The Switch Train: Following a Call — Telephone Switching History

Figure 1 — Selectors installed in an Automatic Electric exchange, from Smith and Campbell's Automatic Telephony (1921). Each vertical shaft is one switch; each shelf of ten is one group sharing a bank multipl… — Figure 1 — Selectors installed in an Automatic Electric exchange, from Smith and Campbell's Automatic Telephony (1921). Each vertical shaft is one switch; each shelf of ten is one group sharing a bank multiple; the cabling between shelves is the trunking that makes the dialled number the route. — A. B. Smith and W. L. Campbell, Automatic Telephony (1921), via Internet Archive

4.1 The Number Is the Route

In a step-by-step office the telephone number is not looked up anywhere. Each digit moves one switch, and the switch it moves is chosen by the digits before it. The equipment a call passes through is called the switch train, and in its classic form it has four kinds of switch:

a line finder (or, in older offices, a line switch), which notices the calling line and connects it to a first selector;
a first selector, which sends dial tone, takes the first digit, and finds a free path to the next stage;
as many group selectors (second, third) as the office’s size needs, each taking one digit;
a connector, which takes the last two digits, finds the called line, tests it, rings it, and feeds it talking battery.

Each switch in the train hands the call on and then stays in it, holding its part of the path until the call ends. This volume follows one call through that train, stage by stage. Volume 5 then opens up the relays that make each stage work.

Figure 2 — The full switch train of a 10,000-line office, dialling 2345. The first selector takes the 2 (the thousands), the second selector the 3 (the hundreds), and the connector the 4 and the 5. Each stage… — Figure 2 — The full switch train of a 10,000-line office, dialling 2345. The first selector takes the 2 (the thousands), the second selector the 3 (the hundreds), and the connector the 4 and the 5. Each stage carries a natural point for a call-progress LED, the idea behind the display on the system this project is building. — Original diagram drawn for this dive

4.2 How Big an Office, How Many Digits

Smith and Campbell’s 1921 textbook builds the system up from its smallest form (Automatic Telephony, pp. 7–13):

Table 1 — Smith and Campbell's 1921 textbook builds the system up from its smallest form ([Automatic Telephony, pp. 7–13](https://archive.org/details/automatictelepho00smit))

Size	Digits	Switch train	Notes
100 lines	2	line switch or finder → connector	”A switchboard for 100 lines consists of 100 lineswitches with their master switch … and about ten connector switches.”
1,000 lines	3	→ first selector → connector	Ten hundred-line groups. Level n of the selector banks is trunked to the connectors of the n-hundreds group.
10,000 lines	4	→ first selector → second selector → connector	”Groups of groups.”
100,000 numbers (several offices)	5	→ first, second, third selector → connector	The first digit picks the office (Kerr’s example: 5-2368).
2,000 lines with 200-point switches	4	200-point finder → first selector → 200-point connector	The second digit, 1 or 2, chooses the connector’s bank; the third steps vertical and the fourth rotary (USAF course, Vol. 3).

Why “about ten connectors” for a hundred lines? Because a line is in use only about “3 or 4 per cent of the time” (Smith and Campbell, p. 7). Expensive switching equipment is shared, and the art of designing a step-by-step office is the art of deciding how much sharing the traffic will bear.

4.3 Stage 1: The Line Circuit

Every telephone has one piece of equipment of its own in the office: its line circuit. In Automatic Electric practice this is a single line-and-cutoff relay, a two-step relay: “the manufacturer frequently refers to this relay as line equipment,” mounted in groups of 20 or 40 (USAF course, Vol. 3 §1-1). In Bell practice the same job is done by two relays, a line relay (L) and a cut-off relay (CO).

When the subscriber lifts the handset, loop current flows through the relay’s windings. That is enough for the first step: the make contacts close, but not the break contacts. The closed contacts mark the line: they ground its level on the line finders’ vertical bank, put battery on its control-bank contact, and start the finder group. When a finder (or a connector, on an incoming call) seizes the line, only the cut-off winding is left across battery; the current rises, the relay operates fully, and its break contacts take the line relay off the loop. The line is now in the hands of a switch, and its grounded sleeve tells every other switch it is busy.

Figure 3 — The line circuit, drawn Bell-style with separate line (L) and cut-off (CO) relays for clarity. Tip goes to ground and ring to −48 V through the line relay, each via a cut-off contact. The line's ti… — Figure 3 — The line circuit, drawn Bell-style with separate line (L) and cut-off (CO) relays for clarity. Tip goes to ground and ring to −48 V through the line relay, each via a cut-off contact. The line's tip, ring and sleeve also appear in every finder bank and connector bank that can reach it. Automatic Electric combined L and CO in a single two-step relay. — Original diagram drawn for this dive

4.4 Stage 2: Finding the Caller

There are two ways to get a calling line to a first selector, and step-by-step used both.

