Wireless Sensor Networks: Characteristics and Applications

With the development of sensor technology, embedded technology, distributed information processing technology, and wireless communication technology, wireless sensor networks (WSN), composed of a large number of micro sensor nodes with microprocessing capabilities, have gradually become a hot research topic.

Compared with traditional wireless communication networks like Ad Hoc networks, the characteristics of self-organization, dynamism, reliability, and data-centric focus of WSN allow them to be applied in areas inaccessible to people, such as battlefields and deserts. Therefore, it can be concluded that the future of wireless sensor networks will have broader prospects.

Wireless Sensor Networks (WSN) are a type of distributed sensing network formed by a large number of stationary or mobile sensors that create a wireless network in a self-organizing and multi-hop manner. They collaboratively sense, collect, process, and transmit information about the objects being sensed within a geographical area covered by the network and ultimately send this information to the network owner. The three elements of a wireless sensor network are sensors, sensed objects, and observers.

The numerous types of sensors in wireless sensor networks can detect a variety of phenomena in the surrounding environment, including earthquakes, electromagnetic fields, temperature, humidity, noise, light intensity, pressure, soil composition, and the size, speed, and direction of moving objects. Potential application areas can be summarized as: military, aviation, explosion-proof, disaster relief, environment, medical care, healthcare, home, industrial, and commercial fields.

Compared to traditional wired networks, wireless sensor network technology has significant advantages, with primary requirements including: low energy consumption, low cost, universality, network topology, security, real-time performance, and data-centricity.

Typical Structure of Wireless Sensor Network Systems

The typical structure of a wireless sensor network system for remote monitoring using a homogeneous network consists of four hardware components: sensor nodes, aggregation nodes, server-side PCs, and client-side PCs. The functions of each component are as follows.

Figure 1: Block Diagram of Remote Monitoring Wireless Sensor Network System Structure

Sensor Nodes

Deployed in the monitoring area (Zone A), they form a wireless network through self-organization. The data monitored by the sensor nodes is wirelessly transmitted along other nodes in a multi-hop manner, reaching the aggregation node (Zone B) after multiple hops.

Aggregation Nodes

This is a network coordinator responsible for constructing the wireless network, which transmits the information and data received from the sensor nodes wirelessly to the server-side PC via SCI (Serial Communication Interface).

Server-side PC

This is a management node located in Zone B and also serves as an independent Internet gateway node. On the LabVIEW software platform, there are two software components: one is the monitoring management software platform VI for the wireless sensor network, which is a virtual instrument VI for monitoring the sensor wireless network; the other is the Web Server software module and Remote Panel technology, which enables the connection between the sensor wireless network and the Internet.

Client-side PC

No software design is required on the client-side PC; users can access the front panel of the server PC’s wireless sensor network monitoring virtual instrument through a browser, enabling remote monitoring and management of the sensor wireless network (Zone A) from a different location (Zone C).

1. Sensors and Their Conditioning Circuits

Sensors should be selected based on the environmental characteristics of the area where the wireless sensor network is located to adapt to the specific requirements such as temperature range and size. The conditioning circuits connected to the sensors convert the output changes from the sensors into voltage signals compatible with A/D converters, typically ranging from 0 to 2.5 V or 0 to 5 V. When operating in areas without grid power supply, both the sensors and their conditioning circuits should be low power.

2. Data Acquisition and A/D Converters with Microprocessor Systems The computer system in the sensor node is a low-power microprocessor system designed to work under harsh conditions in remote areas far from the testing center. For example, the MSP430-F149A ultra-low-power mixed signal processor produced by Texas Instruments (TI) includes a built-in sample/hold circuit and a 12-bit A/D converter, capable of collecting and converting signals as well as controlling instructions and processing data for the entire node system.

3. RF Modules The RF module receives external wireless commands and wirelessly sends out the measured data detected by the sensors, such as the CC2420 wireless transceiver chip from TI.

4. Power Supply The power supply for sensor nodes in wireless sensor networks is a highly specialized and currently hot research topic. If nodes are located in remote areas away from the power grid, battery power or wireless RF power supply methods are generally used.

The aggregation node in the wireless sensor network shown in Figure 1 is a network coordinator that operates the panel controls of the monitoring management software platform on the PC, responsible for configuring and building the wireless sensor network under its instructions, and transmitting the data information received from the sensor nodes wirelessly to the PC. Typically, the coordinator consists of four main components: microprocessor system, RF module, communication interface, and power supply, as shown in the hardware composition block diagram.

Hardware Composition Block Diagram of Wireless Network Coordinator

1. Communication Interface The communication interface in the coordinator is responsible for communicating with the PC. On one hand, when the corresponding controls on the front panel of the wireless sensor network monitoring platform VI on the operating PC are activated, the communication interface transmits the corresponding commands such as network retrieval and data transmission; on the other hand, when the coordinator receives data information wirelessly sent from the sensor nodes, it also uploads this information to the PC via the communication interface.

