*4.3. Interoperability Agents*

The main function of interoperability agents is to ensure the communication between the MAS platform and the physical hardware. The communication is based on the data-centric publish-subscribe (DCPS) model.

There are two main distributed communication models that could be implemented in MG context: Data-centric communications and object-centric communication model. In data-centric communications, data distribution between communicating entities is the fundamental concept. This model is based on data publishers and data subscribers. Publishers pass data of known types to subscribers through a common channel. In the opposite, in object oriented communications, instead of focusing on the data, the main concern is the interface between entities. An interface comprises of a list of predefined methods ready to be used. An object oriented system is based on server and client interface communication. The client interface invokes methods on a named server interface, which the server responds accordingly [41]. Even though the two approaches are complementary, middleware built on data-centric communication are more competitive in the RT applications.

RTI DDS Connect was implemented as middleware for RT interoperability between the agent context and the hardware of the testbed. As explained earlier, data-centric communication is based on data sharing between publishers and subscribers. The RTI DDS Connext allows to user to create publishers and subscribers independently of the programming language. Publishers and subscribers are connected by 'Topics.' A Topic corresponds to a single data type that allows identifying which data is being to be shared between which publishers and subscribers [42]. For instance, if publisher A is publishing value 'x' in Topic 1, subscribers who want to get the 'x' value should connect to the same Topic 1.

In this work, a publisher for each generation units and controllable load were implemented in the MAS context. The MGO computes the references and sends them to the corresponding publisher agents. Once the data is published, the subscribers implemented in the SCADA system of the testbed read the data and operate accordingly. Figure 5 illustrates how the agents implemented in the agent context publish data in RTI DDS topics, and how each generation unit reads the published data through subscribers.

**Figure 5.** Interoperability Communication Setup.
