Reference / Paper · 2024
The step-by-step telephone switching system: Reverting calls
Reverting calls on party lines.
Reference / Paper · 2024
Reverting calls on party lines.
The step-by-step telephone switching system: reverting calls Douglas A. Kerr Issue 2 October 2, 2024 ABSTRACT In a multi-party telephone line (often called by civilians a “party line”), two or more subscribers’ stations are connected to the same pair of conductors to the central office (the “line”). The point is to spread the considerable cost of the line over several subscribers’ service, allowing service to be given at a lower rate. If a subscriber served by a multi-party line attempts (probably unwittingly) to call another subscriber on the same line, the call cannot be completed in the normal way. For one thing, the called line is already busy, and so the switching system will not gladly connect to it. Such a call, if it can somehow be completed, is called a reverting call, because the connection “reverts” to the line on which the call originated. In the step-by-step switching system, various special provisions for completing such calls were in wide use. In this article, we describe their principles and some of the details of their operation. Considerable background is given on pivotal topics. 1 1.1 PREFACE Reader background It is assumed that the reader is generally familiar with the operation of the step-by-step telephone switching system. Extensive information on it can be found in the other articles in this series. 1.2 The context The “universe” of this article is the Bell Telephone System. Non-Bell companies utilized many schemes and details only rarely, if at all, practiced in the Bell System. (But see section 8.) 1.3 Great diversity Although “consistency” was a principal tenet of the Bell System’s operation, that hardly meant “uniformity”. The technical details of the systems and their operation were enormously diverse. In an article like Copyright © 2020, 2024 Douglas A. Kerr. May be reproduced and/or distributed but only intact, including this notice. Brief excerpts may be reproduced with credit. The step-by-step telephone switching system: Reverting calls Page 2 this, one dare not say “always” or “never” or “there were only three different ways such-and-such was done.” The techniques and operational details I describe are intended to give the reader the important concepts of the prominent techniques. But consider every statement to be modified by an unseen “Well, not always”. 2 2.1 INTRODUCTION Multi-party lines In a multi-party telephone line (often called by civilians a “party line”), two or more telephone subscribers are served by the same pair of conductors (the “line”) to the central office. The point is to spread the considerable cost of the line (and the line’s portion of the equipment at the central office) over several subscribers’ service, allowing service to be given at a lower rate. 2.2 Reverting calls If a subscriber having service over a multi-party line calls a number for a station that, as it turns out, is served by the same line (and the calling subscriber might well have no idea of that), the call cannot be completed in the regular way. For one thing, the switching system will find that the called number is busy (because, of course, its line is in use by our hapless subscriber trying to make a call to that number). And, if the switching system could get beyond that, it could not ring the called station because there was already a station off hook on its line, namely the station of our hapless calling subscriber. If the system did apply ringing to the line, it would immediately be “tripped”, as normally happens when the called station answers. And in any case, the ringing signal would be “shorted out” by the off-hook calling station. A call from one station on a line to another station on the same line (if we could make such a thing happen, and we can) is called a reverting call, because the telephone “connection” reverts to the same line on which it originated. Of course the telephone industry did not just wish to declare reverting calls “intrinsically doomed”. So, as with every other need in the telephone network, many schemes and systems for actually accomplishing reverting calling were devised. 3 THIS ARTICLE The overall matter of multi-party lines is unimaginably complicated, and even this “small” aspect of it (reverting calling) is gigantically complicated. The step-by-step telephone switching system: Reverting calls Page 3 In this article, I will first give some background on multi-party line operation, and then I will describe several systems used to make reverting calling possible. Emphasis will be on the overall principles of operation. I will make no effort to give detailed circuit descriptions of the various circuits involved. 4 MULTI-PARTY LINES 4.1 “Station” and “party” The “parties” alluded to in the name “multi-party line” are of course the multiple “users” (actually always spoken of “in the day” as “subscribers”, but more recently as “customers”) given service over the line. Perhaps a four-party line serves Joe, Mary, Bill, and Claire. It is of course not they but their respective telephone “stations” that are connected to the line. And, in certain common turns of phrase, “party” actually refers to the station of one of the parties. Here, I will usually follow that, for consistency with much other writing on this subject. 