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The Strowger switch is the first commercially successful electromechanical stepping switch telephone exchange system. It was developed by the Strowger Automatic Telephone Exchange Company founded in 1891 by Almon Brown Strowger. Because of its operational characteristics, it is also known as a step-by-step (SXS) switch.
Strowger, an undertaker, was motivated to invent an automatic telephone exchange after having difficulties with his telephone service. He became convinced that the manual telephone exchange operators were deliberately interfering with his calls, leading to loss of business. According to the local Bell Telephone Company manager Herman Ritterhoff, Strowger swore to "get even" with the telephone operators and "put every last one of them out of a job." Ritterhoff claimed that the real cause of Strowger's difficulties was a metal sign hung on his wall over his telephone, causing an intermittent short circuit when blown by the wind. [1]
Strowger conceived his invention in 1888, and was awarded a patent for an automatic telephone exchange in 1891. The initial model was made from a round collar box and some straight pins. [2]
While Almon Strowger devised the initial concept, he was not alone in his endeavors and sought the assistance of his brother Arnold, nephew William, and others with a knowledge of electricity and financing to realize the concept. The Strowger Automatic Telephone Exchange Company was founded in 1891. [3] In the original design patent, four keys were added near the telephone, one each for thousands, hundreds, tens and units, with each key having an additional wire connection to the central exchange. Each key had to be tapped the correct number of times to step the switch and make the desired connection. To connect to number 1256, the user would press the first key once, the second key twice, the third key five times and the final key six times. [4]
The company installed and opened the first commercial exchange in his then-home town of La Porte, Indiana on November 3, 1892. [5] [6] The exchange had around 75 subscribers. The installation followed the original patented design, with four keys and four additional line wires connected to the exchange, but not all of the keys were used.
Early advertising called the new invention the "girl-less, cuss-less, out-of-order-less, wait-less telephone". [7]
In 1896 the company patented the finger-wheel dial as an improvement to the existing four-key design. [8]
The Strowger Automatic Telephone Exchange Company became the Automatic Electric Company, which Strowger was involved in founding, although Strowger himself seems not to have been involved in further developments. The Strowger patents were exclusively licensed to the Automatic Electric Company. Strowger sold his patents in 1896 for US$1,800 (equivalent to $55,000in 2022) [9] and sold his share in Automatic Electric in 1898 for $10,000 (equivalent to $300,000in 2022). [9] His patents subsequently sold for $2,500,000 (equivalent to $47,000,000in 2022) [9] in 1916. Company engineers continued development of the Strowger designs and submitted several patents in the names of its employees.
The Strowger system was widely used until the development of the more reliable crossbar switch, an electromechanical switch with a matrix of vertical and horizontal bars and simpler motions. [10]
Strowger's patent [11] specifies dialing equipment at the customer location and the switching equipment at the central office.
The telegraph keys or telephone dial creates trains of on-off current pulses corresponding to the digits 1–9, and 0 (which sends 10 pulses). This equipment originally consisted of two telegraph keys engaged by knife switches, and evolved into the rotary dial telephone.
The central office switching equipment has a two-motion stepping switch. A contact arm is moved up to select one of ten rows of contacts, and then rotated clockwise to select one of ten contacts in that row, a total of 100 choices. The stepping motion is controlled by the current pulses coming from the originating customer's telegraph keys, and later from the rotary dial.
The Strowger switch has three banks of contacts. Toward the upper end of each shaft are two ratchets. The upper one has ten grooves, and raises the shaft. The lower one has long vertical teeth (on the other side, hidden).
