.net/pumpkin 2 HISTORY 2.1 Introduction When telephones were first used, it was typically in pairs, in what we might today describe as “intercom” operation. Perhaps a pair was used between the ranch house and the bunkhouse, or between the sales office and the adjoining warehouse. These phones were purchased by the user (who, especially in rural areas, might order them by mail from Sears, Roebuck or Montgomery Ward). But soon the concept of the telephone exchange arose. Here, various homes and offices would be equipped with telephones linked by wires to a switchboard, on which a human operator could connect any two Step-by-Step—Overview Page 2 of the telephones for a conversation. These switchboards were the nexus of a new kind of enterprise, the telephone company.1 At first, the switchboard might be located in a back room at the general store, but as these systems expanded, the switchboards (would could become quite extensive) were typically placed in purpose-built buildings. These buildings, including the switchboards, were in fact often called a telephone exchange2, but later became known as central offices. Manual telephone systems were not, as some might suppose, “primitive”. There was enormous ingenuity and attention to detail in the design of the systems and of their “hardware” (notably the switchboards themselves), and the overall system was very complex. There was also a gigantic body of thought underlying the procedures and protocols needed to make what turned out to be a nationwide system operate smoothly.3 But, at the bottom line, the system was very labor intensive: it was operated by human beings. The labor cost was gigantic, and as the system expanded, there was concern that there might not be enough personnel available. One pundit had said in about 1910 that soon the telephone system would need to “employ every working-age woman4 in the country as an operator.” 2.2 Mechanization The attention of countless inventors was directed to the prospect of “mechanizing” telephone switching, and zillions of schemes were invented and patented. Many of these were developed into manufacturable, operable systems, and installed here and there for forward-looking telephone companies. 1 At first these often took the form of a cooperative, an enterprise owned by the users. Today, there are still some telephone systems operated by an enterprise with that structural concept. 2 In modern times, in the telephone industry, “exchange” took on a different meaning: a region used to determine the pricing of calls, such as the “Toledo exchange”. Later yet, as the industry was restructured through the breakup of the Bell Telephone System, “exchange” came to take on several other meanings. 3 All this is discussed in considerable detail in the article “Manual Telephone Switching”, by the same author, and probably available where you got this. 4 Yes, how sexist, but in fact, except at the earliest days (when telephone operators were, of course, men), for many decades the profession of telephone operator was almost universally the province of women. The industrial-sociological reasons for this are beyond the scope of this article. Step-by-Step—Overview Page 3 But before long one system had come to the forefront, and became the most widely used system for many years. And it survived many decades—some systems of the type are still in use today. This is the “Strowger” automatic telephone system (known in the Bell Telephone System as the step-by-step system), the one we will learn about in this article. 2.3 Describing mechanized systems Because (except in the earliest systems) mechanized telephone switching systems for some while were controlled by a dial at the subscriber station, it became common to speak of mechanized systems as “dial” systems, to be contrasted with “manual” systems. Especially in the decades in which mechanized telephone switching was spreading throughout the telephone network, it became common (especially in the “more technical” publications) to speak of it as “machine switching”. And in fact a mechanized switching system itself was often spoken of as a “switching machine5”, or, perhaps to distinguish it from other technical facilities located in the same central office building, just “the machine”6. 2.4 Birth of the Strowger system Almon B. Strowger was an undertaker in Kansas City, Missouri, which was of course served by a manual telephone switching system. Folklore has it that Strowger’s interest in emphasizing telephone switching came from his belief that, when someone in his part of town, bereaved over the death of a loved one, picked up the telephone and asked the operator to connect her to Strowger’s undertaking parlor, the operators (one of whom was said to be the wife of an undertaker competitive to Strowger) would often connect the caller instead to the competitor. Strowger thought that if there were no telephone operators involved, this abuse couldn’t happen. And that set him on a quest with results far more reaching that his original objective. His pivotal contribution to what we now think of as the Strowger system was the two motion stepping switch (often called the Strowger switch). In this switch (in the form we will encounter it), the 5 In recent years, it has become common to call a telephone switching system a “switch”. 