Wireless Sensor Networks

Wireless sensor networks provide bridges between the virtual world of information technology and the real physical world. They represent a fundamental paradigm shift from traditional inter-human personal communications to autonomous inter-device communications. They promise unprecedented new abilities to observe and understand large-scale, real-world phenomena at a fine spatio-temporal resolution. As a result, wireless sensor networks also have the potential to engender new breakthrough scientific advances.

Networked wireless sensor devices

As shown in Figure 1.2, there are several key components that make up a typical wireless sensor network (WSN) device:

1. Low-power embedded processor: The computational tasks on a WSN device include the processing of both locally sensed information as well as information communicated by other sensors. At present, primarily due to economic

Figure 1.2 Schematic of a basic wireless sensor network device

constraints, the embedded processors are often significantly constrained in terms of computational power (e.g., many of the devices used currently in research and development have only an eight-bit 16-MHz processor). Due to the constraints of such processors, devices typically run specialized component-based embedded operating systems, such as TinyOS. However, it should be kept in mind that a sensor network may be heterogeneous and include at least some nodes with significantly greater computational power.
Moreover, given Moore’s law, future WSN devices may possess extremely powerful embedded processors. They will also incorporate advanced low-power design techniques, such as efficient sleep modes and dynamic voltage scaling to provide significant energy savings.

2. Memory/storage: Storage in the form of random access and read-only memory includes both program memory (from which instructions are executed by the processor), and data memory (for storing raw and processed sensor measurements and other local information). The quantities of memory and storage on board a WSN device are often limited primarily by economic considerations, and are also likely to improve over time.

3. Radio transceiver: WSN devices include a low-rate, short-range wireless radio (10–100 kbps, <100 m). While currently quite limited in capability too, these radios are likely to improve in sophistication over time – including improvements in cost, spectral efficiency, tunability, and immunity to noise, fading, and interference. Radio communication is often the most power-intensive operation in a WSN device, and hence the radio must incorporate energy-efficient sleep and wake-up modes.

4. Sensors: Due to bandwidth and power constraints, WSN devices primarily support only low-data-rate sensing. Many applications call for multi-modal sensing, so each device may have several sensors on board. The specific sensors used are highly dependent on the application; for example, they may include temperature sensors, light sensors, humidity sensors, pressure sensors, accelerometers, magnetometers, chemical sensors, acoustic sensors, or even low-resolution imagers.

5. Geopositioning system: In many WSN applications, it is important for all sensor measurements to be location stamped. The simplest way to obtain positioning is to pre-configure sensor locations at deployment, but this may only be feasible in limited deployments. Particularly for outdoor operations, when the network is deployed in an ad hoc manner, such information is most easily obtained via satellite-based GPS. However, even in such applications, only a fraction of the nodes may be equipped with GPS capability, due to environmental and economic constraints. In this case, other nodes must obtain their locations indirectly through network localization algorithms.

6. Power source: For flexible deployment the WSN device is likely to be battery powered (e.g. using LiMH AA batteries). While some of the nodes may be wired to a continuous power source in some applications, and energy harvesting techniques may provide a degree of energy renewal in some cases, the finite battery energy is likely to be the most critical resource bottleneck in most WSN applications.

Depending on the application, WSN devices can be networked together in a number of ways. In basic data-gathering applications, for instance, there is a node referred to as the sink to which all data from source sensor nodes are directed. The simplest logical topology for communication of gathered data is a single-hop star topology, where all nodes send their data directly to the sink. In networks with lower transmit power settings or where nodes are deployed over a large area, a multi-hop tree structure may be used for data-gathering. In this case, some nodes may act both as sources themselves, as well as routers for other sources.

One interesting characteristic of wireless sensor networks is that they often allow for the possibility of intelligent in-network processing. Intermediate nodes along the path do not act merely as packet forwarders, but may also examine and process the content of the packets going through them. This is often done for the purpose of data compression or for signal processing to improve the quality of the collected information.