(i) Progressive Control: Step by step system is an example of progressive control. The connection is set up in stages, in response to the digits dialed and each step in setting up the connection is controlled by relays mounted on the selector which operates at the input stage.
(ii) Common Control: Those systems in which the control subsystem is outside the switching network are known as common control switching system. Strowger exchanges are usually direct control systems, whereas crossbar and electronic exchanges are common control system. All stored program control systems are common control systems. Common control is also known as indirect control or register control.
(iii) Stored Program Control: Modern digital computers use the stored programmed concept. Here, a program or a set of instructions to the computer is stored in its memory and the instructions are executed automatically one by one by the processor. Carrying out the exchange control functions through programs stored in the memory of a computer led to the nomenclature stored program control (SPC). An immediate consequence of program control is the full-scale automation of exchange functions and the introduction of a variety of new services to users. Common Channel Signaling (CCS), centralized maintenance and automatic fault diagnosis, and interactive human-machine interface are some of the features that have become possible due to the application of SPC to telephone switching