6 For example, in a large long distance switching center, a technician at a test board where the trunks between this switching center and others around the country were tested, finding that a problem was perhaps caused in the switching system itself (probably located on a different floor of the building), might remark to a colleague, “I’m gonna call the machine and ask them to test that.” Step-by-Step—Overview Page 4 terminals are arranged around a portion of a cylinder, in 10 levels, 10 terminals around on each level, the whole array being called a terminal bank. An electromagnet, in classical “stepping switch” fashion, lifts a shaft carrying a contact wiper in steps, each to a successively higher level. Then a second electromagnet, again in stepping switch fashion, rotates the shaft in steps, each step putting the wiper on a successive terminal. The general central concept (as it had matured greatly into the design used in the actual Strowger system) is shown symbolically in Figure 1. Figure 1. Strowger switch bank and wiper concept 2.5 Strowger’s enterprise Strowger, along with a relative and some colleagues, in about 1891 formed the Strowger Automatic Telephone Exchange Company to develop and produce a working telephone switching system based on Strowger’s seminal switch concept. Interestingly enough, it appears that Strowger himself as not much involved in the ensuing development (not a surprise, really, as he had essentially no training in electrical and mechanical engineering). And of course it was a long way from the switch concept taught in Strowger’s patent to an actual telephone switching system, even a small one. Developing the system, and the switches, and even each little pawl or contact in the switch, was overall a stupendous task. But in fact there were capable people in Strowger’s company, many of whom got to be famous names in the history of this art. Perhaps the most noted of these was Alexander E. Keith, to whom are attributed many important advances in the system. The first commercial Strowger switching systems did not use a dial for control. Rather, if we assume a system with a maximum of 1000 lines (adequate for the small communities where the system was first Step-by-Step—Overview Page 5 launched), the subscriber had three pushbuttons at his station. He would push the first a number of times corresponding to the hundreds digit of the wanted number, then the same with the second button for the tens digits, and then the same with the third button for the units digit. But soon the dial was developed7, and the familiar modus operandi became the premise for the future of the Strowger system. Strowger’s company had several name and structure changes, and finally became Automatic Electric Company, for many years the second largest manufacturer in North America (after Western Electric Company, the Bell Telephone System’s manufacturing arm) of telephone switching systems (and telephone sets as well). The Strowger system was quickly seen as attractive by many telephone companies (outside the Bell Telephone system, as we will see shortly), and began to spread across the US and Canada. 2.6 What about the Bell Telephone System? By the time the Strowger system was hitting its stride, the newly-emergent Bell Telephone System became the elephant in the telephone industry room. American Telephone and Telegraph Company (AT&T), its parent, had established telephone exchange service companies in many medium and large cities, and bought numerous such companies already established by others. All these used manual switching, and that art became greatly refined under AT&T. AT&T was of course earnestly interested in mechanization of telephone service for the economic reasons I mentioned earlier. Automatic Electric saw this as gigantic market for the Strowger system. But AT&T’s priority regarding mechanization was understandably the large cities, where, because of the size of the systems and the amount of traffic, the potential economic gains through mechanization were the greatest. Their sophisticated studies of the matter had shown them that the Strowger system did not have the “intelligence” to provide optimal network behavior in a large metropolitan area context. So they began the development of a switching system using a wholly different 7 Attributed to Alexander E. Keith of the Strowger Company and his two frequent collaborators, John and Charles J. Ericson (not related, as we should guess from their exact names, to Lars Magnus Ericsson, founder of the famous Ericsson telecommunications company). Step-by-Step—Overview Page 6 concept, the panel dial system. And they in effect said to Automatic Electric, “Don’t bother me right now”. By 1920 the Bell System had begun to deploy panel dial systems in many of the large cities in which it operated. But, once the Bell System was well on the way toward mechanizing its large city systems, it turned its attention to the cities in the next tier of size. And, in order to move quickly on that, they began to buy Strowger systems from Automatic Electric (applying many of their own requirements and specifications). Seeing that this was working out well, their next step (in 1919) was to arrange for a license from Automatic Electric under which they could manufacture Strowger systems themselves (that is, by Western Electric). The initial versions were essentially “clones” of the Automatic Electric product, but of course soon they added their own features and designs. (One readily visible change was that they changed the heights of the frames to 11’6” from the lesser height that was standard in Automatic Electric systems.) Still, the basic switch mechanism remained essentially unchanged, and throughout the history of both product lines, switch parts from Automatic Electric and Western Electric were generally interchangeable. In fact, even the somewhat counter-intuitive color codes used by Automatic Electric for switch wring, different from that used on Western Electric products, was retained in Western Electric step-bt-step switches for the rest of that system’s life. Before long, even when the equipment was purchased from Automatic Electric, the Bell System spoke of the system as the “step-by-step” system (often abbreviated as “SXS”) rather than the “Strowger” system. And I will refer to the system by that name from here on. But I may still on some occasions speak of the switch concept as the “Strowger” switch. 