Telephone Switching Concept

Manual Switch – In the telephone exchange all subscribers are connected to a central switch and it is manually operated by an operator. If a person want to call another person first he or she should call to telephone exchange. There is a person who can connect the two subscribers together by plugging in the exchange.

Strowger Switch - Every subscriber line ended at the Strowger switch with a relay, referred to as line relay. This switch had stepping relays that worked according to the dial pulses. Upon completing one digit, the first stepping relay would activate another stepping relay. Complete connection was achieved by serial activation of several stepping relays. The line relays were connected to an electromechanical device called line finder.

Crossbar switch - Crossbar switch employed centralized common control equipment which encompassed a group of equipment engaged in establishing call set up. The equipment was released and was available for setting up other calls once setting up phase of a call was over. Crossbar switch replaced the line finder with line scanner, to check when a telephone line connected to a line link frame was off hook. 

SPC switch – after the evolution of electronics, especially the invention of the transistor, SPC switches came into use. The SPC switch entirely replaced the electromechanical common control equipment of the cross bar switch with electronic circuits, while retaining the same switching matrixes. These systems employ memories based on RAM (Random Access Memory) technology, tape and disk storage. SPC switching system was space division switching systems.