## 2 Initial Phase of NCS Prior to 2000  
The NCS concept is not a recent phenomenon. It emerged in the early 1970s, with the progress of computation, as well as communication, technologies. However, the computation and communication technologies were still in the nascent stages and so the NCSs were designed as decentralized control systems, which remained prominent for the next three decades. One of the earliest works on decentralized NCS was proposed by Reference [11], where algebraic and geometric methods were discussed to obtain stable results. A dual-mode decentralized scheme for the networked control of a commodity's flow has been presented in Reference [12]. First, local controllers located at network nodes, exchange simple messages with their neighbors to determine the confined optimum flows. Second, the local controllers adjust their flows to reach the optimal equilibrium state in finite time. The stabilization of the decentralized linear time-invariant multivariable (LTI-MultiVar) system was proposed in Reference [13] by employing numerous local feedback control rules. A necessary and sufficient condition was obtained from the above rule for the stabilization of the given system. Later, in Reference [14], the properties of the closed loop were studied for both controllable and observable _k-channel_ linear systems, by applying the consequences of decentralized feedback. In addition, the theory of a complete system was established. Complete systems can be made both controllable and observable for all channels by applying non-dynamic decentralized feedback. Some development in the field of decentralized control was achieved in 1980s. In Reference [15], a new dynamic interaction measure was defined by using the perception of structured singular value (SSV) for multivariable systems under feedback. To predict the stability of the decentralized systems and to measure the loss of performance, dynamic interaction measure was used. Later, in Reference [16], [PERSON] et al. presented several important studies on decentralized control systems. The decentralized regulation and synchronization for partially surveyed discrete-time-event systems was studied. In Reference [17], which was illustrated using a simple production system. This study was later extended in Reference [18], and it was concluded that decentralized supervision would be easier to design and use. In the 1990s, several NCS-related ideas evolved based on the concepts developed in the recent years, such as smart networks for control, decentralized control of complex systems, supervisory control, control in multimedia wireless networks, quasi-decentralized control, scheduling, event-triggered control, stability analysis, and so on [19; 20; 21; 22; 23; 24; 25; 26; 27; 28; 29; 30; 31]. The decentralized control of systems, which is complex in nature, were described in Reference [19], wherein topics, such as stabilization, optimization, estimation and control, output control, decompositions, and reliable control, were discussed in detail. In Reference [20], the author discussed the role of distributed, as well as centralized, networks for control. The authors also differentiated the above two categories in different ways. It also discussed the benefits and drawbacks of data networks, as well as the control networks. In a discrete-time system [21], the events must be within the imposed time constraints. To achieve a given control objective, a controller can hinder, permit, or pressurize several events in the system. In Reference [22], the network-level management and control issues were discussed. The quasi-decentralized control for complex systems was analyzed in Reference [23] using the concept of power system stabilization to design a sample system. Three types of data: periodic data, sporadic data, and messages, have been proposed in Reference [24] for the planning of the NCS. As a basic parameter, delay bound was used, which guarantees the stability of the given system using [PERSON]'s theorem.