2. Microprocessor System The microprocessor in the coordinator is the main controller of the entire wireless sensor network and serves as the core of the coordinator.

3. RF Module This RF module receives the data information wirelessly sent by the sensor nodes and uploads it to the PC via the communication interface; on the other hand, it sends operational commands to the sensor nodes using wireless transmission.

Wireless Sensor Network Communication Protocol

Currently, the IEEE 802.15 working group for wireless personal area networks has established the following standards:

Medium-speed wireless personal area network standard IEEE 802.15.1—Bluetooth;

High-speed wireless personal area network standard IEEE 802.15.3—Ultra-Wideband (UWB);

Low-speed wireless personal area network standard IEEE 802.15.4. Low-speed wireless personal area networks mainly provide simple low-cost network connections for wireless applications with limited power capabilities and low throughput requirements. The main goal is to construct a short-range wireless communication network that is simple, flexible, reliable in data transmission, extremely low in device costs, and minimal in energy consumption.

Low-speed wireless personal area networks meet the requirements of wireless sensor networks regarding low energy consumption, low cost, universality, network topology, security, real-time performance, and data-centricity. Therefore, the physical layer and MAC layer protocols of wireless sensor networks currently being researched and applied mostly use the IEEE 802.15.4 standard.

The network layer protocols based on the IEEE 802.15.4 standard mainly include the ZigBee protocol stack proposed by the ZigBee Alliance established in September 2001 and the embedded micro IPv6 protocol stack suitable for wireless sensor network nodes. Among them, the ZigBee protocol is widely researched and applied due to its low cost and compatibility with products from different manufacturers.

Interconnection of WSN Wireless Sensor Networks with the Internet

Homogeneous networks introduce one or more wireless sensor network sensor nodes as independent gateway nodes to connect to the Internet, placing the interface with the Internet’s standard IP protocol at the gateway nodes external to the wireless sensor network. This approach aligns well with the data flow patterns of wireless sensor networks, making management easier without requiring significant adjustments to the wireless sensor network itself; however, it may cause excessive energy consumption near the gateway and potentially lead to some degree of information redundancy.

The heterogeneous network is characterized by: some high-energy nodes being assigned IP addresses as interfaces with the Internet’s standard IP protocol. These high-capacity nodes can perform complex tasks and bear more loads, but defining what constitutes a ‘high capacity’ node is challenging. Additionally, facilitating communication between IP nodes through other ordinary nodes presents a technical challenge.

Characteristics and Advantages of WSN Wireless Sensor Networks

WSN cannot simply be understood as a network of multiple sensor nodes using wireless communication; it has its own characteristics and advantages.

1. Large Network Scale (Many Nodes)

For example: monitoring forest and grassland fire prevention, wildlife activity, and environmental conditions often requires deploying a large number of wireless sensor nodes, covering areas far exceeding those of typical local area networks.

The advantages of deploying a large number of wireless sensor nodes include:

(1) Improved overall monitoring accuracy

(2) Reduced precision requirements for individual nodes

(3) The presence of numerous redundant nodes provides the system with strong fault tolerance.

2. Self-Organizing Network

Unlike the deployment of local networks, the positions of wireless sensor nodes cannot be predetermined (e.g., deployed by aircraft or randomly placed by personnel), and the neighbor relationships between nodes cannot be established beforehand.

This requires wireless sensor nodes to have self-organizing capabilities, enabling automatic configuration management. The method to achieve this is through topology control mechanisms and network routing protocols that automatically form a multi-hop wireless network system capable of forwarding data.

3. Dynamic Network

The topology of wireless sensor networks frequently changes. Reasons include:

(1) Passive changes: sensor nodes running out of power; environmental changes causing communication failures; sensor nodes failing.

(2) Active changes: adding new nodes; changes made based on routing algorithm optimizations.

4. Strong Reliability

(1) Reliable hardware structure of sensor nodes

During deployment: may be deployed by aircraft, randomly placed by personnel

During operation: exposed to wind, sunlight, rain, extreme cold, and heat.

Maintenance: extremely difficult (nearly impossible).

(2) Reliable network structure

Self-organizing and dynamic characteristics ensure basic information transmission operates normally.

(3) Reliable software

(4) Strong information confidentiality

5. Data-Centric

In the Internet, terminals, hosts, routers, and servers all have their own IP addresses. To access resources on the Internet, one must first know the IP address of the server storing the resources. Thus, the Internet is an address-centric network. In contrast, wireless sensor networks are task-oriented networks.

In WSN, although nodes also have identifiers, whether these identifiers are uniform across the entire WSN depends on specific needs. Moreover, there is no inherent relationship between node identifiers and their physical locations. When users query events in WSN, they report the events of interest to the entire network rather than to a specific node. Often, the focus is on the data results rather than which node generated the data.

