# A Methodology for the Design of Application-Specific Cyber-Physical Social Sensing Co-Simulators
## 2 State of the Art
### Social, Physical and Network (Cyber) Simulation  
Although systems including five different domains in CPSS have been reported [1], typically they only include three subsystems: physical world, cyber world and social world [2]. Thus, in the context of CPSS simulation, basically three types of proposals should be taken into account: the physical processes simulators, the social simulators and the IT simulators. The **physical processes simulators** are based on mathematical engines which calculate numerically the evolution of the system integrating the differential equations which describe the system's behavior [63]. These simulators are the oldest [69; 70] as they were extremely used in manufacturing, avionics or metal factories. Nowadays, it is difficult to find research works on this topic, as a great variety of powerful commercial products may be found. Simulink [71], MapleSim [72], OpenModelica [73], Wolfram SystemModeler [74] or xcos [75] are some of the most famous and used examples. All these solutions provide a graphic interface and a programming environment which recovers the mathematical details and enable high-level developers work comfortably. In this area, current research works try to include some aspects of the virtual world in the physical simulations. These solutions are known as mixed simulation [76]. However, most of these proposals are focused on sanitary scenarios and are difficultly extended to other applications, as specific knowledge and configurations is required. On the other hand, first CPS (Cyber-Physical Systems [77]) simulators have been proposed [78; 79; 80]. In the final design, these simulators will include the physical and the cyber world; however, nowadays they only consider the physical world and some characteristics about the cyber world. Furthermore, they cannot recover the mathematical details, so experience in numerical integration is required in order to correct manage the simulations. In both cases, mixed and CPS simulations, the techniques used in order to include the cyber world in the physical one consist of deep modifications in the numerical algorithms and models. Moreover, in general, physical processes simulators integrate the dynamical models for total amount of simulated time and, once obtained the results (consisting of one or various discrete signals); they are showed in the graphic environment or returned in the method callback. Table 2 compares all the simulators described in respect with the most important characteristics. It is important to note that other types of simulator may be found. For example, very recently various cyber physical systems modeling frameworks aiming at modeling different physiological processes have been proposed [81; 82]. Moreover, prior works in Cyber Physical systems which aimed at modeling stochastic variables with long tails which are more adequate to model human behavior have also appear [23]. **Social simulation** studies the interaction among social entities taking into account their psychology and their behavior, their interactions with other entities in the environment and the behavior of all entities as a group [83]. Mainly, there are two different types of social simulations. First, social-level simulations analyze situations, usually a society, as a whole, including its evolution when a change happens. Second, agent-based simulations study each person in a more detailed manner, designing a specific model for each agent (for example, the movement becomes an important factor in the simulation). In CPSS environments, agent-based simulations are preferable, as may include factors such as the movement of the individuals. On the contrary, social-level simulations are not suitable at all as important characteristics of people (movement, etc.) cannot be considered. In agent-based simulations every element in the simulation is modeled as an agent, a unit that acts on their own, without external direction, in response to situations they encounter during the simulation [84].