sentences:  Early electronic switching systems (ESS) developed in the 1960s and 1970s almost invariably used centralized control.Although many present day exchange design continue to use centralized SPC, with advent of low cost powerful microprocessors and VLSI chips such as programmable logic array (PLA) and programmable logic controllers (PLC), distributed SPC became widespread by the early 21st century.In centralized control, all control equipment is replaced a central processing unit.