# A Comprehensive Review of Sensor-Based Smart Building Monitoring and Data Gathering Techniques
## 2 Smart Building Monitoring and Control System Architecture  
In advancing building management technologies, the deployment of building automation and control systems signifies a crucial evolution in the administrative and operational oversight of facility infrastructures. These systems encompass both hardware and software components designed to facilitate the automated control and monitoring of diverse building systems.  
This section outlines the technical architecture of BACSs, emphasizing their role in enhancing connectivity for efficient communication, decision making, and automated control. The BACS architecture consists of three layers: management, automation, and field, as illustrated in Figure 1.  
The management layer includes the human-machine interface, which is usually integrated into the organization's enterprise network. This layer includes operator and monitoring units, along with additional peripheral computing devices, all connected to a central data processing system. It manages the data from across the automation layer to facilitate decision making and reporting. Multiple autonomous systems can be integrated to enhance the human interface for monitoring and management purposes. The adoption of a three-layer architecture reduces the network traffic on the management level; however, the system scale should be considered. For example, the separation of the networks may be an expensive solution for smaller systems [10].  
The automation (supervisory) layer serves as the main hub for communication and control. The automation layer includes both hardware and software components, with the software representing the messages or signals that convey information about the device's status [12]. Connectivity is established through various communication networks that link and integrate numerous discrete devices. Some of the most widely used BACS automation protocols include BACnet, LonWoks, KNX, and Modbus [10].  
Figure 1: Three-layer BACS architecture [10].  
The field level involves physical input sensors, actuators, and output activators. It encompasses application-specific controllers that are connected to the plant and equipment for monitoring and controlling the environment. The field devices are distributed throughout the building, providing monitoring and control functions. They are installed throughout the BACS to continuously measure and monitor physical parameters [10; 12].  
Sensors detect and respond to environmental changes, revealing the value of a physical property either directly or indirectly. Subsequently, the data generated are eventually collected, transmitted, assessed, and stored for further analysis [13; 14]. Different systems and applications utilize various types and quantities of sensors to gather data; however, sensor fusion technology can integrate sensory data from diverse sources [15]. We will briefly describe some of the common sensor fusion techniques in Section 5.  
Integrating several subsystems within a building's automation framework, regardless of their proprietary protocols or manufacturers, requires a supervisory level. In three-layer BACS architecture, it is referred to as a neutral supervisor (Figure 1). A neutral supervisor operates above the automation layer and is designed to be vendor-neutral, ensuring interoperability among different systems and devices [12]. This integrated architecture forms the backbone for achieving optimal operational efficiency and precision in building management.