WSN utilizes micro sensor nodes to collect information, with nodes having self-organizing and collaborative working capabilities. The network employs wireless multi-hop communication, offering the following advantages over traditional sensor networks (SN):

1. High Accuracy: Achieves dense spatial sampling and close-range monitoring that a single sensor cannot.

2. High Flexibility: Once deployed, no human intervention is required.

3. High Reliability: Avoids single-point failure issues.

4. High Cost-Effectiveness: Reduces wired transmission costs, and with technological advancements, sensor costs have decreased.

WSN Application Areas

Due to the particularity of WSN, its application areas are significantly different from those of ordinary networks, mainly including the following categories:

(1) Military Applications

WSN can be rapidly deployed, self-organized, and has strong concealment and high fault tolerance, making it widely applicable on the battlefield.

Includes: monitoring enemy forces and weapons, real-time battlefield surveillance, target location and locking, and battle outcome assessment, etc.

(2) Emergency and Temporary Situations

After a natural disaster, fixed communication network facilities may be completely destroyed or unable to function normally. In remote or secluded areas, or natural reserves where vegetation cannot be disturbed, it is impossible to use fixed or pre-set network devices for communication. These situations can be addressed using the rapid deployment and self-organization features of WSN.

(3) Environmental Monitoring

For example: monitoring irrigation conditions in farmland, soil composition, environmental pollution, forest fire alarms, water level monitoring, temperature monitoring, and data collection on lighting duration, etc.

(4) Medical Care

Includes: collection of patients’ physiological data, management of medical equipment, distribution of medications, and tracking and locating key personnel, etc.

(5) Smart Homes

Embedding sensor nodes of WSN in home appliances and connecting them to the Internet can provide a more comfortable, convenient, and user-friendly home environment.

(6) Factory Monitoring

For instance: chemical, petroleum, electricity, machinery processing, textile dyeing, and other industries can easily implement monitoring using WSN technology.

Issues and Research Hotspots in WSN Applications

In the design and application process of wireless sensor networks, various foundational technologies are critical to supporting sensor networks in completing tasks. Addressing these key technologies is essential for ensuring the normal operation of network user functions.

(1) Network Protocols

In the research of network protocols for wireless sensor networks, MAC protocols and routing protocols are the focus of study.

Commonly used MAC protocols include: IEEE802.15.4, S-MAC, and T-MAC protocols; routing protocols include: SPIN, DO, GEM, LEACH, and others.

(2) Time Synchronization

In WSN, sensor nodes often need to cooperate to complete complex detection and sensing tasks, which requires maintaining time consistency among nodes to facilitate time-related operations; some energy-saving schemes in WSN also achieve their goals through time synchronization.

Currently, the more mature time synchronization protocols applied in WSN include RBS (Reference Broadcast Synchronization), Tiny/mini-Sync (Tiny/Mini Synchronization), and TPSN (Timing-Sync Protocol for Sensor Networks).

(3) Localization Technology

In WSN, the detection data obtained by sensor nodes is generally associated with location, and users are interested in knowing from which location the received data is sent. In some applications, measuring functionality requires coordination among sensors located at different spatial positions. Therefore, localization technology in WSN includes both node localization and target localization.

Localization technology can utilize existing GPS and other positioning technologies or adopt suitable and effective localization algorithms based on the characteristics of WSN. The main algorithms currently include DV2hop algorithm, location distribution algorithm, DV2distance algorithm, etc.

(4) Data Fusion

Due to various limitations of WSN, there is a need to process monitoring data to meet user demands, saving communication bandwidth and energy while improving information collection efficiency. From an energy consumption perspective, communication energy consumption between nodes is much higher than that of computational processing.

Currently, there are many methods for data fusion, commonly used ones include: average method, Kalman filter method, Bayesian method, neural network method, statistical decision theory, fuzzy logic method, production rules, and D-S evidence theory.

(5) Energy Management

Charging and replacing batteries in WSN nodes is difficult. Therefore, energy efficiency is crucial in design, making energy management an important research topic in WSN.

Currently, the main energy management strategies include sleep mechanisms and data fusion, mainly applied in the computing, storage, and communication units. Sleep mechanisms can be achieved through various measures, including corresponding hardware chips, network protocol coordination, dynamic power management, and dynamic voltage scheduling.

(6) Security Management

In WSN, security management mainly involves communication security and information security. Communication security considers node security, passive resistance to intrusions, and active countermeasures against intrusions, while information security focuses on data confidentiality, data authentication, data integrity, and timeliness.

Currently, the security research content in WSN mainly includes: a) efficient encryption algorithms at the physical layer, spread spectrum anti-jamming, etc.; b) secure MAC protocols at the data link layer; c) secure routing protocols at the network layer; d) key management and secure multicast at the application layer. Currently, dedicated security protocols in WSN include: SNEP (Secure Network Encryption Protocol) and uTESLA (Micro-Timed Efficient Stream Loss-Tolerant Authentication Protocol).