A line switch (Keith plunger or rotary) gives every line its own small switch, which hunts forward over trunks to an idle first selector.
A line finder works the other way round. Every line is multipled into the banks of a small pool of two-motion switches, and when a line calls, one of them hunts backward to find it.

Line switches give quicker dial tone and suit heavy traffic, and the British Post Office kept uniselector line switches throughout the Strowger era. Line finders put the expensive switch on the trunk side, a handful per hundred lines, and win on cost when each line calls rarely. The Bell System adapted the two-motion switch as a line finder in 1926 (BTL Switching Technology, ch. 3) and made it the norm (Kerr, The line finder switch).

Figure 4 — Two ways to connect a caller to a first selector. Left: a preselector (line switch) on every line, each hunting outward over the trunks to the first selectors. Right: lines wired into a shared bank… — Figure 4 — Two ways to connect a caller to a first selector. Left: a preselector (line switch) on every line, each hunting outward over the trunks to the first selectors. Right: lines wired into a shared bank, with a few line finders, each permanently tied to its own first selector, hunting inward for whichever line is calling. — Original diagram drawn for this dive

4.4.1 Distributors and Group Relays

A pool of line finders needs something to decide which finder answers the next call, and to keep that decision ready before the call arrives. In Automatic Electric offices that job belongs to the distributor and its group relays. The USAF course describes the standard arrangement in detail (Vol. 3, §1-1 and §1-2):

A line-finder shelf “is normally equipped to service 200 lines at 10 percent trunking”: 20 line finders, split into groups A and B. A typical shelf carries “200 line-cutoff relays, two distributor switches, two group relays [sets], and a fuse.”
The distributor is “a 25-point, non-homing-type rotary stepping switch” with six bank levels: guard, finder start, vertical stepping, level test, and lower and upper bank cut-through. Each of its positions controls one line finder. In the course’s words, it “connects the group relays to any one of the linefinders of its group … disconnects from the linefinder just as soon as it has seized the calling line … [and] assigns [the finder] for the next line to follow.” If the next finder in line is busy, ground on the guard level drives the distributor on past it by itself.
The group relays, a set of about a dozen on their own base for each group, start the assigned finder, pulse its vertical magnet, stop it when its vertical wiper reaches the marked level, pulse its rotary magnet until it finds the marked line, release the distributor once the line is seized, and transfer the call to the partner group if a finder fails or all of a group’s finders are busy. “Because the group relays and the distributor likewise function together, they are referred to as group-distributor equipment.”
Preferential hunting. Group A’s banks are wired levels 1 to 0; group B’s are wired in reverse, so a line on level 3 of group A sits on level 8 of group B. Each group normally searches only its lower five levels, which keeps hunting short, and passes a call it cannot reach to its partner.

Each line finder is wired permanently, “back-to-back”, to one first selector, forming a finder–selector link. In an office small enough to need no selectors, the finder is wired to a connector instead.

The word for this job varies by maker. Stromberg-Carlson’s XY offices called the same device an allotter: “In a 100 line group there are 14 linefinders, 2 allotters, and 100 line circuits” (USAF course, Vol. 4), and patents use “allotter” generically, for example US 2,901,546, “Line finder and allotter system” (Google Patents). Bell offices did it with a relay group circuit and a start chain, with no stepping switch at all (Kerr). The “distribution switches” in my own collection are almost certainly Automatic Electric distributors.

Figure 5 — An Automatic Electric 200-line finder shelf: 200 line circuits multipled to 20 line finders in two groups. Each group's distributor points at the next idle finder before the call arrives, and its g… — Figure 5 — An Automatic Electric 200-line finder shelf: 200 line circuits multipled to 20 line finders in two groups. Each group's distributor points at the next idle finder before the call arrives, and its group relays step that finder to the caller. Group B's banks are wired in reverse, and a call overflows to the partner group when its own is busy. — Original diagram drawn for this dive

Figure 6 — A line finder from the Waltham demonstration band. Mechanically it is the same two-motion switch as a selector, but its bank holds subscribers' lines, and it hunts for the one that is calling rathe… — Figure 6 — A line finder from the Waltham demonstration band. Mechanically it is the same two-motion switch as a selector, but its bank holds subscribers' lines, and it hunts for the one that is calling rather than for a free trunk. — Daderot, via Wikimedia Commons (CC0)

4.5 Stage 3: The First Selector

The moment the finder cuts through, its first selector is in the caller’s loop, and the selector’s A relay operates on the loop current. That is seizure. The selector sends dial tone: the first selector’s shelf is wired with a dial-tone transformer in the A relay’s battery feed, where a second selector has plain ground instead (USAF course, Vol. 3 §2-2). Kerr puts it neatly: dial tone is “permanently connected to the relay that will feed the station ‘talking battery’ while the line is in the hands of the first selector” (Overview).

The first selector is “seminumerical”. The first digit steps its wipers up to a level under dial control. Then, “right after the dial has stopped momentarily and before it is pulled for the next digit,” it rotates by itself along that level, testing each trunk’s sleeve, and stops on the first one that is not grounded (USAF course). Every trunk on level 3 of every first selector leads to the switches that serve the 300s, so it does not matter which idle trunk the selector chooses, only that it is idle. If none is, the selector runs off the end to the 11th step and returns all-trunks-busy tone.

Group selectors (second, third) are the same switch wired without dial tone. “Central offices that serve more than 1000 lines … normally use second selectors … In large commercial installations, third selectors are also often used” (USAF course). A digit-absorbing selector can swallow an unwanted digit, such as a prefix, without advancing the call (Kerr, The selector switch).

Figure 7 — Dialling 345. On the first selector the digit 3 lifts the wipers to level 3, and the selector then hunts along the level past busy trunks to the first idle one. On the connector reached through tha… — Figure 7 — Dialling 345. On the first selector the digit 3 lifts the wipers to level 3, and the selector then hunts along the level past busy trunks to the first idle one. On the connector reached through that trunk, the 4 lifts the wipers to level 4 and the 5 turns them to contact 5: line 345. — Original diagram drawn for this dive

4.6 Stage 4: The Connector

The connector is “a fully numerical switch”. Both of its motions are dial-controlled, so it has no interrupter springs, and it is recognisable by “the large number of relays mounted on its switch base” (USAF course, Vol. 3 ch. 3). It takes the last two digits: the tens digit steps it up to a level and the units digit steps it round to a contact. That contact is the called line.

What it does next is the whole of an operator’s job, done in a fraction of a second:

Busy test. A moment after the last pulse, the connector tests the called line’s sleeve. Ground means the line is in use: the connector returns busy tone and goes no further.
Seize and ring. Battery through the line’s cut-off winding means idle. The connector grounds the sleeve, making the line busy to every other switch and operating its cut-off relay, and connects ringing to the line and ringback to the caller.
Ring trip. When the called party lifts the handset, direct current flows in the ringing path and operates the ring-trip relay. Ringing stops and the talking path is completed.
Battery feed and supervision. The connector feeds talking battery to both parties through a transmission bridge, two relays joined by capacitors, and reverses the caller’s battery when the called party answers.

Volume 5 opens up each of these steps.

Figure 8 — The connector of the Waltham demonstration band. The crowd of relays on its base is the visible difference from a selector. Between them they test, ring, trip, feed battery and supervise. — Daderot, via Wikimedia Commons (CC0)

4.6.1 Special Connectors

The USAF course lists four kinds of connector:

Regular, as above.
Trunk-hunting, “sometimes called PBX connectors”. All the lines of a business’s group sit consecutively on one level, and only the first, the pilot number, is listed. If it is busy, the wipers hunt on to the next, and the next; if all are busy, they stop on the last and return busy tone. Kerr describes the circuit: a fourth bank contact, H, is strapped to the sleeve on every line of the group except the last, and a busy line with a grounded H makes the switch take another rotary step. Automatic Electric also built level-hunting connectors, which could step up a level and carry on hunting, so that one listed number could serve thirty or more lines (Kerr, The connector switch).
Executive right-of-way, which can break into a busy line.
Test, at the left end of each connector shelf, used by the test desk through a test distributor and not wired for ringing.

4.7 The Call, End to End

Figure 9 — One local call in a 1,000-line office, from lifting the handset to hanging up: thirteen events across the six pieces of equipment involved. The right-hand column says what happens at each step; gre… — Figure 9 — One local call in a 1,000-line office, from lifting the handset to hanging up: thirteen events across the six pieces of equipment involved. The right-hand column says what happens at each step; green dots mark the events a call-progress LED can show. — Original diagram drawn for this dive

Following the USAF course’s circuit descriptions, and Smith and Campbell’s own 1,000-line example, here is a caller dialling 345 in a 1,000-line Automatic Electric-pattern office:

Table 2 — Following the USAF course's circuit descriptions, and Smith and Campbell's own 1,000-line example, here is a caller dialling 345 in a 1,000-line Automatic Electric-pattern office

