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 matrices. These systems
employ memories based on RAM (Random Access Memory) technology, tape and disk
storage. SPC switching system was space division switching systems.