4.2 4.2.1 Ringing signals for multi-party operation Introduction So that the subscriber at a called station will recognize when a call is coming over the line for that station, different ringing “signals” are used for the different stations. These are distinguished by one or more of the following distinctions: a. Applying the ringing signal from the ring or tip conductor to ground. (This is spoken of as “divided” ringing.) b. Using a signal in which the DC component of the voltage is negative or positive. (This is spoken of as “superimposed” ringing.)1 c. Sending different patterns (cadences) of ringing. (This is spoken of as “code” ringing.) This was the premise of that old tag line, “That’s my ring!” A typical set of ringing codes is shown here. 1 The term comes from the fact that this ringing signal has both AC and DC components, and can be thought of as an AC voltage “superimposed” on a DC voltage. The irony is that the “ordinary” kind of ringing, used for individual lines and for two-party lines via the distinction in item a, is every bit entitled to the description “superimposed”. But the term “superimposed” is rarely used in connection with that ringing signal, usually being only used in connection with ringing signals drawing upon the distinction in item b. And the “ordinary” kind of ringing voltage is most often called “AC-DC” ringing. The step-by-step telephone switching system: Reverting calls Page 4 Figure 1. d. Having the AC component of the ringing signal at different frequencies. This was only very rarely used in the Bell Telephone System (although very important elsewhere), and I will ignore it from here on. 4.3 Selectivity This refers to the matter of whether the ringer at a certain station only rings on calls for that station. In a full-selective ringing system, only the ringer at the called station rings. (This is often spoken of as just “selective”) In a semi-selective ringing system, at a given station, the ringer will ring for calls to that station and certain other stations on the same line, but not for all stations. (The distinction to the subscriber between what is heard for a call to his station and what is heard for a call to another station is by way of different ringing codes.) In a non-selective ringing system (although that term is rarely used), the ringers at all stations ring for calls to any stations. Again, the distinction to the subscriber is made by way of different ringing codes. Schemes that only use divided ringing and/or superimposed ringing are inherently full-selective. Schemes that use divided ringing and/or superimposed ringing plus code ringing are inherently semi-selective. Schemes that only use code ringing are inherently non-selective. The step-by-step telephone switching system: Reverting calls 4.4 Page 5 Commonly-provided services The following multi-party line services were widely provided in the Bell Telephone system. 1. Individual. (This is a “single party” line, and is included in the list only for completeness.) Ringing can be thought of as on the ring lead and with a negative DC component, but the ringers ignore the latter. This ringing system is not usually named. 2. Two-party (full-selective). This uses distinction a (in Section 4.2). The ringing system is called “divided” (although that term is not commonly used in connection with this scheme). 3. Four-party, semi-selective. This uses a combination of distinctions a and b (two ringing codes used, “one-ring” and “two-ring”). The ringing system is sometimes called “divided code” (although that term is not commonly used in connection with this scheme). 4. Four-party, full-selective. This uses a combination of distinctions a and b. The ringing system is called “four-party full-selective”. 5. Eight-party (semi-selective). This uses distinctions a, b, and c (with two ringing codes). The ringing system is called “eight party semi-selective”. 6. 10-party (semi-selective). This uses a combination of distinctions a and c (with five ringing codes). The ringing system is called “10-party semi-selective” or “10-party divided code”, or often just “10-party”. 4.5 4.5.1 Terminal-per-line and terminal-per-station operation Introduction In step-by-step switching systems, two different basic architectures were used for handling multi-party lines. These cast shadows on the systems used for handling reverting calls, so I will discuss them briefly here. 4.5.2 Terminal-per-line (TPL) operation This was primarily used in smaller offices, typically in rural areas. It was deprecated in later times. In such systems, all the parties on a given line had telephone numbers that were identical except for the last digit. That last digit directly indicated the ringing signal to be used for that party. In such systems, all the dialed digits except the last three lead the connection to a connector switch in the group handling that block of The step-by-step telephone switching system: Reverting calls Page 6 1000 numbers (which embraced 100 lines); the last three digits operate the connector (not two as for a classical connector). The first two of those last three digits move the connector to a certain terminal (just like the last two digits would in the more familiar arrangement). The very last digit then operates a small stepping switch in the connector. It puts into effect the ringing signal implicated by that digit, which will be used when that line is rung. All calls to any station on that line went by way of that connector bank terminal (thus the name of the arrangement). Note that with this kind of connector (operating on the last three dialed digits), for a certain length of the station numbers, there will be one less stage of selectors in the network. 