# SEIRD COVID-19 Formal Characterization and Model Comparison Validation
## 3 SDL Conceptual Model  
Specification and Description Language (SDL) is an object-oriented, formal language that was defined by the International Telecommunication Union-Telecommunication Standardization Sector (ITU-T), The document that defines the language structure is the Recommendation Z.100 [8]. Originally, the language was designed to specify event-driven, real-time, complex interactive applications that involve many concurrent activities. The mechanisms used to perform the communication between the different model elements are discrete signals [9; 10]. In addition, the use of SDL to perform the conceptualization of the simulation model allows the extension of the model to be able to integrate CA [11; 12] or MAS [13] simulation paradigms, combining them in a single model, or connecting the model with other simulation models or simulators. It also has strong capabilities to integrate IoT [14], which will be interesting in future developments of this research to interact with different sensors. Moreover, the language can be transformed into other well-known formal languages, like DEVS [15], allowing the common formalism approach [16] for complex simulation. The conceptualization of a simulation model needs to define the next components: (i) Structure: system, blocks, processes, and the hierarchy of processes. (ii) Communication: signals, including the parameters and channels that the signals use to travel. (iii) Behavior: defined through the processes. (iv) Data: based on Abstract Data Types (ADT). (v) Inheritances: describe the relationships between and the specializations of the model elements. At least the first two-components, the structure, and the behavior must be described in a formal language to be able to do a correct conceptualization of a simulation model. The language has four levels: (i) system, (ii) blocks, (iii) processes, and (iv) procedures. The hierarchical decomposition of SDL is shown in Figure 4. Regarding the notation we will use to describe the SDL diagrams, every time we refer to an SDL element, we write it all in CAPS, while the name of the elements will be in _Italics_. A SYSTEM diagram represents the uppermost level of the structure of the model, and the communications between them, the channels. Since the SYSTEM is the outermost agent, it may communicate with the environment. The SYSTEM, in SDL terminology, is an AGENT but is not the only AGENT we can find, the BLOCK and the PROCESS that we will find in the lower levels are also AGENTS. Figure 3: Preliminary values for the System Dynamics SEIRD model. Both bidirectional and unidirectional channels are allowed in SDL. The communication channels are joined to the objects through ports. Ports are important elements because they guarantee the independence of the different AGENTS we will use in our models. This is important to allow simulation objects, AGENTS, reuse. An object only knows its ports, which are the doors through which it communicates with its environment. An AGENT only knows that it sends and receives events using a specific port. SYSTEM diagrams and BLOCK diagrams represent the model structure, the hierarchical decompositions of the different model elements; some good examples can be reviewed in [10]. However, a PROCESS diagram defines the behavior of the model. When an AGENT PROCESS receives a specific SIGNAL, as a trigger, a set of actions are executed in no time. The time is related to the SIGNAL as a delay needed to travel to another AGENT. A PROCESS diagram uses different graphical elements to represent its behavior. In the next lines, we describe synthetically some of the more important elements to simplify the understanding of the posterior pandemic model definition. Start. This element defines the initial condition for a PROCESS diagram. State. The _state_ element only contains the name of a state, defining the states of behavioral diagrams, such as PROCESS diagrams. Input. Input elements describe the type of events that can be received by the process.