## 2 Peer-to-Peer Approach for Decentralized MG's Control  
One of the key points for integrating DERs into microgrids is the design of a control architecture that coordinates each one of the DER units. Traditionally, the control tasks are carried out by means of a three level hierarchical structure [27; 28; 29; 30; 31], i.e., primary, secondary and tertiary control. The Field Level (Primary Control) is implemented in local DER. It is responsible for stable operation conditions in each DER. The Management Level (Secondary Control) is controlled by a Microgrid Central Controller (MGCC), which ensures the synchronization between the MG and the main grid or the quality of the frequency and voltage of MG, among other tasks. Finally, Grid Level (Tertiary Control) carries out the management of MG when it operates in the market providing an economically optimal operation of the microgrid. Primary, Secondary and Tertiary levels can be combined or not with communication systems. However, to evolve towards the development of the intelligent microgrid, communication interfaces among the different control levels should be created. Thus, a bi-directional communication and channels are stablished in order to allow information transfer between the different controllers [32] for knowing the state of the whole system. Today's microgrid installations use centralized architectures to implement the three level hierarchical scheme and data transfers between entities. These centralized architectures where a central controller take decisions and provides communications between all microgrid resources have been implemented, for many years, by Supervisory Control and Data Acquisition (SCADA) systems [33]. In addition to the SCADA systems, other alternative technologies also used for control and management of centralized microgrids are Power Line Carrier (PLC) and Wireless Networks (WNs). PLC technologies use the electric power lines as a medium that enables the bidirectional data exchange. This provides a vast coverage and in terms of infrastructure is the most cost-effective technology since the lines already exist. However, PLC technology has negative effects in the communication channel such as a noisy medium, distortion, frequency impedance alterations and the risk of signal attenuation [34]. Besides, wireless technologies have emerged as an alternative for centralized wired technologies due to their low cost, low power, flexibility and easy deployment [35]. Some studies about wireless networks in power systems for monitoring and control of segments in power transmission and delivery have been presented in [36; 37; 38; 39; 35]. However, the use of wireless technologies in power system environments presents a number of challenges such as reliability concerns, wireless signal disruption due to electromagnetic interference (EMI), faded signals due to large transmission distances or obstacles in the line-of-sight; overloading of bandwidth; quality or latencies degradation, among others [35; 40; 41]. In Information and Communication Technology (ICT) terminology, these centralized structures (see Figure 2a) are based on client-server communications architectures where every device of the network plays one of two roles: server or client. The server is the central point that receives status information from the whole unit and it is able to calculate an optimal global control strategy. Besides, the client shares its resources via direct links to the server. This control structure causes inefficiencies in the communication system of the microgrid, which are provoked by several causes. On one hand, a failure in the centralized control point could lead to several faults or even the shutdown of the entire system. In addition, there are difficulties in managing a wide range of devices in real time [42; 43], because the system is hardly scalable. As a result, these disadvantages can lead to provide poor services, bottlenecks or under-utilization of the network resources.