4.5.3 Terminal-per-station (TPS) operation This was the more common approach in larger central offices and those in metropolitan areas. In such systems, the numbers of the various parties on any given line were essentially independent. All the digits of the number except the last two lead the connection to a connector in the group handling that block of 100 numbers. The last two digits move the connector to a certain terminal, which corresponds to the telephone number of the called station (thus the name of the arrangement). In one of several ways (in part described in Section 4.6), the connector “knows” the ringing signal for the station with that number, which will be used when that line is rung. 4.6 4.6.1 Control of the ringing signal Introduction In the step-by-step switching system using the terminal-per-line architecture for multi-party lines, with respect to “regular” calls, the proper ringing signal for the called station was most often produced by the application of one or more of the following, as applicable to the ringing plan for a certain line. 4.6.2 The “side of the line” aspect Applying the ringing signal to the ring or tip conductor is often spoken of as applying it to one side of the line or the other. The connector switch (the last switch in the connection) normally applies the ringing signal to what it sees as the ring lead of the terminal for the called number. But for all stations for which the assigned ringing signal is to be sent over the tip conductor, the wiring The step-by-step telephone switching system: Reverting calls Page 7 inside the central office from the connector terminal to the line itself is “turned over”2. Thus when, for such a number, the connector applies the ringing signal to what it sees as the ring of the line, that is actually the tip. 4.6.3 The “DC polarity” aspect Commonly different “hundreds blocks” of numbers are reserved for stations whose ringing signals either negative and positive DC components. The groups of connectors accessing those different hundreds blocks of numbers are wired to either use a ringing signal with a negative or positive DC component (both these “waveforms” being distributed from the “ringing plant”). 4.6.4 The “ringing code” aspect—two codes When only two codes are involved, commonly different hundreds blocks of numbers are reserved for stations whose ringing signals will include “1-ring” or “2-ring” ringing codes. The groups of connectors accessing those different hundreds blocks of numbers are wired to sources of ringing voltage with a “1-ring” or “2-ring” ringing code. 4.6.5 Pick up operation The “2-ring” connectors include a special feature not normally found in the “1-ring” connectors. If certain party’s ringing code were “2-ring” (two medium-length bursts separated by a short quiet period, the pairs being separated by a longer quiet interval), and the connection was established just as the ringing interrupter for the two ring code (which runs continuously) was between the two bursts of a two ring sequence or during the second burst, the subscribers on the line might first hear a single (perhaps shortened) burst of ringing. If a subscriber not being called had a “one-ring” ringing code, he might think this call was for him, and before hearing the next cycle (in which the two ring pattern was clearly evident) might answer the call, which was not for him. 2 This is done with the “jumpers,” on one or the other of two distributing frames, that make semi-permanent connections inside the central office to configure lines for service. The step-by-step telephone switching system: Reverting calls Page 8 To avert this, usually when code ringing is used, and a call will use a code other than “one ring”, the connector switch is arranged not to actually apply ringing until the beginning of a ringing cycle. This involves a special lead running from the ringing interrupter system to all connectors. It carries a brief ground pulse just before the beginning of the master ringing cycle. This is called the “pick up” pulse, and the lead is called the “pick up lead” (PU or PKU). In general, pick up operation is not used when the ringing code is one-ring. With that code, there is no risk that if a partial burst is the first sent it will be mistaken for some other code. If pick up operation were used anyway, it would lead to an average delay in the onset of ringing of perhaps about 1.3 seconds over all such calls. And in the telephone network, time is money. 4.6.6 The “ringing code” aspect—five codes 10-party operation (semi-selective) usually involves the “side of the line” distinction as well as a set of 5 different ringing codes (perhaps as seen in figure 1). Rather than having five groups of connectors, serving five different sets of number hundreds blocks, with each group of connectors wired to apply a different ringing code, usually a special connector (a 10-party connector) is used. It can apply any of 5 different ringing codes, depending on the station called. (The “side of the line” aspect of the ringing signal is handled as discussed in Section 4.6.2.) In such a connector, at each station terminal, in addition to contacts for the usual tip, ring, and sleeve (T, R, and S) leads, there is a fourth contact (A). This carries, in the form of ground pulses, the actual ringing code cadence for the code assigned to that station. This operates a relay in the connector that “keys” the ringing signal. Usually, “pick up” control of the start of ringing is used regardless of the code involved. (The connector really “doesn’t know” what ringing code is being used.) 5 5.1 THE ODD NATURE OF REVERTING CALLS Introduction All the reverting call handling scheme we will learn of here revolve around maneuvers that run wholly counter to the general principles of call handling in a telephone system. My challenge here is to teach the details of cricket to people who have only thought about