Wireless sensor network is a fully distributed system no central node. Served by random manner, many sensor nodes are densely deployed in the monitored area. The sensor-integrated sensor nodes, the data processing unit and a communication module, which is connected through a wireless channel, constitutes self-organizing network system.
Sensor nodes by means of its built-in a variety of sensors to measure where the surrounding environment of heat, infrared, sonar, radar and seismic signals, including temperature, humidity, noise, light intensity, pressure, soil composition, the size of the moving object, speed and direction, and many people are interested in physical phenomena. Between the sensor nodes have a good collaboration, through local data exchange to complete the overall task.
Due to the characteristics of the sensor network node requirements, multi-hop, peer communication than the traditional single-hop, master-slave communication method is more suitable for wireless sensor networks, but can also effectively prevent long-range wireless signal propagation encountered in the process signal fading and interference problems. Through the gateway, sensor networks can also connect to the existing network infrastructure, so that the collected information to a remote loopback end users. Wireless sensor networks have the following characteristics.
(1) large-scale networks
In order to obtain accurate information in the monitoring area typically deploy a large number of sensor nodes, the number of sensor nodes may reach hundreds of thousands, or even more.
Of large-scale sensor networks include two aspects:
On the one hand it is the sensor nodes are distributed over a large geographic area, such as the use of sensor networks in the primeval forests of forest fire prevention and environmental monitoring, the need to deploy a large number of sensor nodes;
On the other hand, the sensor nodes are densely deployed in the area is not a lot of space, a large number of densely deployed sensor nodes. Of large-scale sensor networks have the following advantages: information obtained through different spatial perspective has a greater signal to noise ratio; through distributed processing large amounts of information collected can improve the accuracy of monitoring and reduce the accuracy of the single sensor node requirements; there are a lot of redundant nodes, the system has strong fault tolerance; a large number of nodes can be increased monitoring of the area covered, reduce blind spots.
(2) self-organizing network
In the sensor network applications, sensor nodes are typically placed in the absence of local infrastructure. Position sensor nodes can not be precisely set in advance, the mutual relationship between the neighbor nodes do not know in advance, such as by a large number of sensor nodes seeding aircraft to the vast area of ​​virgin forest, or randomly placed into one area inaccessible or dangerous.
This requires that the sensor node has the ability to self-organization, can automatically configure and manage, automatically form a multi-hop wireless network system monitoring data forwarding through topology control mechanisms and network protocols. During use sensor networks, in part because the sensor node energy depletion or environmental factors that cause failure, there are some nodes in order to make up for the failed node to increase monitoring accuracy and added to the network, so that the number of nodes in a sensor network on the dynamic increased or decreased, so that the resulting network topology changes dynamically. Self-organizing sensor networks to be able to adapt to the dynamic changes of this kind of network topology.
(3) multi-hop routing
The limited distance communications network node, typically in the range of tens to hundreds of meters, a node can only communicate directly with its neighbors. If a node outside its radio coverage wish to communicate, they need to be routed through intermediate nodes. The use of multi-hop routing network gateways and routers to achieve, and wireless sensor networks in multi-hop routing is completed by a common network node, there is no specialized routing equipment. So that each node can be either the originator of information can also be forwarded by the information.
(4) dynamic network
Sensor network topology may change because of the following factors:
Sensor nodes â‘ energy depletion caused by environmental factors or a malfunction or failure;
â‘¡ changes in environmental conditions may cause the wireless communication link bandwidth changes, and even when broken through;
â‘¢ sensor network sensor sensing the object and the observer these three elements are likely to have mobility;
â‘£ new node is added. This requires the sensor network system to be able to adapt to these changes, with dynamic system reconfiguration.
(5) reliable network sensor networks particularly suitable for deployment in harsh environments or humans should not reach the area, the sensor nodes can work in the open air, subject to exposure to the sun or wind and rain, even the destruction of unrelated persons or animals. Sensor nodes are often random deployment, such as seeding or deployed by aircraft to launch the “shells” to the designated area.
These sensor nodes require very strong, difficult to damage, to adapt to harsh environmental conditions. Due to the huge number of restrictions and monitoring of regional environmental sensor nodes, can not artificial “care” to each of the sensor nodes, network maintenance is difficult or can not be maintained. Confidentiality of communications and security sensor networks is also very important to prevent the monitoring data being stolen and forged to obtain monitoring information. Therefore, the sensor network hardware and software must have robustness and fault tolerance.
(6) data-centric networks
Sensor network is mission-based network, from sensor networks sensor nodes makes no sense to talk about. Sensor network node in the node number using signs, node numbers need to design the entire network depends on the network communication protocol. Since the sensor so Random deployment of sensor networks and the relationship formed between the node number is completely dynamic, showing the node number and the node position are not necessarily linked. When the user query sensor network events directly to the event of interest to the network through the report, rather than give a notice to determine the number of nodes. After obtaining the information specified network event reporting to the user.
Such as a query to the data itself or transmit thought cue closer natural language communication habits. It is usually said sensor network is a data-centric networks. For example, when applied to target tracking sensor networks, tracking targets may appear anywhere on the target of interest only concern the location and time of the target appears, do not care which node monitored targets. In fact, in the process of moving the target, the target must be provided by a different node position message.
(7) application related network
Sensor network for sensing objective physical world, the physical world to obtain information. Objective physical world more diverse, inexhaustible. Different sensor network applications concern different quantities, therefore the application of the sensor system and there are many diverse kinds of requirements. Background different applications require different sensor networks, hardware platforms, software systems and network protocols are bound to have a big difference. Therefore, the sensor network is not the same as the Internet, a unified communications protocol platform.
For different sensor network applications, although there are some common problems, but in the development of sensor network applications, more concerned about the difference in sensor networks. Only the system closer to the application, in order to make the most efficient Gao target system. For each specific application to study sensor network technology, which is different from the traditional network sensor networks salient features.
