Network everything. That seems to be the trend in wireless as in all other communications technologies. It’s difficult to identify any segment of electronics today that isn’t networked.
Local-area networks (LANs), personal-area networks (PANs), metro-area networks (MANs), wide-area networks (WANs), the Internet, and the forthcoming Smart Grid all envelop us. And now a newer form of network is finally being widely deployed: the wireless sensor network (WSN) or, more precisely, wireless sensor and actuator networks (WSANs).
Both have been discussed extensively over the years and have been the subject of intensive research and development in university, military, and other research labs around the globe. It’s only now that we’re beginning to see the many useful possibilities, especially for the home-area network (HAN) that is going to be the core of the coming Smart Grid rollout.
WSANs DEFINED
A WSAN is a network infrastructure that can sense its environment and react to specific conditions of interest. It can monitor and control its environment within its design capability. In many cases, it also can be set up to do some amount of relevant computing.
Many, if not most, WSANs are sense-only networks. As a result, they’re called WSNs since they don’t involve controlling functions within the environment. Some organizations refer to WSANs as wireless data acquisition or wireless telemetry. In these traditional functions, a major consideration is the recording and storage of the collected data along with some analysis and display.
The network is made up of miniaturized nodes that consist of a sensor and its related signal conditioning circuitry, a radio transceiver, some memory, and an embedded controller. The battery-powered unit is designed for very low power consumption. These nodes can communicate with a central master control point or with one another.
A central controller or master node with more extensive computing capability collects the information gathered by the sensors and passes it along to some data center, usually through the connection to some other network like a company LAN or the Internet. The nodes are usually stationary but could be mobile. They also could be location-aware.
The nodes can monitor any physical characteristic for which an electronic sensor has been developed. The most common sensors are for temperature, pressure, light, sound, motion, humidity, and pollutants. Some WSNs can accommodate video input. As for control, the actuators may be lights, motors, fans, valves, relays, solenoids, pumps, appliances, or any other electromechanical device.
A primary consideration of any WSAN is network topology. The two most widely used topologies are the star and mesh. The star network (Fig. 1a), also called multipoint-to-point (MPP), has a central master control node with computing power with multiple nodes. The nodes only talk to the controller rather than to one another.
In the mesh network (Fig. 1b), the nodes communicate with one another and offer a multi-hop capability back to a central collection point. In the mesh topology, the nodes report the status of their own sensors and act as relay points that simply retransmit the data from nearby nodes.
The method allows sensors to be spread over a wider range than the single-node range. It also provides a form of network reliability. If a node’s battery dies or its signals are blocked, the network automatically and dynamically reroutes the data through other adjacent nodes. WSANs can use other hybrid forms of network topologies as required as well. These may be a mix of tree, star, or mesh.
THE HARDWARE AND SOFTWARE
The main hardware element is the node. Nodes also are known as “motes,” a mote being a tiny particle, such as dust. The sometimes stated goal of WSNs is to make the nodes that small. Nodes as small as a dime or quarter are fairly common, but that’s about as small as they get today.
The node’s basic architecture (Fig. 2) has an embedded controller and memory at its core. The controller hosts a small operating system that runs the networking software and manages the I/O (see “Interfacing The Sensor”). The sensor, its signal conditioning, and the analog-to-digital converter (ADC) comprise another major section, while the radio transceiver with its antenna form yet another. In some cases, there may be multiple sensors and related circuitry.
An essential part of the node is the power-management portion. The power source is a battery, of course, but power management is critical to long battery life. Some of this control may be handled by the MCU.
The software consists of a small specialized operating system (OS) and all the related drivers and applications programs. More than a dozen OSs are associated with WSANs. A popular one is TinyOS and its related programming language called network embedded system C (nesC), an extension to C. TinyOS is an event-driven OS that calls event drivers for specific tasks as opposed to a threading OS. Other software is related to the sensor such as the communications media access controller (MAC), the protocol and networking functions, and any application software that performs related data manipulation.
RADIO TECHNOLOGY AND STANDARDS
Many existing wireless networking technologies are suitable for use in WSANs (see “Important Wireless Facts To Keep In Mind”). The most widely used are IEEE 802.15.4, ZigBee, Bluetooth, Z-Wave, and 802.11 Wi-Fi. There are also other proprietary technologies including RFID.
If any one technology dominates the WSAN arena, it’s IEEE 802.15.4 and the enhanced version known as ZigBee. The IEEE standard defines the physical layer (PHY) and MAC layer of the system while ZigBee adds the upper network and applications layers. This wireless technology is based on direct-sequence spread-spectrum (DSSS) and uses the carrier sense multiple access with collision avoidance (CSMA/CA) channel access method.
The standard defines several different modulation methods based on phase-shift keying (PSK). It also defines three primary operating bands using unlicensed spectrum. First is the 868.3-MHz frequency in which a maximum data rate of 20 kbits/s can be achieved with raised-cosine binary phase-shift keying (BPSK) modulation. The maximum range is about 1 km. This version is used primarily in Europe.