## 3 Future Directions of MES Development
### Overview of MES
#### 3.2.2 MES Standardization  
Different organizations related to MES define and categorize the functionalities of MES into the so-called MES functions, which, in turn, are standardized by different organizations, such as the Manufacturing Enterprise Solutions Association (MESA), the International Society of Automation (ISA), or the Verein Deutscher Ingenieure (VDI) [37]. The MESA defined \(11\) functions that MES should contain, such as resource allocation and status, operation/detail scheduling, dispatching production units, document control, data collection/acquisition, labor management, quality management, process management, maintenance management, product tracing and genealogy, and performance analysis [38]. MESA created the list in the late 1990s and updated it along the following decades as a union of all the functionalities that MES may have. However, a factory does not have to implement all the 11 functions to be optimally functional. In fact, the implementation of MES functions depends on several factors such as the industry type or the automation level in the factory [39]. In May 2004, to meet the needs of the enterprise collaborative manufacturing strategy, MESA released the \"MESA Next Generation Collaborative Manufacturing Execution System Model\" white paper (MESA International, 2004) that formally proposed the concept of c-MES and the c-MES function model, which contained eight modules: production planning and scheduling, data acquisition, process management, quality management, human resource management, product tracking and inventory, performance analysis and resource allocation, and status monitoring [40]. As an increasing number of software systems (sensor, actuator, SCADA, PLC, ERP, MES, PLM, etc.) were used in the manufacturing industry, the integration of these systems became an issue that needed to be solved. Likewise, new emerging technologies such as IoT, CPS, and cloud also need to be connected and integrated. From 2000 to 2013, ISA published five parts of the ISA-95 standard for the integration of enterprise control. Four control levels were defined in the ISA-95 standard [41]:  
* Level 0 and 1, the actual physical processes and their sensing and actuation;
* Level 2, manufacturing processes, especially SCADA, programmable logic controller (PLC), and distributed control system (DCS);
* Level 3, systems which manage the batch workflow, continuous or discrete production operations, and MES;
* Level 4: business planning and logistics systems that manage the business-related activities of production. MES fills this gap in the automation chain, serving as a mediator or central layer integrating higher and lower levels of the ISA-95 automation pyramid [39]. Having emerged from the process industry [36], MES has become a commonly used approach for the integration of manufacturing planning, which is level 4, and control tasks, which are level 2 in the ISA-95 automation pyramid, and it has been applied in various types of industry. Surveys show the significant increase in the number of vendors and the variety of industrial applications [42]. Because of the software heterogeneity in the manufacturing environment, to ensure a smooth information flow within the enterprise, the adoption of communication interfaces among different software systems is the primary reason limiting the MES implementation, integration, and maintenance [34]. Next, we conducted a comparative analysis in Table 1 on the types of MES systems applied to various industries through literature review. The MES system for application to various industries such as automotive parts, mold, electronic, jewelry, and solar industry was presented, and a number of examples targeting SMEs were presented. In addition, we analyzed that the main function of MES was selectively implemented functions suitable for the industry and process among the 11 functions presented by MESA. Furthermore, we found that there were many MES systems using RFID as a lot tracking method, and some methods using barcodes were also suggested.