The Strowger switch uses two telegraph-type keys on a telephone set for dialing. Each key requires a separate wire to the exchange. The keys are tapped to step the switch in two stages. The first set of incoming pulses raises the armature of an electromagnet to move a shaft which selects the desired level of contacts, by engaging a pawl with the upper ratchet. Another pawl, pivoting on the frame, holds the shaft at that height as it rotates. The second set of pulses, from the second key, operates another electromagnet. Its pawl engages the (hidden) vertical teeth in the lower ratchet to rotate the shaft to the required position. It is kept there against spring tension by a pawl pivoted on the frame. When the switch returns to its home position, typically when a call is complete, a release magnet disengages the pawls that hold the shaft in position. An interlock ensures that the spring on the shaft rotates it to angular home position before it drops to its home position by gravity.
The commercial version of the Strowger switch, as developed by the Strowger Automatic Telephone Exchange Company, used a rotary dial for signalling to the exchange. The original final selector (connector) switch which connected to 100 customers was supplemented by preceding group selector stages, as the "cascading" enabled connection to many more customers, and to customers at other exchanges. Another requirement for commercial systems was a circuit to detect a busy connection (line) and return a busy signal to the calling subscriber.
Instead of dedicating an expensive first-stage selector switch to each customer as in the first exchange, the customer was given access to the first-stage switch of a telephone network, often by a line-finder which searches "backward" for the calling line; so requiring only a few relays (in most cases two, a Line, and a Cut-off relay), for the equipment required for each customer line.
Later Strowger (SXS) exchanges often use a subscriber uniselector as part of the line equipment individual to each line, which searches "forward" for a first selector. This is more economical for higher calling-rate domestic or business customers, and has the advantage that access to additional switches can readily be added if the traffic increases (the number of linefinders serving a group is limited by the wiring multiple installed). Hence exchanges with subscriber uniselectors were usually used at British exchanges with a high proportion of business customers, e.g. director exchanges, or in New Zealand where the provision of local free calling meant that residential customers had a relatively high calling rate.
The fundamental modularity of the system combined with its step-by-step (hence the alternative name) selection process and an almost unlimited potential for expansion gives the Strowger system its technical advantage. Previous systems had all been designed for a fixed number of subscribers to be switched directly to each other in a mesh arrangement. This became quadratically more complex as each new customer was added, as each new customer needed a switch to connect to every other customer. In modern terminology, the previous systems were not "scalable".
From 1912, the British General Post Office, which also operated the British telephone system, installed several automatic telephone exchanges from several vendors in trials at Darlington on 10 October 1914 and Dudley on 9 September 1916 (rotary system), Fleetwood (relay exchange from Sweden), Grimsby (Siemens), Hereford (Lorimer) and Leeds (Strowger). [12] The GPO selected the Strowger switches for small and medium-sized cities and towns. The selection of switching systems for London and other large cities was not decided until the 1920s, when the Director telephone system was adopted. The Director systems used SXS switches for destination routing and number translation facilities similar to the register used in common-control exchanges. Using similar equipment as in the rest of the network was deemed beneficial and the equipment could be manufactured in Britain.
A rotary dial is a component of a telephone or a telephone switchboard that implements a signaling technology in telecommunications known as pulse dialing. It is used when initiating a telephone call to transmit the destination telephone number to a telephone exchange.
A telephone switchboard was a device used to connect circuits of telephones to establish telephone calls between users or other switchboards, throughout the 20th century. The switchboard was an essential component of a manual telephone exchange, and was operated by switchboard operators who used electrical cords or switches to establish the connections.
In telecommunications, common control is a principle of switching telephone calls in an automatic telephone exchange that employs shared control equipment which is attached to the circuit of a call only for the duration of establishing or otherwise controlling the call. Thus, such control equipment need only be provided in as few units to satisfy overall exchange traffic, rather than being duplicated for every subscriber line.
Pulse dialing is a signaling technology in telecommunications in which a direct current local loop circuit is interrupted according to a defined coding system for each signal transmitted, usually a digit. This lends the method the often used name loop disconnect dialing. In the most common variant of pulse dialing, decadic dialing, each of the ten Arabic numerals are encoded in a sequence of up to ten pulses. The most common version decodes the digits 1 through 9, as one to nine pulses, respectively, and the digit 0 as ten pulses. Historically, the most common device to produce such pulse trains is the rotary dial of the telephone, lending the technology another name, rotary dialing.
