Step-by-Step switching system

Step-by-Step switching system. The Step-by-Step system, also known as the Strowger system, was the first commercially successful automatic telephone exchange technology. Invented by Almon Brown Strowger, an undertaker from Kansas City, it directly addressed the problem of human operators, whom he suspected of misdirecting calls to his competitors. This electromechanical system, deployed widely by the Bell System and other operators, formed the backbone of telephone networks for much of the 20th century, automating call routing through a direct-control method where the subscriber's dial pulses directly stepped switches.

Overview

The fundamental innovation was its direct-control architecture, where pulses from a subscriber's rotary dial directly activated a series of electromechanical switches to establish a connection. This stood in contrast to later systems like the crossbar switch or Electronic Switching System which used common control. The primary components were the line finder, selector, and connector switches, each performing a specific stage in the call path. Major manufacturers included the Automatic Electric Company, founded by Strowger, and later Western Electric, the manufacturing arm of the Bell System.

Operation

A call originated when a subscriber lifted the handset, activating a line finder that searched for and connected to their line. Dialing the first digit sent a series of pulses that stepped a first selector switch vertically, aligning with a bank of contacts corresponding to a group of numbers, such as an exchange prefix. The next digit pulses stepped the selector's wiper horizontally to select a specific connector switch. Finally, the last digits pulsed the connector switch to step vertically and then horizontally, making the final connection to the called subscriber's line within that group. This process created a metallic path through the exchange using Strowger switches, also known as two-motion selectors.

Development and deployment

Following his patent in 1891, Strowger established the Strowger Automatic Telephone Exchange Company, with the first commercial installation occurring in La Porte, Indiana in 1892. The technology was refined by engineers like Alexander E. Keith and John J. Carty, leading to its widespread adoption. Independent telephone companies, often in competition with the Bell System, were early adopters. The Bell System itself began large-scale deployment after acquiring related patents, with major installations in New York City and Philadelphia. The system saw global proliferation, with variations manufactured by companies like Siemens AG in Germany and the General Post Office in the United Kingdom.

Technical details

The core component was the Strowger switch, an electromechanical device with a wiper arm that could move in two dimensions under control of solenoids and a ratchet mechanism. A typical exchange required vast arrays of these switches, along with associated relay logic for functions like off-hook detection and busy signal generation. The system used a decimal stepping logic directly driven by dial pulses, with no memory or programmable control. Maintenance was labor-intensive, requiring teams of switchmen for adjustments and repairs, and the systems were notorious for their audible noise, often described as a constant clicking and clacking.

Impact and legacy

The deployment revolutionized telephony by enabling direct subscriber dialing, eliminating the need for manual operators for local calls and dramatically increasing network capacity and privacy. It entrenched the North American Numbering Plan and similar numbering schemes worldwide. While eventually supplanted by more efficient crossbar switch systems in the mid-20th century and then Electronic Switching Systems like the Number One Electronic Switching System, Step-by-Step offices remained in service in some areas into the 1990s. Its influence is evident in telephony terminology, such as "step office," and it represents a critical milestone in the history of automation and telecommunications infrastructure.

Category:Telephony equipment Category:Telecommunications history Category:American inventions