baseball. To help the reader follow the details of the various schemes, I will first describe models of two of these maneuvers that play a role in all the schemes. The step-by-step telephone switching system: Reverting calls 5.2 Page 9 A model “first act” Imagine a residential metropolitan area where two-party service is offered. Suppose Billy (who has two-party service) got Jane’s telephone number in their biology class. After supper, Billy tries to call Jane, but the line is busy. He tries two more times that night, with the same result. “Wow”, he thinks, “she must really be popular.” The next night he tries again several times, with the line always busy. Billy calls the operator. He tells her that he was trying to call BOulevard 00753, but it has always been busy for two days, and he thought the line might be in trouble. The operator asks Billy for his number, does some checking, and tells Billy, “That is because that number is on your party line (!).” (Billy of course would have had no way to know that.) The operator tells Billy how he can complete the call. How that is depends on the system that applies. We’ll see the details of various systems shortly. If there is no “mechanized” reverting call handling provision to which the caller can be commended, the operator herself may need to complete the call (in a way that requires her to school the caller on how this is going to happen). 5.3 A model “last act” After some preliminaries (varying with the system—we’ll see the details shortly), the calling subscriber hangs up. The system then rings both stations. If code ringing is involved, the called party will hear “his” code. The calling party may hear his usual ringing code, or the called party’s code, or perhaps a special cadence that is not any station’s code. In any case, this tells the calling subscriber that the reverting ringing is proceeding. The called subscriber, responding to the ringing at his station (in his code, if code ringing is involved), answers, not realizing that this is not a “regular” call. When he answers, ringing is “tripped” just as for a regular call. The ringing is stopped, and the line is connected to a source of “talking battery”, which will allow two stations on the line to communicate in a sort of intercom fashion. When the calling subscriber notices that the ringing (in whatever cadence) has ceased, that tells him that the called party has answered. So he picks up his phone and greets the called subscriber, 3 Actually, many years ago, the number of a Jane. The step-by-step telephone switching system: Reverting calls Page 10 who has probably earlier said “hello” (which the calling subscriber might well not have heard). When the conversation is done, and both subscribers hang up, there is no longer any DC current in the line. The battery feed circuit notices this, and causes the connection to be released. 5.4 What if the called station does not answer? The instructions given subscribers for reverting calls say that if the called subscriber does not answer (the calling subscriber knows this because the ringing signal he hears continues unabated), in due time the calling subscriber should lift up his handset (thus tripping ringing. just as if the called subscriber had answered) and then hang up (thus releasing the connection). 5.5 Good grief! We can see, from the “model” first and last maneuvers just described, that the handling of reverting calls is, compared to the handling of “regular” calls, pretty clumsy. The good news, in, for example, a residential metropolitan area, is that it is very unlikely that someone you wanted to call was in fact served on the same line as yours. The other party(ies) on your line probably lived a block or two away. Maybe one of them is a classmate, but you never had any reason to call her. In rural areas, the story might be quite different. In an extreme case, 15 farms along a certain rural road might all be served by two eight-party lines. It would be very likely that you might want to call a neighbor, and there was perhaps a 50% chance that his station was on the same line as yours. So, clumsy or not, the residents of that road learned how to use the reverting call handling system that was available. 6 6.1 THE REVERTING CALL SELECTOR Introduction The reverting call selector scheme for handling reverting calls was widely used in step-by-step offices for handling reverting calls on most types of multi-party line. The step-by-step telephone switching system: Reverting calls 6.2 6.2.1 Page 11 In terminal-per-line offices Introduction Here I describe the operation of this system as used in step-by step central offices providing party line operation on a terminal-per-line basis (see Section 4.5.2). 6.2.2 Basic operation Recall that in such systems, all stations (parties) on a line had the same number except for the last digit. The last digit of the number indicated the ringing signal for the station (unique as to the side of the line, the polarity of the DC component, and/or the ringing code). We note that a moderately alert subscriber, wanting to call a certain number, can recognize it is on the same line as his station by the fact that all digits of the two numbers but the last were the same, and may thus recognize that this will be a reverting call. For now we will assume he knows how to do that (perhaps having diligently read the instructions in the front of the directory), and as a consequence, will not end up calling the operator for guidance. The calling subscriber dials a service code (typically three digits4), presumably found in the front of the directory, to reach the reverting call selector (of the type appropriate to his type of line). Note that, unlike the other selectors in the step-by-step system, which advance the emerging connection to the next stage, the reverting call selector is itself the destination of this connection. (And thus it might better have been called a “reverting call trunk”, but because it is physically made like a selector, and possibly has the same central mechanism… ). In some cases, the caller receives a tone signal (perhaps dial tone or some variant) to confirm that he has reached the reverting call selector. The caller dials the last digit of his number, and then the last digit of the number he wants to reach. These set the two motion switch mechanism in the selector to a position where its wipers set up the selector to apply, on behalf of the calling station, its ringing signal, and, on behalf of the called line, its ringing signal. Then the caller hangs up. 4 “In the day”, when step-by-step systems used service codes of the form “11x”, the code for the reverting call selector was often “119”. The step-by-step telephone switching system: Reverting calls Page 12 The selector then, back over the connection that brought the calling station to it, rings the calling and called stations, using their respective ringing signals. Even if the ringing signals are applied on opposite sides of the line, the ringing bursts are “interleaved”, as for various reasons it is not desirable to simultaneously apply ringing voltage to both sides of the line.5 The subscriber at the called station hears ringing (if coded ringing is involved, with his familiar ringing code), and recognizes this as a call for his station. The calling subscriber hears ringing (usually, if coded ringing is involved, with his familiar ringing code). This tells him that the reverting call is proceeding. When the called subscriber answers, ringing is “tripped”. All ringing is halted, and the line is connected to a “taking battery” supply, which allows both stations on the line to communicate in an “intercom-like” way. The caller, noting that ringing has ceased, also picks up his phone, and the two persons can converse. At the end of the conversation, when both parties have hung up, there is no longer any DC current in the line. That is detected by the battery feed relay in the system, causing the selector to release, which in turn releases the connection leading to it. 6.2.3 But if the subscriber is not that alert But suppose that the calling subscriber doesn’t notice that the number he wants to call and his are the same except for the last digit, or notices that and just considers it coincidence, and in any case has not read the description in the front of the directory of what reverting calls are and now to make them. So after repeatedly getting a busy signal when trying to call that number, like Billy in the parable above, he calls the operator. When she hears he is trying to call 8-6158, and his number is 8-6154, she advises him that this is a reverting call and schools him on how he should make it. 6.2.4 Ringing the calling party first Where possible, when the system begins to ring both stations, the first ringing “burst” that is sent is for the calling station. The reason is that if the first burst were for the called station (and assuming the under the ringing system in use that was not heard by the calling party), and the subscriber at the called station was right by the telephone, and answered right away, ringing would be tripped before 5 Although some systems do just that. The step-by-step telephone switching system: Reverting calls Page 13 the calling party heard any ringing, and so the final act scenario would fail. This timing relationship is typically enforced by way of the pick up functionality described in Section 4.6.5. 6.2.5 Special ringing codes Sometimes, where the multi-party line uses code ringing, this scheme involves the use of ringing codes that, while sounding like familiar ones, actually had slightly different timing, to make the “interleaving” work out. For example, this shows the typical “1-ring” and “2-ring” codes used in 4-party and 8-part semi-selective ringing systems. Figure 2. But, in reverting call operation, these codes are often used: Figure 3. We see that the 2-ring code is slightly “compressed” from its normal cadence so that two phases of it (one for use for the calling party, one for use for the called party) can be provided within the usual 6-second ringing cycle. In some cases, the ringing sent to the calling party uses a special distinctive code (“reverting call ringing”), so that the caller clearly The step-by-step telephone switching system: Reverting calls Page 14 realizes that in fact his reverting call is underway (and that he is not, by an accident of timing, receiving an ordinary call from someone else). Here we see it used in conjunction with 1-ring and 2-ring codes. Figure 4. Only the 1-ring code or 2-ring code would be used on any given call, depending on the code used for the called station, with the calling station always receiving the “reverting” code. We see that. with this plan, the 2-ring code retains its normal pattern. The short duration of the burst in the reverting code assures that it can be interleaved (always on opposite sides of the line; see Section 6.2.6) with either the 1-ring or 2-ring codes in their normal patterns. In fact, this same concept can be used when there are five codes (as for 10-party line operation). This shows the principle (I am not certain of the typical codes actually used for this scheme). Figure 5. The step-by-step telephone switching system: Reverting calls 6.2.6 Page 15 Another wrinkle If the ringing signals for the two stations are both on the same side