# Protocol for Energy-Efficiency in Networked Control Systems Based on WSN
## 2 General Background  
Efficient energy consumption is a major concern in the WSN field and the specialized literature reveals a rich variety of manners for increasing the lifetime of a sensor network. Clearly, the adoption of different materials or hardware components may allow significant energy savings. In [22], [PERSON] et al. show how the choice of one kind of node over another can more than double the lifetime of a simple sensor application. Actually, different hardware will bring different consumptions for the very same applications, but the way in which those components are used (the logical part) is also relevant [23]. At the source level, energy-efficient data processing and storage algorithms must be adopted [24; 25; 26; 27], while, at the network level, communication protocols at different network layers work for transporting the information between the source and the sink nodes through (potentially) several different intermediate nodes, in the most efficient manner (considering nodes restrictions and the application nature) [28; 29; 30]. Besides, note that even different deployment choices will impact the operation of the WSN [31], and, as a consequence, the consumption of the batteries. This work investigates the problem of energy-aware WSN over NCS. Generally speaking, NCS are related to the use of communication channels shared by many interconnected units (so a network), with the purpose of closing one or several control loops [3]. This concept can be applied in a wide range of applications, such as in factory automation, tele-operation of robotic systems, health-care, smart cities, etc. The case of wireless NCS was already investigated in [32], identifying some associated problems such as packet loss and delay. The authors present a timing scheme and an adaptive control loop to cope with variations in network conditions. The reference wireless standard was IEEE 802.11b, which is designed to bring connectivity in Wireless Local Area Networks (WLAN) at data rates up to 11 Mbps. This standard is still used in laptops, cellphones and other mobile devices. The current paper is focused on WSN technologies which, as already mentioned, are most based on the IEEE 802.15.4 standard, focusing on low-rate data communications with low power consumptions. WSN-based NCS have been studied in many works. For instance, in [12], the authors propose a networked control system considering WSN issues. The controller is event-driven (event is a packet arrival) and includes a state predictor based on classical Extended Kalman Filter (EKF) to deal with delay. All the references mentioned in this paragraph consider WSN as responsible for producing data loss and delay, but the complexity of the network protocols and, particularly, the multi-hop case, are not considered. The investigation proposed in this paper treats various problems found in WSNs, including multi-hop transport of data and energy consumption. Even when not explicit in mathematical models, the packet loss is discussed in all related works as an inherent and relevant problem of WSN-based NCS. Among the different ways of considering packet loss, the most common ones can be classified as:  
* **Unreliable protocols and robust codification. ** In many cases, real-time requirements or resource limitations may force the use of unreliable protocols, i.e., protocols that do not assure the correct reception of all transmitted packets. In this case, the receiving application must be able to work with partial quantities of data. In many WSN-based applications, reliability is not a requirement because there exists a high correlation between the gathered data over time (or space). For instance, in weather monitoring applications, where sensors nodes send measuresthat do not present fast variations over time (thus losses can be easily estimated with the correctly received data). Another example is multimedia communication, where high correlations between a measure (e.g., a pixel on an image, or a level of sound) and the surrounding ones exist (on space or time).