First, we must distinguish common control from direct progressive control described in Section 184.108.40.206. With direct progressive control a subscriber dialed a digit, and the first relay bank stepped to the dialed digit; the subscriber dialed a second digit, and the second stepping relay bank actuated, stepping to that digit level, and so on, through the entire dialed number. With common control, on the other hand, the dialed number is first stored in a register.13 These digits are then analyzed and acted upon by a marker, which is a hard-wired processor. Once the call setup is complete, the register and marker are free to handle other call setups. The marker was specifically developed for the crossbar switch. Such marker systems are most applicable to specialized crossbar switching matrices of crossbar switches. Stored program control (SPC) is a direct descendent of the crossbar common control system. SPC is described below.
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