What Is a Wireless Sensor Network?

Introduction

A wireless sensor network (WSN) is a wireless network consisting of spatially distributed autonomous devices that use sensors to monitor physical or environmental conditions. These autonomous devices, or nodes, combine with routers and a gateway to create a typical WSN system. The distributed measurement nodes communicate wirelessly to a central gateway, which provides a connection to the wired world where you can collect, process, analyze, and present your measurement data. To extend distance and reliability in a wireless sensor network, you can use routers to gain an additional communication link between end nodes and the gateway. 

National Instruments Wireless Sensor Networks offer reliable, low-power measurement nodes that operate for up to three years on 4 AA batteries and can be deployed for long-term, remote operation. The NI WSN protocol based on IEEE 802.15.4 and ZigBee technology provides a low-power communication standard that offers mesh routing capabilities to extend network distance and reliability. The wireless protocol you select for your network depends on your application requirements. To learn more about other wireless technologies for your application, read the “Selecting the Right Wireless Technology” white paper.

WSN Applications

Embedded monitoring covers a large range of application areas, including those in which power or infrastructure limitations make a wired solution costly, challenging, or even impossible. You can position wireless sensor networks alongside wired systems to create a complete wired and wireless measurement and control system.

A WSN system is ideal for an application like environmental monitoring in which the requirements mandate a long-term deployed solution to acquire water, soil, or climate measurements. For utilities such as the electricity grid, streetlights, and water municipals, wireless sensors offer a lower-cost method for collecting system health data to reduce energy usage and better manage resources. In structural health monitoring, you can use wireless sensors to effectively monitor highways, bridges, and tunnels. You also can deploy these systems to continually monitor office buildings, hospitals, airports, factories, power plants, or production facilities. 

WSN System Architecture

In a common WSN architecture, the measurement nodes are deployed to acquire measurements such as temperature, voltage, or even dissolved oxygen. The nodes are part of a wireless network administered by the gateway, which governs network aspects such as client authentication and data security. The gateway collects the measurement data from each node and sends it over a wired connection, typically Ethernet, to a host controller. There, software such as the NI LabVIEW graphical development environment can perform advanced processing and analysis and present your data in a fashion that meets your needs. 

Figure 2. Common Wireless Sensor Network Architecture

Power and Network Standards

A WSN measurement node contains several components including the radio, battery, microcontroller, analog circuit, and sensor interface. In battery-powered systems, you must make important trade-offs because higher data rates and more frequent radio use consume more power. Today, battery and power management technologies are constantly evolving due to extensive research.

Often in WSN applications, three years of battery life is a requirement, so many of the WSN systems today are based on ZigBee or IEEE 802.15.4 protocols due to their low-power consumption. The IEEE 802.15.4 protocol defines the Physical and Medium Access Control layers in the networking model, providing communication in the 868 to 915 MHz and 2.4 GHz ISM bands, and data rates up to 250 kb/s. ZigBee builds on the 802.15.4 layers to provide security, reliability through mesh networking topologies, and interoperability with other devices and standards. ZigBee also allows user-defined application objects, or profiles, which provide customization and flexibility within the protocol.

In addition to long-life requirements, you must consider the size, weight, and availability of batteries as well as international standards for shipping batteries. The low cost and wide availability of carbon zinc and alkaline batteries make them a common choice. Energy harvesting techniques are also becoming more prevalent in wireless sensor networks. With devices that use solar cells or collect heat from their environment, you can reduce or even eliminate the need for battery power. 

Processor Trends

To extend battery life, a WSN node periodically wakes up to acquire and transmit data by powering on the radio and then powering it back off to conserve energy. The WSN radio must efficiently transmit a signal and allow the system to go back to sleep with minimal power use. Likewise, the processor must also be able to wake, power up, and return to sleep mode efficiently. Microprocessor technology trends for WSNs include reducing power consumption while maintaining or increasing processor speed. Much like your radio choice, the power consumption and processing speed trade-off is a key concern when selecting a processor for WSNs. This makes PowerPC and ARM-based architectures a difficult option for battery-powered devices. A more common architecture option includes the TI MSP430 MCU, which is designed for low-power operation. Depending on the specific processor, power consumption in sleep mode can range from 1 to 50 µW, while in on-mode the consumption can range from 8 to 500 mW.

Networking Topologies

You can use several network topologies to coordinate the WSN gateway, end nodes, and router nodes. Router nodes are similar to end nodes in that they can acquire measurement data, but you also can use them to pass along measurement data from other nodes. The first, and most basic, is the star topology, in which each node maintains a single, direct communication path with the gateway. This topology is simple but restricts the overall distance that your network can achieve.

To increase the distance a network can cover, you can implement a cluster, or tree, topology. In this more complex architecture, each node still maintains a single communication path to the gateway but can use other nodes to route its data along that path. This topology suffers from a problem, however. If a router node goes down, all the nodes that depend on that router node also lose their communication paths to the gateway.

The mesh network topology remedies this issue by using redundant communication paths to increase system reliability. In a mesh network, nodes maintain multiple communication paths back to the gateway, so that if one router node goes down, the network automatically reroutes the data through a different path. The mesh topology, while very reliable, does suffer from an increase in network latency because data must make multiple hops before arriving at the gateway.

Figure 3. WSN Network Topologies 

The NI Wireless Sensor Network Advantage

With the National Instruments WSN platform, you can customize and enhance a typical WSN architecture to create a complete wired and wireless measurement system for your application. NI software integration provides the flexibility to choose a Windows-based host controller for your WSN system or a real-time controller such as NI CompactRIO, giving you the ability to integrate reconfigurable I/O with your wireless measurements. With either host controller, you can use LabVIEW and the NI-WSN software with LabVIEW project integration and drag-and-drop programming to easily configure your WSN system, extract high-quality measurement data, perform analysis, and present your data.

In addition, LabVIEW integration delivers the ability to stretch the connectivity of your WSN from the enterprise and database level all the way through the Internet to end client devices, such as an iPhone or laptop. You can use this complete system architecture to acquire data from virtually anywhere with an NI wireless sensor network, process and host that data on a server, and then access the data conveniently and remotely from a wireless smart device. To learn more about the options in WSN measurement system architectures, visit the NI WSN Measurement Systemswhite paper.