A wireless sensor network is a fully distributed system with no central nodes. Through random delivery, many sensor nodes are densely deployed in the monitoring area. These sensor nodes integrate sensors, data processing units, and communication modules that are connected by wireless channels to form a network system.
The sensor node measures the heat, infrared, sonar, radar and seismic signals in the surrounding environment by means of its built-in variety of sensors, including temperature, humidity, noise, light intensity, pressure, soil composition, size of moving objects, Many physical phenomena of interest, such as speed and direction. The sensor nodes have good cooperation ability to complete global tasks through local data exchange.
Due to the node characteristics of the sensor network, multi-hop and peer-to-peer communication methods are more suitable for wireless sensor networks than traditional single-hop and master-slave communication methods, and can also effectively avoid encountering during long-distance wireless signal propagation. Signal fading and interference issues. Through the gateway, the sensor network can also be connected to the existing network infrastructure to pass back the collected information to remote end users. Wireless sensor networks have the following characteristics.
(1) Large-scale network
In order to obtain accurate information, a large number of sensor nodes are usually deployed in the monitoring area, and the number of sensor nodes may reach tens of thousands or even more.
The large-scale nature of sensor networks includes two aspects:
On the one hand, sensor nodes are distributed in a large geographical area. For example, in the original large forest, sensor networks are used for forest fire prevention and environmental monitoring, and a large number of sensor nodes need to be deployed;
On the other hand, sensor nodes are densely deployed, and a large number of sensor nodes are densely deployed in a space that is not very large. The large-scale nature of the sensor network has the following advantages: the information obtained through different spatial perspectives has a larger signal-to-noise ratio; the distributed processing of a large amount of collected information can improve the accuracy of monitoring and reduce the accuracy requirements for a single node sensor; The existence of a large number of redundant nodes makes the system have strong fault tolerance performance; a large number of nodes can increase the coverage of the monitored area and reduce the blind area.
(2) Self-organizing network
In sensor network applications, sensor nodes are typically placed where there is no infrastructure. The position of the sensor nodes cannot be accurately set in advance, and the mutual neighbor relationship between the nodes is not known in advance, such as spreading a large number of sensor nodes through an aircraft to a vast original forest, or randomly placing them in an unreachable or dangerous area.
In this way, the sensor node is required to have self-organizing capability, can be automatically configured and managed, and automatically form a multi-hop wireless network system for forwarding monitoring data through a topology control mechanism and a network protocol. During the use of the sensor network, some sensor nodes fail due to energy exhaustion or environmental factors, and some nodes are added to the network to compensate for the failed nodes and increase the monitoring accuracy, so that the number of nodes in the sensor network is dynamic. The ground is increased or decreased, so that the topology of the network changes dynamically. The self-organization of the sensor network should be able to adapt to the dynamic changes of this network topology.
(3) Multi-hop routing
The communication distance of nodes in the network is limited, generally in the range of tens to hundreds of meters, and the node can only communicate directly with its neighbors. If you want to communicate with nodes outside of its RF coverage, you need to route through the intermediate nodes. The multi-hop routing of the network is implemented by using gateways and routers, and the multi-hop routing in the wireless sensor network is completed by ordinary network nodes, and there is no special routing device. In this way, each node can be either the initiator of the information or the forwarder of the information.
(4) Dynamic network
The topology of the sensor network may change due to the following factors:
1 The sensor node fails or fails due to environmental factors or exhausted power;
2 changes in environmental conditions may cause changes in the bandwidth of the wireless communication link, even when the time is off;
3 Sensors, sensing objects and observers of the sensor network may be mobile;
4 new nodes are added. This requires the sensor network system to be able to adapt to this change, with dynamic system reconfigurability.
(5) Reliable network sensor networks are particularly suitable for deployment in harsh environments or areas unsuitable for humans. Sensor nodes may be exposed to open air, exposed to the sun or wind and rain, or even destroyed by unrelated people or animals. Sensor nodes are often deployed in random, such as by aircraft flying or launching "cannonballs" to a designated area for deployment.
These require the sensor nodes to be very rugged, not easily damaged, and to adapt to a variety of harsh environmental conditions. Due to the limitations of the monitoring area environment and the large number of sensor nodes, it is impossible to manually “care†each sensor node, and the maintenance of the network is very difficult or even impossible to maintain. The confidentiality and security of the communication of the sensor network is also very important. It is necessary to prevent the monitoring data from being stolen and the forged monitoring information. Therefore, the hardware and software of the sensor network must be robust and fault tolerant.
(6) Data-centric network
The sensor network is a mission-based network, and there is no point in talking about sensor nodes out of the sensor network. The nodes in the sensor network adopt the node number flag. Whether the node number needs the whole network or not depends on the design of the network communication protocol. Since the sensor makes the point randomly deployed, the relationship between the constructed sensor network and the node number is completely dynamic, and the node number is not necessarily related to the node position. When a user queries a event using a sensor network, the event of interest is directly advertised to the network instead of being advertised to a node with a certain number. The network reports to the user after obtaining the information of the specified event.
This idea of ​​using the data itself as a query or transmission clue is closer to the habit of natural language communication. So usually the sensor network is a data-centric network. For example, in a sensor network applied to target tracking, the tracking target may appear anywhere, and the user interested in the target only cares about the location and time of the target, and does not care which node monitors the target. In fact, in the process of moving the target, it is necessary to provide the location message of the target by different nodes.
(7) Application related network
The sensor network is used to perceive the objective physical world and acquire the amount of information in the physical world. The physical quantity of the objective world is diverse and cannot be exhausted. Different sensor network applications are concerned with different physical quantities, so there are various requirements for the application system of the sensor. Different application backgrounds have different requirements for sensor networks, and their hardware platforms, software systems, and network protocols are bound to vary greatly. Therefore, the sensor network cannot have a unified communication protocol platform like the Internet.
Although there are some common problems for different sensor network applications, in the development of sensor network applications, the differences in sensor networks are more concerned. Only by bringing the system closer to the application can the most effective target system be made. Sensor network technology is studied for each specific application, which is a distinguishing feature of sensor network design from traditional networks.
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