Telephone switching (Inventions)

The invention: The first completely automatic electronic system for switching telephone calls.

The people behind the invention:

Almon B. Strowger (1839-1902), an American inventor Charles Wilson Hoover, Jr. (1925- ), supervisor of memory
system development Wallace Andrew Depp (1914- ), director of Electronic Switching
Merton Brown Purvis (1923- ), designer of switching matrices

Electromechanical Switching Systems

The introduction of electronic switching technology into the telephone network was motivated by the desire to improve the quality of the telephone system, add new features, and reduce the cost of switching technology. Telephone switching systems have three features: signaling, control, and switching functions. There were several generations of telephone switching equipment before the first fully electronic switching “office” (device) was designed.
The first automatic electromechanical (partly electronic and partly mechanical) switching office was the Strowger step-by-step switch. Strowger switches relied upon the dial pulses generated by rotary dial telephones to move their switching elements to the proper positions to connect one telephone with another. In the step-by-step process, the first digit dialed moved the first mechanical switch into position, the second digit moved the second mechanical switch into position, and so forth, until the proper telephone connection was established. These Strowger switching offices were quite large, and they lacked flexibility and calling features.
The second generation of automatic electromechanical telephone switching offices was of the “crossbar” type. Initially, crossbar switches relied upon a specialized electromechanical controller called a “marker” to establish call connections. Electromechanical telephone
switching offices had difficulty implementing additional features and were unable to handle large numbers of incoming calls.


Electronic Switching Systems

In the early 1940′s, research into the programmed control of switching offices began at the American Telephone and Telegraph Company’s Bell Labs. This early research resulted in a trial office being put into service in Morris, Illinois, in 1960. The Morris switch used a unique memory called the “flying spot store.” It used a photographic plate as a program memory, and the memory was accessed optically. In order to change the memory, one had to scratch out or cover parts of the photographic plate.
Before the development of the Morris switch, gas tubes had been used to establish voice connections. This was accomplished by applying a voltage difference across the end points of the conversation. When this voltage difference was applied, the gas tubes would conduct electricity, thus establishing the voice connection. The Morris trial showed that gas tubes could not support the voltages that the new technology required to make telephones ring or to operate pay telephones.
The knowledge gained from the Morris trial led to the development of the first full-scale, commercial, computer-controlled electronic switch, the electronic switching system 1 (ESS-1). The first ESS-1 went into service in New Jersey in 1965. In the ESS-1, electromechanical switching elements, or relays, were controlled by computer software. A centralized computer handled call processing. Because the telephone service of an entire community depends on the reliability of the telephone switching office, the ESS-1 had two central processors, so that one would be available if the other broke down. The switching system of the ESS-1 was composed of electromechanical relays; the control of the switching system was electronic, but the switching itself remained mechanical.
Bell Labs developed models to demonstrate the concept of integrating digital transmission and switching systems. Unfortunately, the solid state electronics necessary for such an undertaking had not developed sufficiently at that time, so the commercial development

Almon B. Strowger

Some people thought Almon B. Strowger was strange, perhaps even demented. Certainly, he was hot-tempered, restless, and argumentative. One thing he was not, however, was unimaginative.
Born near Rochester, New York, in 1839, Strowger was old enough to fight for the Union at the second battle of Manassas during the American Civil War. The bloody battle apparently shattered and embittered him. He wandered slowly west after the war, taught himself undertaking, and opened a funeral home in Topeka, Kansas, in 1882. There began his running war with telephone operators, which continued when he moved his business to Kansas City.
With the help of technicians (whom he later cheated) he built the first “collar box,” an automatic switching device, in 1887. The round contraption held a pencil that could be revolved to different pins arrange around it in order to change phone connections. Two years later he produced a more sophisticated device that was operated by push-button, and despite initial misgivings brought out a rotary dial device in 1896. That same year he sold the rights to his patents to business partners for $1,800 and his share in Strowger Automatic Dial Telephone Exchange for $10,000 in 1898. He moved to St. Petersburg, Florida, and opened a small hotel, dying there in 1902. It surely would have done his temper no good to learn that fourteen years later the Bell system bought his patents for $2.5 million.
of digital switching was not pursued. New versions of the ESS continued to employ electromechanical technology, although mechanical switching elements can cause impulse noise in voice signals and are larger and more difficult to maintain than electronic switching elements. Ten years later, however, Bell Labs began to develop a digital toll switch, the ESS-4, in which both switching and control functions were electronic.
Although the ESS-1 was the first electronically controlled switching system, it did not switch voices electronically. The ESS-1 used computer control to move mechanical contacts in order to establish a conversation. In a fully electronic switching system, the voices are digitized before switching is performed. This technique, which is called “digital switching,” is still used.
The advent of electronically controlled switching systems made possible features such as call forwarding, call waiting, and detailed billing for long-distance calls. Changing these services became a matter of simply changing tables in computer programs. Telephone maintenance personnel could communicate with the central processor of the ESS-1 by using a teletype, and they could change numbers simply by typing commands on the teletype. In electromechanically controlled telephone switching systems, however, changing numbers required rewiring.

Consequences

Electronic switching has greatly decreased the size of switching offices. Digitization of the voice prior to transmission improves voice quality. When telephone switches were electromechanical, a large area was needed to house the many mechanical switches that were required. In the era of electronic switching, voices are switched digitally by computer. In this method, voice samples are read into a computer memory and then read out of the memory when it is time to connect a caller with a desired number. Basically, electronic telephone systems are specialized computer systems that move digitized voice samples between customers.
Telephone networks are moving toward complete digitization. Digitization was first applied to the transmission of voice signals. This made it possible for a single pair of copper wires to be shared by a number of telephone users. Currently, voices are digitized upon their arrival at the switching office. If the final destination of the telephone call is not connected to the particular switching office, the voice is sent to the remote office by means of digital circuits.
Currently, voice signals are sent between the switching office and homes or businesses. In the future, digitization of the voice signal will occur in the telephone sets themselves. Digital voice signals will be sent directly from one telephone to another. This will provide homes with direct digital communication. A network that provides such services is called the “integrated services digital network” (ISDN).
See also Cell phone; Long-distance telephone; Rotary dial telephone; Touch-tone telephone.

Next post:

Previous post: