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1. Differences in perceived nodes of different Internet of Things applications are very large. A sensing node can be as small as an object that is barely visible to the naked eye, or it can be a large building; it can be a very small chip or it can be A desktop computer-sized intelligent measurement and control device; it can be fixed or mobile; it can be alive or inanimate; it can be human or animal. Some nodes can sense physical parameters such as temperature, humidity, sound, and pressure in the physical world. Some nodes can sense chemical parameters such as oxygen, carbon dioxide, and other chemical components. Some nodes can sense location information, and some nodes can serve as objects. Identity tag. All sensor nodes in the Internet of Things have a common feature, that is, they are installed with a sensor chip or device, and have the ability to automatically sense external environment changes and communication capabilities. However, the perception nodes of different Internet of Things applications are very different. Figure 2-1 shows different types of IoT sensing nodes.
2. Sensing and controlling capabilities of sensor nodes People describe RFID as a technology that allows objects to “openâ€. The RFID tag stores the information of the object and transmits its stored data to the RFID application system through the wireless channel. General sensors only have the ability to sense the parameters of the surrounding environment. For example, in an environmental monitoring system, a temperature sensor can transmit its measured ambient temperature in real time, but it has no control over ambient temperature. The designer of a fixed-site water sensor node in a precision agricultural IoT application system hopes that it can automatically turn on the switch and water the fruit trees or vegetables when the monitored soil moisture is below a certain set value. Perceived nodes also have control capabilities. In the application system for emergency handling of IoT emergencies, the robot for nuclear leakage on-site treatment can enter a designated location according to the instruction, and the related nuclear leakage related parameters are measured by the sensor and transmitted to the command center. The robot opens a switch or closes a switch according to the command of the command center. From this example, it can be seen that as a sensor node with intelligent processing capabilities, it must have both sensing and control capabilities, and at the same time have the ability to adapt to the surrounding environment. Therefore, from a simple RFID tag chip, a temperature sensor or a measurement and control device, to a complex intelligent robot, the most important difference between them is: Does the sensing node need to have both the sensing ability and the control and execution capability? What kind of control and execution capabilities are needed?
3. Discussion on the Development of Perceptual Layer Technology When we discuss Perceptual Layer technology, we need to pay attention to two issues:
First, because some real-world IoT application systems require the sensor nodes at the end to have the ability to sense and control at the same time, some technical materials also call the perceptual layer the "perceptual control layer." This book uses a general representation method, referred to as the "perceptual layer."
Second, the Internet of Things that we currently discuss mainly involves the application of RFID and sensors with large-scale and low-cost, which is very natural and necessary during the first phase of the development of the Internet of Things. However, as an information technology researcher, we cannot but notice the development of intelligent robot technology that countries around the world are researching vigorously and the application of intelligent robots in military, disaster prevention, disaster relief, security, aerospace and other special fields. Intelligent robots have a strong ability to sense the external environment, adaptive and collaborative capabilities, and intelligent processing capabilities. At present, the research of robot clusters that control a large number of intelligent robots in cooperation through the network is demonstrating step by step that it can more effectively expand the application prospects of human beings' ability to sense the world and handle problems intelligently. When smart robot technology matures and is applied, it will inevitably enter the Internet of Things and become an important member of the Internet of Things. Therefore, in discussing the characteristics of the perception layer, we must anticipate this problem in a forward-looking manner. 
Understanding the characteristics of the sensory and sensory nodes requires attention to the following issues.