of the line and with the same DC polarity, but differ only in terms of the ringing code, to follow the principle exactly we would have to send, on that side of the line, with that DC polarity, both ringing codes. The resulting cadence, if that were even possible, would be a mish-mash, and the called subscriber could not be reasonably be expected to recognize his station’s code in it and realize that this was a call for him. So in this situation, the system sends, on that side of the line, with that DC polarity, just the code for the called station. The called station subscriber recognizes that, and will answer. At the calling station, the subscriber hears this and knows this is not his station’s code, but he knows that this is a reverting call scenario, and so any ringing (with whatever code) will tell him that the reverting call is preceding, and when it stops, will tell him that the called station has answered. If there are other stations on the line, whose ringing is on that same side of the line and with that same DC polarity (but having a different code), they will also hear this ringing (just as if this were a regular call to this called party), and they will ignore the ringing. 6.3 6.3.1 In terminal-per-station systems Introduction In central offices providing multi-party operation on a terminal-per-station basis (see Section 4.5.3), the reverting call selector works the same technically, but there is a variation as to how the user may come to deal with the situation. 6.3.2 Typical scenario of operation Imagine that a subscriber, like Billy in the parable above, finds a number he wants to call repeatedly busy. Like Billy, he calls the operator. She gives him the reason for his lack of success, schools him in how to use the reverting call selector, and gives him a specific two-digit number he will need to call that specific station. (I will sometimes speak of this jocularly as the “magic number”.) The first digit of that number is an arbitrary digit that identifies the ringing signal for the calling station. The second digit is an arbitrary digit that identifies the ringing signal for desired called number. So after reaching the reverting all selector, the caller dials that “magic number”. Its two digits set the two motion switch mechanism in the selector to a position where its wipers set up the selector to apply, on The step-by-step telephone switching system: Reverting calls Page 16 behalf of the calling station, its ringing signal, and, on behalf of the called line, its ringing signal. Then the caller hangs up. The rest of the story is just as described for operation in a terminal-per-line office. 6.4 For four-party full-selective ringing When the lines involved use four-party semi-selective ringing, only a one-digit “magic number” is needed to define the ringing for both stations. Accordingly, the mechanism of the selector is a degenerate form of the regular step-by-step two motion switch mechanism, only able to step in the rotary direction, that done in response to the one dialed digit. It might seem at first that there would be 12 possible combinations of (two different ones of) the four distinct ringing signals for the two stations,6 and those combinations could not all be separately invoked by a single digit. But in fact there are only 6 different cases. They are listed here in terms of arbitrary “case numbers”7. Polarity on Case Ring Tip 1 – – 2 – + 3 + – 4 + + 5 +.– nil 6 nil +,– 6 The signal for the calling station could be any of four, and for each of those cases, for the called line (which by definition has a different signal) the signal could be any of the remaining three, a total of 4 × 3 = 12 possibilities. 7 I do not know for this arrangement how those combinations are mapped onto digits of the “magic number.” The step-by-step telephone switching system: Reverting calls Page 17 The reason there are half as many as we might have thought is that, when the rubber meets the road, it does not matter which of the two stations has which combination. They both will be rung. Can we in fact send the ringing signal for both stations (having different polarities) even if they both use the same side of the line? Sure. All we have to do is time-interleave the “negative” and “positive” bursts. 6.5 For four-party semi-selective ringing The story here is very similar to that described in Section 6.4. Again it might seem at first that there would be 12 possible combinations of (two different ones of) the four distinct ringing signals for the two stations, and those combinations could not all be separately invoked by a single digit. But in fact, only 8 different situations are needed, which we see in this table. They are listed according to the digit dialed in the “magic number” for that combination. Code (s) sent on Digit Ring Tip 1 1R 1R 3 2R 1R 4 1R 2R 6 2R 2R 7 1R nil 8 nil 1R 9 2R nil 0 nil 2R When the two stations involved in this operation have ringing on the same side of the line, we usually send only the ringing code for the called station. That way the called subscriber will recognize that the call is for him. As to the calling subscriber, all that is necessary that he hears some ringing, so that when it stops he knows that the calling station has answered. But, when the two stations have ringing on opposite sides of the line, each receives its own code (a nicety, not a necessity). The step-by-step telephone switching system: Reverting calls Page 18 Note the odd numbering for the various “stations” I’m not sure where that comes from. 6.6 For two-party ringing For a two-party line, only a single ring code (one-ring) is used, sent on one side of the line or the other for the two stations. Thus, in a reverting call, the ringing signals for the two stations are the same except for being on opposite sides of the line. There is no need for the calling subscriber to acquire, and dial, any “magic number”. In the ringing phase of a reverting call, the selector always sends bursts of ringing voltage on the two sides of the line (interleaved in time). Each station hears its familiar ringing code (always “one-ring”). This does not guarantee that the first ringing burst is for the calling subscriber. 