3 SOME TECHNICAL BACKGROUND 3.1 Local battery and common battery operation In most telephones used until late in the 20th century, the transmitter (‘microphone”) was of the variable resistance type, and required DC current through it to “energize it”. In the earliest telephones of this type, this was provided by a battery of two or three quite large dry cells (the type later identified as the “Number 6”), typically located on the lower part of the telephone set housing. As telephone exchanges emerged, the telephone companies generally insisted on providing the telephone sets themselves, and with that came the responsibility of Step-by-Step—Overview Page 7 feeding them, that is, replacing these “batteries” periodically, rather a pain. Relief from this came from the concept of providing DC current over the telephone line from the central office to energize the transmitter at the station. This scheme was at first called the “central energy” scheme, but soon the term “common battery operation” became most widely used. Then, to allow the “original” way of working to be distinguished, it became known as “local battery” operation. 3.2 Line and loop Early telephone lines used one conductor (“wire”), and the circuit returned to ground at each end. This made the line very susceptible to noise from various phenomena. Soon it was concluded that it would be much better to use two conductors for the line. Transmission of speech signals8 was on a balanced basis: identical AC signals (as would be measured with respect to ground), but with opposite polarity, appeared on the two conductors. Seen from conductor to conductor, the two signals added, and it was this net signal to which the telephone set responded. When a DC voltage is applied to the line at the central office (as in common battery operation), in order to energize the transmitter (“microphone”) at the station, and to allow the central office to determine whether the station is “off hook” or not (see sections 3.1 and 3.5), the current flow is out one conductor and back through the other. Thus it became common to view this circuit as a “loop”, and as a consequence, a telephone line) especially as it exists outside the central office) is, even today, often called a loop. 3.3 Tip, ring, and sleeve By way of a tradition referencing the contacts on the plug used in manual telephone switchboards, the two conductors of a telephone circuit are usually spoken of as the tip and ring conductors (and often we just say “tip” and “ring”, not including “conductor”). When speaking of things inside a switching system (or even inside a telephone set), it is common to say “lead” (pronounced “leed”) rather than ”conductor”. 8 Other than in the telephone system context we would probably say “audio” signals, but that term is not typically used in “traditional” telephone system discussions. Step-by-Step—Overview Page 8 Inside a switching system, the tip and ring leads are almost always accompanied by a third lead, used for various control purposes, and called (by virtue of the same tradition) the sleeve conductor. These three leads are usually designated on drawings and the like as T, R, and S, respectively. 3.4 Battery Central office equipment runs from a DC voltage that is supplied by a large storage battery, which is continuously charged by a rectifier system or the equivalent. In reality, under normal operation, the current to run the equipment comes from the rectifier, and the battery just ”floats”. Most central office equipment (including most step-by-step systems) operate from a nominal voltage of 48 V. For an esoteric but very important reason, this voltage is usually negative with respect to ground (-48 V). Because of the way in which the power source is organized, the operating voltage is spoken of as “battery”. 3.5 Off hook and on hook In early telephones, the earliest separate hand-held receivers (often today spoken of by civilians as the ”potato masher” style) had a large screw eye on their “butt end”. This was hung on a hook extending from the telephone set (which of course was on the wall). The hook was movable, and operated contacts inside the telephone set proper. When the hook was pulled down by the weight of the receiver, these contacts disconnected the telephone set circuit from the line, and opened the circuit from the dry cell battery in the base of the telephone set to the transmitter (“microphone”) circuit. Later, when common battery operation was adopted, in which the DC to energize the transmitter was fed from the central office, thus eliminating the need for the dry cell battery at the station, a real maintenance pain, a further advantage was that the central office could easily tell when the subscriber lifted the receiver “off the hook” to place a call, since this allowed current to flow in the line from the DC voltage applied at the central office, which could easily be perceived with a relay. So, even today, the contact set operated in a modern telephone set when the handset is placed in the cradle is called the hookswitch9. 