Wireless sensor networks

What is a sensor network?

Sensor networks can continuously or periodically monitor many physical conditions. Some examples: air temperature humidity and pressure, levels of solar ultraviolet and ionising radiation, sound levels, weight/pressure, wind speed and direction, acceleration and vibration, electrical current, soil temperature and moisture content, proximity of objects by ultrasound and infrared, location by GPS, the levels of specific gasses, and the presence of Bluetooth and Wi-Fi devices. Sensors are connected by a data network to computers that can record their information, enable it to be examined at any location, draw attention to events, and initiate automatic action on them. They all include some amount of wired data network, but wireless sensor networks use less and have distinct characteristics that may make them more suitable in some situations.

Why use a sensor network?

Most systems can be made more effective by knowing extra information about how they are working, and often more effective still by knowing it in real-time. Commonly this resolves to issues of time, materials, problems and hazards. For example, saving time by quickly finding portable assets and key personnel required at various points in a process, or monitoring materials and items used in a manufacturing process, or detecting potentially harmful substances in the environment. As the ability of sensor networks increases and their prices fall so they are becoming useful in more scenarios.

Why use a wireless sensor network?

Recent advances in technology, particularly wireless transceivers batteries solar cells and power usage, have made wireless sensor networks useful in more situations. Relative to wired sensor networks, wireless sensors have the advantage of requiring less wired network infrastructure, saving time money disruption and risk in installation. Some can relay information from other more remote sensors to the computers, thereby further reducing infrastructure requirements. Some can operate for long periods on battery power, or indefinitely on solar power. Some can even scavenge the power they require from other environmental conditions such as temperature differences and movement. Collectively these properties make them quick and easy to deploy, particularly for ad hoc requirements and rapid response to events. Wireless sensors networks may be more practical or the only viable solution in some situations, such as measuring conditions at multiple locations distributed across large indoor and outdoor sites, for portable and mobile use, and in environments that are difficult to wire or tend to degrade wiring.

How does a wireless sensor network interact with other wireless networks?

Sensors have been developed to use a variety of proprietary wireless networking protocols, and at least four different wireless networking specifications are based on the IEEE 802.15.4 standard published by the IEEE 802.15 WPAN Task Group 4. One wireless networking specification gaining traction currently is Zigbee. In the UK Zigbee is allowed to work in two frequency ranges. One is the same range as used by most Wi-Fi equipment and many other devices, specifically it uses 2.4 GHz to 2.483.5 GHz divided into 16 channels. It is capable of data rates up to 250 kbps. The other is from 868.0 MHz to 868.6 MHz divided into 3 channels and capable of data rates up to 100 kbps. The latter may suffer less from interference and potentially has a greater range. DASH7 is a Zigbee alternative that is based on the ISO/IEC 18000-7 standard. It operates at 433.92 MHz and has greater range, lower power requirements, and a data rate up to 200kbps.

How reliable are wireless sensor networks?

Some wireless networks like Zigbee support a mesh network structure, where each device in the network can forward information to multiple other devices. Consequently, as with the design of the Internet, data can take multiple routes through the network making its delivery more reliable. However in some environments radio waves can suffer from interference. Interference from metal, water, and other transmitters is not a problem with the Rubee protocol (IEEE 1902.1 and 1902.2) which modulates the magnetic component of its electromagnetic carrier wave, although Rubee is currently less well developed.