Sensor nodes utilize their built-in diverse sensors to measure thermal, infrared, sonar, radar, and seismic wave signals in the surrounding environment, including temperature, humidity, noise, light intensity, pressure, soil composition, and the size, speed, and direction of moving objects, among many other physical phenomena of interest. The sensor nodes exhibit good collaboration capabilities, completing global tasks through local data exchange. Due to the requirements of the characteristics of sensor network nodes, multi-hop, peer-to-peer communication methods are more suitable for wireless sensor networks than traditional single-hop, master-slave communication methods, effectively avoiding issues such as signal fading and interference encountered during long-distance wireless signal propagation. Through gateways, sensor networks can also connect to existing network infrastructures, allowing the collected information to be relayed back to remote end users. Wireless sensor networks have the following characteristics. (1) Large-scale Network To obtain accurate information, a large number of sensor nodes are usually deployed in the monitoring area, with the number of sensor nodes potentially reaching thousands or even more. The largeness of sensor networks includes two aspects: One aspect is that sensor nodes are distributed over a large geographic area, such as deploying sensor networks for forest fire prevention and environmental monitoring in virgin forests, requiring the deployment of a large number of sensor nodes; The other aspect is that sensor nodes are densely deployed, with a large number of sensor nodes densely placed in a relatively small area. The largeness of sensor networks has the following advantages: information obtained from different spatial perspectives has a greater signal-to-noise ratio; distributed processing of a large amount of collected information can improve monitoring accuracy and reduce the accuracy requirements for individual sensor nodes; the presence of a large number of redundant nodes gives the system strong fault tolerance; a large number of nodes can increase the coverage of the monitoring area and reduce blind spots. (2) Self-organizing Network In sensor network applications, sensor nodes are typically placed in areas without infrastructure. The exact positions of sensor nodes cannot be predetermined, nor are the mutual neighbor relationships between nodes known in advance, such as when a large number of sensor nodes are scattered over a vast virgin forest by aircraft or randomly placed in inaccessible or hazardous areas. This requires sensor nodes to have self-organizing capabilities, allowing them to automatically configure and manage themselves, forming a multi-hop wireless network system for forwarding monitoring data through topology control mechanisms and network protocols. During the use of sensor networks, some sensor nodes may fail due to energy depletion or environmental factors, and some nodes may be added to the network to compensate for failed nodes and enhance monitoring accuracy, resulting in a dynamic increase or decrease in the number of nodes in the sensor network, thereby dynamically changing the network’s topology. The self-organizing nature of sensor networks must adapt to these dynamic changes in network topology. (3) Multi-hop Routing The communication distance between nodes in the network is limited, typically ranging from several dozen to several hundred meters, and nodes can only communicate directly with their neighbors. If communication is desired with nodes outside their radio frequency coverage, routing through intermediate nodes is required. Multi-hop routing in the network is implemented using gateways and routers, while multi-hop routing in wireless sensor networks is completed by ordinary network nodes without dedicated routing devices. Thus, each node can act as both an initiator of information and a forwarder of information. (4) Dynamic Network The topology of sensor networks may change due to the following factors: ① Sensor nodes may fail or become non-functional due to environmental factors or energy depletion; ② Changes in environmental conditions may cause variations in the bandwidth of wireless communication links, leading to intermittent connectivity; ③ The three elements of sensor networks—the sensors, the sensed objects, and the observers—may all have mobility; ④ The addition of new nodes. This requires sensor network systems to be able to adapt to such changes, possessing dynamic system reconfigurability. (5) Reliable Network Sensor networks are particularly suitable for deployment in harsh environments or areas inaccessible to humans, where sensor nodes may operate outdoors, subjected to sunlight, wind, and rain, or even vandalism by unrelated persons or animals. Sensor nodes are often deployed randomly, such as through aerial scattering or launching “shells” into designated areas. All these require sensor nodes to be very robust, not easily damaged, and adaptable to various harsh environmental conditions. Due to the limitations of the monitored area environment and the vast number of sensor nodes, it is impossible to manually “care for” each sensor node, making network maintenance very challenging or even unmanageable. The confidentiality and security of communication in sensor networks are also very important, requiring prevention against data theft and the acquisition of counterfeit monitoring information. Therefore, the hardware and software of sensor networks must possess robustness and fault tolerance. (6) Data-Centric Network Sensor networks are task-oriented networks; discussing sensor nodes outside of sensor networks is meaningless. Nodes in sensor networks use node numbering for identification, and whether node numbering needs to be globally unique depends on the design of the network communication protocol. Since sensor nodes are randomly deployed, the relationship between the constructed sensor network and node numbering is entirely dynamic, with node numbering having no necessary connection to node location. When users query events using the sensor network, they directly notify the network of the events they are concerned about, rather than notifying a specific node with a defined number. The network reports back to the user after acquiring information about the specified event. This data-centric approach to querying or transmitting clues is more aligned with the habits of natural language communication. Therefore, it is often said that sensor networks are data-centric networks. For example, in sensor networks applied to target tracking, the tracked target may appear anywhere, and users interested in the target only care about the location and time of its appearance, not which node detected the target. In fact, during the movement of the target, it is inevitable that different nodes will provide the location information of the target. (7) Application-Specific Network Sensor networks are used to perceive the objective physical world, obtaining information about the physical world. The physical quantities of the objective world are diverse and inexhaustible. Different sensor network applications focus on different physical quantities, leading to various requirements for sensor application systems. Different application contexts impose different requirements on sensor networks, resulting in significant differences in their hardware platforms, software systems, and network protocols. Thus, sensor networks cannot have a unified communication protocol platform like the Internet. Although there are some common issues among different sensor network applications, the development of sensor network applications is more concerned with the differences in sensor networks. Only by making the system closer to the application can the most efficient target system be created. Researching sensor network technology for each specific application is a significant feature that distinguishes sensor network design from traditional networks.