9 Actually, curiously enough, the modern term is most often “switchhook”. I guess this is like the hair style feature made famous by General Burnside becoming called “sideburns”. Step-by-Step—Overview Page 9 And the state of the telephone set is spoken of as being off hook (when it is active) vs. on hook (when it is idle). And in fact, in the broader field of signaling over telephone trunks, the two states of a binary “signaling channel” are usually designated off hook and on hook. 3.6 Multi-party lines In a multi-party line (often called by civilians a “party line”), two or more subscriber stations are connected to the central office over one line. This is a gigantic topic overall, with many different schemes being used. In an important class of schemes, each “party” has a distinct telephone number. We may say that a step-by-step switching system with a certain number of stages, fully “fleshed out”, can handle up to 10,000 lines, but what we really mean is “up to 10,000 numbers”. But to avoid complicating our discussions, In this article I will assume that all subscribers are served by individual lines (single-party lines). Then we can speak rather interchangeably of “lines” and “numbers”. 3.7 Talking battery Because the original motivation for providing DC onto the line from the central office was to energize the station transmitter, the DC that is applied onto a connection from different places in the step-by-step system (as the connection advances) is often spoken of as “talking battery”. The irony of the term is that, even though the provision of this battery is critical to switching system operation at its various stages, until the connection is completed there is no one to “talk” to. 3.8 The dial Although (as mentioned in section 2.5) the earliest Strowger systems use pushbuttons at the station to “enter” the desired number, soon the dial (what we today might speak of as a “rotary dial) was introduced for that purpose. The signaling principle is this. If the user wants to “enter” the digit 7, he puts his finger in a hole in the dial finger wheel marked “7”, pulls the finger wheel around (against a spring) until his finger hits a stop, and releases the finder wheel. The spring pulls the finger wheel back to the resting position, at a speed controlled by a little “flyball” governor. As the wheel returns. a cam operates a contact that opens the continuity of the telephone set circuit (and thus the flow of current through the line) 7 times, at a rate Step-by-Step—Overview Page 10 of about 10 interruptions per second. These interruptions are called pulses. The “code” is mostly straightforward: 2 pulses for a “2”, 7 pulses for a “7”, and so forth, except that for a “0” (since zero pulses could not be perceived), 10 pulses are sent. 4 THE SWITCHES THEMSELVES There are three principal types of switch used in the step-by-step system, all based on the Strowger concept, and in fact having almost identical mechanisms and overall construction: the line finder, the selector, and the connector. The three types, however, differ considerably in their complement of relays; their circuitry; their role in the switching network; and how they operate to fulfill it. Figure 2 shows a typical step-by-step switch. This one happens to be a selector switch, whose role you will hear of shortly. Figure 2. Typical selector switch At the bottom we see the banks. Plural? Yes. When I introduced the concept of the Strowger bank and wiper system, I was intentionally vague about the nature of the bank “terminals”. It would be easy to Step-by-Step—Overview Page 11 assume that each terminal of the bank had three contacts (for the tip, ring and sleeve conductors) and that the wiper had three contact-making members to connect to them. But in reality, we can only readily have two contacts at a bank position. So in fact the “bank” of the switch is actually an assembly of two banks, as we see in some detail in Figure 3. Figure 3. Three-conductor bank assembly On the lower bank, at each position are two contacts, rather thin, lying opposite one another on a thin insulating phenolic sheet. At each position, these contact pairs carry the ring and tip leads of the circuit. There is a thicker phenolic sheet, not shown in the drawing, between these “sandwiches” at the various levels. On the upper bank, at each position, a single contact, a bit thicker than the contacts on the lower bank, projects out from between two thick phenolic sheets. At each position, this contact carries the sleeve leadof the circuit. For either bank, the entire stack of phenolic sheets and metal contacts, for all ten levels, is clamped together between steel pates at the top and bottom by long screws extending from top to bottom. What we have thought of as “the wiper” of the switch is actually two wipers, one running on the lower bank and one on the upper bank. Each wiper has an upper and lower contact “leaf”. On the bottom wiper, the two contact leaves are insulated from one another (since they will touch separate contacts, for the tip and ring leads). On the top wiper, the two leaves are electrically connected together, since they will both be touching the top and