To overcome the limitations of manual switching; automatic exchanges, having Electro-mechanical components, were developed. Strowger exchange, the first automatic exchange having direct control feature, appeared in 1892 in La Porte (Indiana). Though it improved upon the performance of a manual exchange it still had a number of disadvantages, viz., a large number of mechanical parts, limited availability, inflexibility, bulky in size etc. As a result of further research and development, Crossbar exchanges, having an indirect control system, appeared in 1926 in Sundsvall, Sweden. The Crossbar exchange improved upon many short- comings of the Strowger system. However, much more improvement was expected and the revolutionary change in field of
