difference between common control and direct control switching system
Common control Systems which have control subsystem as an integral part of the switching network itself were known as direct control switching systems. Systems in which the control subsytem is outside the switching network are known as Common control systems. Strowger exchanges are usually direct control systems, whereas crossbar, electronic exchanges including all stored program control systems are common control systems. Common control is also known as indirect control or register control.
Robust control Informally, a controller designed for a particular set of parameters is said to be robust if it also works well under a different set of assumptions. High-gain feedback is a simple example of a robust control method; with sufficiently high gain, the effect of any parameter variations will be negligible. From the closed loop transfer function perspective, high open loop gain leads to substantial disturbance rejection in the face of system parameter uncertainty.