bottom of a single contact, for the sleeve lead. Step-by-Step—Overview Page 12 The bank assembly is fastened to the switch, in rather precise alignment, by way of the threaded studs seen at the tips of the bank rods (with the nuts seen there). In Figure 4 we see a “two-contact” wiper. Figure 4. Two-contact wiper As we see, the wiper (which of course moves around quite a bit) is connected to the switch circuitry with two very flexible cords. Their actual conductors use what is called “tinsel” construction. A very thin ribbon of conducting material is wound around a small fabric core, this whole thing then covered by a durable but flexible woven cover. This style of cord is able to withstand literally millions of cycles of flexure and twisting as the switch goes through its motions. Toward the top of the switch “chassis” we see five relays. The different kinds of switch (line finder, selector, and connector, whose roles will be described shortly) have different numbers of relays, and their functions vary between the type of switch. These relays are of a basic design long associated with the step-by-step system. Although, for example, the switching systems designed by the Bell System (and made by Western Electric) used, from the outset, new types of relay, more compact and less costly to manufacture, -switches made by Western electric (as well as those made by Automatic Electric) continued to use the relay design we see here. Just above the bank assembly is the switch mechanism. Its heart is three electromagnets (generally just called “magnets”). One steps the switch in the vertical direction, one steps the switch in the rotary direction, and the third releases the pawls that hold the switch in place (after it has stepped up and around) and allows a spring to rotate the shaft back to it home angular position, and then allows the shaft to then drop by gravity to its home position. There are several pawls, latches, and the like involved in this operation. Step-by-Step—Overview Page 13 There are also various contact assemblies that do things like detect if the switch is at its home position or not. These play various roles in the logic executed by the relays in controlling the switch movement and otherwise managing the emerging connection. 4.1 Switches in situ Figure 5 shows a “shelf” of connector switches. (The connector is the last switch in a connection.) Figure 5. Connector shelf We see the switches themselves individually covered by the iconic “mailbox-shaped” covers.10 The covers have a depression on each side (near the top), allowing the cover to be grasped by one hand for removal or replacement. The two ribs near the bottom stiffen the cover in cross section (toward the top, that is done by the top panel of the cover). The shelf accommodates 11 connectors, of which the leftmost one is never part of an actual customer connection but is used for test purposes. Of the remaining 10 positions, only 8 switches are installed at the moment, that presumably being the number reckoned to be needed “now” to accommodate the traffic through this branch of the network with the adopted grade of service11. But note that all positions are equipped with banks. The bank wiring (with 300 conductors) which runs from bank to to bank (we’ll see why shortly) is so complex that it would not be practicable to wire in a new bank each time a switch was to be added to the shelf. 10 11 Sometimes spoken of as “lunchbox-shaped” covers. Grade of service is a target limit for the probability that a call will be blocked at a certain stage of the network because of the lack of an available resource of some sort, such as a connector switch in the group that serves the called line. Step-by-Step—Overview Page 14 I mentioned earlier that the banks were fastened to the switch proper. If one switch is removed (perhaps to replace it with another), that bank is temporarily bereft of support from a switch, but the multiple wiring on each side will adequately support the bank for that period. But when the shelf is made, and first installed (before any switches are emplaced), or even when (as we see in the figure) a few switch positions are, for the time being, unequipped with switches, we must provide proxies for the switches in their role of supporting the banks. Thus we see, in the two unequipped positions, light-colored sheet metal brackets (bank supports), fastened by one screw to the shelf framework, and which support the banks by way of their usual mounting studs. To the right we see a terminal strip for all the leads of the bank wiring. At the lower left is a fuse panel, which has positions for the unique flat-type alarm indicating fuses (“grasshopper fuses”, the style used for telephone equipment until the mid-1950s) for each switch in the shelf. Also at the left are test jacks and alarm indicator lamps. Now we can return to the matter of switching network architecture and operation. 5 THE STEP-BY-STEP SYSTEM 5.1 Classification The step-by-step system (in the form we will speak of) is classified as a direct progressive control system. Direct means that the switch is moved (in stepping switch fashion) directly by the individual pulses in a dialed digit or digits. Progressive means that the successive stages of the overall switching network12 are moved by successive digits of the dialed number, so the connection progresses “digit by digit”. 