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The purpose of a switching system is to establish a temporary link between callers. In the first simple exchanges, all calls were handled manually by a switchboard operator, who plugged in the lines linking subscribers.
In 1892 Strowger introduced the automatic exchange operated on the step by step system. Many of these exchanges are still operating. Next came the crossbar exchanges, developed in Sweden.They consisted of a series of vertical and horizontally crossed bars, operated electromechanically. Although this system is still found throughout the world, it involves thousands of wear-prone exposed moving parts, which are costly to install and maintain.
In 1948, the invention of the transistor led to the first semi-electronic telephone exchange. Nowadays many exchanges are built around electronic components. The latest digital switching centres have many advantages over old exchanges, reducing the cost, size of equipment, installation time, and increasing reliability and number of customer services.