Step	What happens	Equipment
Off-hook	The line relay operates and marks the line. The distributor has already chosen an idle finder; the group relays step it up to the marked level and round to the line. The cut-off relay operates.	Line circuit, line finder, distributor
Dial tone	The finder’s permanently paired first selector is seized. Its A and B relays operate; B grounds the sleeve back to the finder to hold it; dial tone flows from the A relay’s feed.	First selector
Dial 3	A follows three breaks. The C relay and vertical magnet lift the shaft to level 3. C releases in the pause, and the selector hunts along level 3 (trunks to the connectors of the 300s) until the sleeve wiper finds an ungrounded trunk. The cut-through relay F operates and extends the call.	First selector
Dial 4	The connector’s A pulses; its vertical magnet lifts the shaft to level 4. C releases, and pulsing moves to the rotary side.	Connector
Dial 5	The rotary magnet steps five times to contact 5 on level 4: line 345. E holds through the digit, then releases: the busy test.	Connector
Idle line	H operates. It grounds the sleeve (line 345 is now busy, and its line-and-cutoff relay operates fully), sends ringing through F, and connects ringback to the caller.	Connector
Answer	Direct current through the answering telephone trips F. D feeds the called line and reverses battery to the caller; speech passes through the bridge capacitors.	Connector
Hang-up	The caller hangs up: A and B release, the sleeve ground is removed, and the first selector and finder fall back. In the Automatic Electric circuit the connector releases only when the called party has hung up too.	All

Smith and Campbell’s own example has subscriber 261 calling 805. The selector lifts to level 8 and hunts a trunk to an idle connector in the 800 group; dialling 0 lifts the connector to level 0, the tenth, and 5 turns it to contact 5 (p. 9).

4.7.1 Holding: “The Basic Principle of Step-by-Step”

One detail in that sequence deserves its own name. When a selector cuts through to the next switch, its own A relay drops out of the loop, and its slow B relay starts to release. B’s slow release keeps ground on the sleeve “long enough to allow relay B of the next switch to operate and place a multiple ground on the C lead.” From then on, the sleeve ground that holds every switch behind it comes from the switch ahead. The course calls this “the basic principle of step-by-step automatic switching”: “This method of having a safety ground and extending holding and guarding circuits from one switch to the next is the basic principle of step-by-step automatic switching” (USAF course, Vol. 3 §2-2). At the end of the train, only the connector’s B relay holds the whole path, so taking that one ground away releases everything behind it.

4.8 Frames and Cross-Connections

The bank positions are not the telephone numbers by accident. In a real office both are set by jumpers:

The main distributing frame (MDF) connects each outside line to its connector-bank position, and so assigns its telephone number.
An intermediate distributing frame (IDF) assigns its line-finder position separately, so traffic can be balanced between finder groups. “There is not a fixed relationship between the numbers of the lines and their ‘positions’ on the line finder banks” (Kerr, Overview).
Selector banks are cross-connected to the trunks ahead at a distributing terminal assembly, “the equipment where the system grading is provided” (USAF course). Grading is the craft of sharing trunks between selectors so that each level’s trunks are used evenly.

In the course’s model office, selectors are mounted ten to a shelf, ten shelves to a bay, and each switch plugs into a shelf jack. A switch can be pulled for repair without unsoldering a single wire, and a test set can be plugged in where it was.

Figure 10 — Racks of Strowger selectors from a German local exchange, preserved at the Fernmeldemuseum Aachen. Shelf above shelf of identical switches, each plugged into its own jack, multipled to its neighbou… — Figure 10 — Racks of Strowger selectors from a German local exchange, preserved at the Fernmeldemuseum Aachen. Shelf above shelf of identical switches, each plugged into its own jack, multipled to its neighbours, and cross-connected to the next stage. — Túrelio, via Wikimedia Commons (CC BY-SA 3.0 de)

4.9 In the Reference Library

Sources

USAF Extension Course Institute, CDC 36251, Telephone Switching Equipment Repairman (Electromechanical), Vols. 3–4, ERIC ED194781. https://archive.org/details/ERIC_ED194781
A. B. Smith and W. L. Campbell, Automatic Telephony, 2nd ed., 1921, pp. 7–13, 56. https://archive.org/details/automatictelepho00smit
D. A. Kerr, The step-by-step telephone switching system: Overview, The line finder switch, The selector switch, The connector switch. http://dougkerr.net/Pumpkin/
US 2,901,546, “Line finder and allotter system” (filed 1955). https://patents.google.com/patent/US2901546A/en