5.2 Implications of the 5-digit number In the discussions here, we will assume a system that uses five-digit telephone numbers. Typically, five-digit telephone numbers were used (before the universal use of 7-digit numbers) in cities with more than one central office but just a few. The first digit (the “central office code” tells the central 12 Network here is used in the sense of the structure in a switching system through which calls flow from one line to another across the system, and not in the sense of a number of interconnected witching systems.. Step-by-Step—Overview Page 15 office that serves the line. The numbers are customarily shown, in telephone directories and in advertising and the like this way: 5-236813. This of course would be a line served by the “5” office. A line with number 8-5746 would be served by the “8” office. To keep things simple for a while, we will at first consider the operation of the system on a call placed by a line in the “5” office to a line also served by the “5” office. This is called an intraoffice call. When we do, certain arrangements for carrying the connection from one central office to another drop out of the picture, allowing us to more clearly see the important concepts. The way the system works in this case makes it just seem that this is an isolated central office with no more than 10,000 lines but which for some reason uses 5-digit, rather than 4-digit, numbers. 5.3 Network architecture at a glance In Figure 6, we see, in the “5” office, in block diagram form, the path through the switching network for established connection from a line in the “5” office (whose telephone number is of no interest to us) to the line whose number is 5-2368 (also served by the “5” office). Figure 6. Completed 5-digit connection The symbols for the various switches are intended to be evocative of the switch proper (the square box) and its bank (the array or horizontal lines, each representing the 10 terminals on one level of the bank). The bent line with the arrow head represents the wipers. The paths between a terminal of the bank of a selector to the succeeding switch are called trunks. We are of course probably klost familiar with that term as it applies to a circuit between central offices (interoffice trunks), or even between the originating side of one office and its terminating side (intra-office trunks). Bu those in fact are special cases of the broader use of “trunk” that we will see here. 13 But, in Lorain Ohio, with a handful of central offices, the telephone officials recognized (quite correctly) that the number presented in this form: 89-191 (for a line served by the “8” central office) was even easier to grasp, and so that was done in telephone directories (and by the subscribers in newspaper ads and the like). Step-by-Step—Overview Page 16 On each trunk, I have shown the digits that would appear there as the calling line dials 5-2368. Of course the hyphens are not digits and are not “sent”; I include them here just to clarify the structure of the number in a familiar way. By virtue of the direct, progressive nature of the step-by-step system, the number of stages in the network depends on the number of digits in the number. Here, we have 4 stages (the line finder is not counted as a stage)–we’ll see why in a moment. The number of selector stages (here, 3) is 2 less than the number of digits in the telephone numbers. We see that the connection involves 5 switches: a line finder, three selectors, and a connector 5.4 5.4.1 Handling of the call—overview The line finder and 1st selector When the calling subscriber lifts the handset, through a process to be described later, the call is assigned a line finder in the group that can access this line (and, for the moment let’s say 99 other lines). Those lines are connected to terminals on the banks all the line finders in the “group” serving that station (think perhaps 10 altogether in the group), but we only see the one that was called to service for our connection. When this line finder is called to service, it moves its wipers to the terminal to which the calling line is connected. Each line finder is essentially-permanently tied to a selector in the first stage, called a 1st selector, and there is now a path from the calling line to the “input” of the 1st selector tied to “our” line finder. That input provides “talking battery” to the calling line through the windings of a relay that will let the selector see if any current flows in the line (and of course at first it will). The first digit dialed (that would be”5”) causes the 1st selector to extend the connection to one of a number of 2nd selectors in a group we label “5XXXX”. That selector only ever takes part in a connection that is destined to reach a number starting with “5”. (in this notation, we do not include the hyphen that is used in “displaying” 5-digit telephone numbers. Why are there several “5XXXX” 2nd selectors? For the simple reason that, at any given time, there might be more than one connection needed to lines with numbers of the form “5XXXX” (all of which must pass through a “5XXXX” 2nd selector). Step-by-Step—Overview Page 17 How does our 1st selector manage to extend the connection to a particular one of the “5XXXX” 2nd selectors, one that is not already busy? We will learn the broad answer while looking at Figure 7, where we zoom in on the area of the 2nd selectors. Figure 7. “Fanout” from a 1st selector We see here “our” 1st selector, as well as three groups of “NXXXX” 2nd selectors, the “1XXXX”, “2XXXX”, and “5XXXX” groups, with 10 selectors in each group (this being