7 7.1 THE REVERTING CALL CONNECTOR Introduction This system is primarily used for 10-party service, but the basic principle would be equally applicable to other multi-party line types. It is typically more convenient for the calling subscriber than the reverting call selector method. This system does not require the calling party to, in advance, be aware that the number he wishes to reach is in fact served on his line, or to deduce that from getting a lot of busy signals. Nor does he need to contact an operator to get a “magic number” to use in the process. He does, however, need to know how to work the reverting call system when it is encountered. (There will typically be a tutorial in the front of the directory.) 7.2 Some basics A reverting call connector is not only used for reverting calls. When reverting call connectors are used, all connections to 10-party lines end up in a 10-party reverting call connector. The terminal reached on a certain connection might be on the same line as the calling station (a reverting call situation), but most often would not. Thus, on each connection where the called line is found busy, the switch must ascertain whether this will be a reverting call or not, and behave quite differently in the two cases. The way it does that depends on the fact that, if the called number is for a station on the same line as the calling station, the sleeve lead of the called station (reached by the connector wipers) is the same lead The step-by-step telephone switching system: Reverting calls Page 19 as the sleeve lead of the calling station (brought to the connector over the connection through the earlier switches); it is the single sleeve lead for the line on which both stations are served. On every call, the connector, finding the called line apparently busy, in effect makes a “continuity check” between those two sleeve leads. If that reveals that they are indeed the same lead, the connector goes into its “reverting call” mode. We will see in some detail how that is done in the two following sections. 7.3 7.3.1 Call to a number whose station is not on the same line as the calling station (not a reverting call) Conventions In this figure and those to follow, a relay core shown shaded indicates that the relay is operated. The relays in the line finder and selectors I have arbitrarily designated CT in the figures are the cut through relays. They extend the connection to the next switch, and, when operated, prevent their switch from releasing. They hold to the S (sleeve) lead. CO is the designation for the cutoff relay in a line circuit. When operated it frees the line from the L (line) relay in the line circuit (not shown), which detects when an idle line has some off hook to request service. 7.3.2 Connection completed The following figure shows a connection that has been established from a certain multi-party line (its numbers are 2415 and 5569) to the number 2368 (which is not on the same line as the calling station). Although it doesn’t really matter here, imagine that the station calling is the one whose number is 2514. Figure 6. The step-by-step telephone switching system: Reverting calls Page 20 The path we see through the earlier switches is that for the S (sleeve) lead of the calling line, which plays a critical role in this story. Of course it is accompanied by the matching R (ring) and T (tip), which we don’t see yet, and which don’t really figure in this story. In this figure, the connection has been built up through the line finder and several selectors to a connector in the proper group (the one accessing 23xx numbers). When the connection reaches the connector, its A relay (not seen) operates and it operates the B relay. The B relay grounds the incoming sleeve lead, which holds operated the CT relays in the line finder and the various selectors, which in turn hold the respective switches “up”, and it holds operated the CO relay in the line circuit. The last two digits have then been dialed into the connector, driving it to terminal 2368 (not the number of a station on the same line as our calling station). The connector tests the sleeve of that terminal through the winding of the G relay to battery, to ascertain if that sleeve is grounded, which would indicate that the called station is busy. 7.3.3 The called number tests busy In this example, we assume that the called number’s line is indeed busy, by way of a wholly-unrelated existing call through another connector. That other connector has grounded the sleeve of the line serving that number. This operates the CO (cutoff) relay in the line circuit for that line, freeing that line from its L relay, and the ground on the sleeve “marks” the line as busy, so no other connector will attempt to connect to it if another caller dials that number. So “our” connector finds, when its G relay operates, that the sleeve of the line for the called number is indeed grounded; thus that line is busy. The immediate result is that the connector returns busy tone back though the connection to the caller. The calling subscriber, believing that the called number is busy (and in this case it is) will hang up. And if our connector did not