Almon Brown Strowger was an American inventor who gave his name to the Strowger switch, an electromechanical telephone exchange technology that his invention and patent inspired.
This timeline of the telephone covers landline, radio, and cellular telephony technologies and provides many important dates in the history of the telephone.
Automatic Electric Company was an American telephone equipment supplier primarily for independent telephone companies in North America, but also had a worldwide presence. With its line of automatic telephone exchanges, it was also a long-term supplier of switching equipment to the Bell System, starting in 1919. The company was the largest manufacturing unit of the Automatic Electric Group. In 1955, the company was acquired by General Telephone and Electronics (GT&E). After numerous reorganization within GTE, the company's assets came under the umbrella of Lucent in the 1990s, and subsequently part of Nokia.
The director telephone system was a development of the Strowger or step-by-step (SXS) switching system used in London and five other large cities in the UK from the 1920s to the 1980s.
A sender is a type of circuit and system module in 20th-century electromechanical telephone exchanges. It registered the telephone numbers dialed by the subscriber, and transmitted that information to another part of the exchange or another exchange for the purpose of completing a telephone call. Some American exchange designs, for example, of the No. 1 Crossbar switch used originating senders and terminating senders. The corresponding device in the British director telephone system was called a "director" and, in other contexts, "register".
In electrical engineering, a stepping switch or stepping relay, also known as a uniselector, is an electromechanical device that switches an input signal path to one of several possible output paths, directed by a train of electrical pulses.
Loop start is a telecommunications supervisory protocol between a central office or private branch exchange (PBX) and a subscriber telephone or other terminal for the purpose of starting and terminating a telephone call. It is the simplest of the telephone signaling systems, and uses the presence or absence of loop current to indicate the off-hook and on-hook loop states, respectively. It is used primarily for subscriber line signaling. An extension of the protocol that adds disconnect supervision is often called kewlstart.
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The rotary machine switching system, or most commonly known as the rotary system, was a type of automatic telephone exchange manufactured and used primarily in Europe from the 1910s. The system was developed and tested by AT&T's American engineering division, Western Electric, in the United States, at the same time when Western Electric was also developing the Panel switch. When AT&T selected the Panel System for large American exchanges, development and sales of the No. 7-A Machine Switching System, its formal commercial name, were transferred to Western Electric's international division in Belgium. In Europe and other continents the system was met with considerable commercial success.
The push-button telephone is a telephone that has buttons or keys for dialing a telephone number, in contrast to having a rotary dial as in earlier telephone instruments.
A telephone exchange, also known as a telephone switch or central office, is a crucial component in the public switched telephone network (PSTN) or large enterprise telecommunications systems. It facilitates the interconnection of telephone subscriber lines or digital system virtual circuits, enabling telephone calls between subscribers.
The Cities of Manchester and Salford and some surrounding urban areas such as Stockport, Oldham and Bolton,, were the location of several advances in the development of telephony in the United Kingdom. A Manchester company installed the country's first regular telephone system in the city, wiring solutions which were adopted across Britain were initiated in Manchester and Stockport, the city was (arguably) the site of the UK's first telephone exchange and a service which was the forerunner of modern mobile phones was controlled from the city.
Dialling is the action of initiating a telephone call by operating the rotary dial or the telephone keypad of a telephone.
The Panel Machine Switching System is a type of automatic telephone exchange for urban service that was used in the Bell System in the United States for seven decades. The first semi-mechanical types of this design were installed in 1915 in Newark, New Jersey, and the last were retired in the same city in 1983.
The Automatic Telephone and Electric Company was a British telephone exchange manufacturer established in 1911. After several name changes and acquisitions, the company was merged into Plessey in 1961.
NANP telephone switches | |
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| Early automatic & crossbar switches | |
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