of course only “typical”). As we saw before, the “5XXXX” selectors, for example, will only be involved in connections to lines with numbers of the form “5XXXX”. The heavy lines each represent a set of 10 circuits (3 leads each), running (separately) from the 10 terminals on one level of our illustrative 1st selector to the “inputs” of the 10 second selectors in a group. Now when, in our scenario, our caller dials the fist digit “5”, the five dial pulses (detected by the release of the “talking battery feed” relay in the 1st selector) directly cause the “vertical” magnet to step 5 times, taking it to level 5 of the selector bank14. The figure graphically suggests that. Now, the switch begins to autonomously step in the rotary direction, the wiper in effect scanning the terminals of all the 2nd selectors (10 on this case) that are connected to terminals on that level. The terminal for a busy 2nd selector will have ground on the sleeve contact, and when the selector sees that, it keeps on stepping. 14 In discussions at this level it is customary to speak of “bank” in the singular, meaning what is called in more mechanically-detailed discussions the “bank assembly”. Step-by-Step—Overview Page 18 (Selectors 1-3 are assumed to be busy, indicated by the shading on their symbols.) But when it get toselector number 4 of the group, which is idle, there is no ground on the sleeve lead. The 1st selector, sensing this, stops stepping, and extends the connection, via its wipers and the contacts of that bank terminal, to the input of that lucky 2nd selector (its good fortune is shown here by the heavy line on its symbol). Now, it is this 2nd selector that supplies talking battery to the called line. It is now, for a little while, responsible for “managing” the nascent connection. We introduce on this figure an important symbol in this kind of work, the multiple symbol15, a short diagonal line with a free end. It means that an lead or multi-lead path shown as connected to an element is also connected to other elements of that same kind and situation. We see this symbol on the calling line, where it is shown connected to the bank of a line finder. The multiple symbol means that the line is also connected to the banks of other line finders (of course, in the group that serves this line). The “free” end of the symbol points toward the element that “there are more of what this is connected to”. We also see multiple symbols on the paths leading from our 1st selector to the three groups of 2nd selectors. That reminds us that there are a number of 1st selectors in the same group as ours, and all of their bank terminals (in this case, on levels 1, 2, and 5) are connected to the same groups of 2nd selectors as our 1st selector. 5.4.2 Moving on You may wish to refer to Figure 6 again. In the very same way we saw for the 1st selector, the 2nd digit dialed, “2”, moves the “5XXXX” 2nd selector involved (number 4) to its 2nd level, and in the same way we saw before, it finds an idle “52XXX” 3rd selector. The 3rd digit dialed, “3”, moves that 3rd selector to its 3rd level. Here, however, it does not then hunt over 10 4th selectors (there are no such things) but, (in exactly the same way), hunts over 10 connectors (these in the group “523XX”). When it finds an idle one, just as before, the connection is extended to the selected connector. 15 The basis for that name is discussed in section A.2.1 in Appendix A. Step-by-Step—Overview 5.4.3 Page 19 The connector As we might suspect, it is now this connector that supplies talking battery to the calling line. And now, and for the duration of the connection, it is this connector that is responsible for managing the connection, which is soon to be completed. Connectors are the last switch in the connection train, and they differ substantially from selectors. For one thing, a connector is stepped in the vertical direction by the first dialed digit it receives (always the next-to-last digit of the called line number, its tens digit) and then is stepped in the rotary direction by the next digit it receives (always the last digit of the called station number, its units digit). The result is that the connector’s wipers land on the bank terminal for the called line. Just as for the links from selectors to switches in the following stage, a busy line shows ground on its sleeve lead. Now a relay in the connector, by testing the sleeve lead of that terminal, sees if the station is busy (it might be on an existing connection), and if so (i.e., if the sleeve shows ground), the connector returns busy tone “upstream” to the calling subscriber. But is the line is idle, battery (through a relay winding in the line circuit) appears on the sleeve contact. In that case, the connector applies ringing voltage to the line through a special “ring trip” relay. That relay can determine when the calling station answers, and when that happens, that relay removes ringing voltage from the line, provides talking battery to the called line, and establishes a transmission path between the calling and called lines. 5.5 The line finder–a closer look In the system operation overview just above, I described the use of the line finder as the way that a line wishing to make a call “enters” the system. In fact, this is only one of two conceptually-distinct ways this is done, one (the “line switch” approach) most in vogue in the earlier days of the step-by-step system, and another (the “line finder” approach )most commonly used in “modern” t