A State-Based Programming Model for Wireless Sensor Networks

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22 Chapter 2. Wireless Sensor Networks power but also high-bandwidth Bluetooth radio. The ZebraNet [67] sensor node deploys a short-range and a long-range radio. Optical Communication. Light as a communication medium has radically different characteristics. One significant drawback is that because of the mostly directional propagation of light, optical communication requires a line of sight. Also, ambient light can interfere with the signal and may lead to poor link quality. Therefore optical communication may be suitable only for specific applications. However, it has several benefits over RF communication. Because of lower path loss it is more suitable for long-range communications. Also, light signals do not require sophisticated modulation and demodulation, thus optical transceivers can be realized with less chip complexity and are more energy efficient compared to radio transceivers. Also, they can be built very small. At the University of California at Berkeley, the Smart Dust [68, 120] project examines the use of laser light for communication in their Smart Dust sensor nodes. The project aims to explore the limits of sensor-node miniaturization by packing all required subsystems (including sensors, power supply, and wireless communication) into a 1 mm 3 node (see Fig. 2.2 for a conceptual diagram). Optical communication has been chosen to avoid the comparatively large dimensions for antennas. A receiver for optical signals basically consists of a simple photodiode. It allows the reception of data modulated onto a laser beam emitted by another node or a base station transceiver (BST). For data transmissions, two schemes are being explored in Smart Dust: active and passive transmission. For active transmission, the sensor node generatesr a lase beam with a laser diode. The beam is then modulated using a MEMS steerable mirror. For passive transmission, the node modulates an unmodulated laser beam with a so-called corner-cube retroreflector (CCR). The CCR is a MEMS structure of perpendicular mirrors, one of which is deflectable. Incident light is reflected back to the light source, unless deflected. Several prototypes of the optical transceiver system and entire Smart Dust sensor nodes have been built. We will present one of them in the following section. Sensors, Sensor Boards and Sensor Interface Application-specific sensor-node designs typically include all or most sensors. General-purpose nodes, on the other hand, include only few or no sensors at all, but provide a versatile external interface instead. This approach is beneficial since the required sensor configurations strongly depend on the target application. The external interface allows to attach various sensors or actuators directly or to attach a preconfigured sensor board. The sensors most commonly found in sensor node and sensor board designs are for visible light, infrared, audio, pressure, temperature, acceleration, position (e.g., GPS). Less typical sensor types include hygrometers, barometers, magnetometers, oxygen saturation sensors, and heart-rate sensors. Simple analog sensors are sampled by the processor via an analog-to-digital converter (ADC). More sophisticated sensors have digital interfaces, such as serial ports (e.g., USARTs) or bus systems (e.g., I2C). Over this digital interface a proprietary application protocol is run for sensor configuration, calibration and sampling. The processor of the sensor node typically implements these protocols in software. Actuators found in WSN include LEDs,
2.3. Sensor Nodes 23 Sensors Mirrors Analog I/O, DSP, Control Passive Transmitter with Corner-Cube Retroreflector 1-2mm Interrogating Laser Beam Laser Lens Mirror Active Transmitter with Beam Steering Power Capacitor Solar Cell Thick-Film Battery Incoming Laser Communication Photodetector Photodetector and and Receiver Receiver Figure 2.2: Smart Dust conceptual diagram (from [119]). buzzers, and small speakers. 2.3.3 Selected Sensor-Node Hardware Platforms Today there are a number of sensor nodes available for use in WSN research and development. Some designs have been commercialized and are available even in large numbers. Others designs have been used in limited numbers only in research projects. Still other designs are alpha versions with just a few prototypes built or which exist only in hardware simulators. Depending on their intended application scenarios, sensor nodes have to meet vastly different (physical) application requirements, like sensor configurations and durability. However, we focus mainly on the characteristics that are relevant for developing and running software. In the following we present a few selected sensor nodes which represent various design points and different stages of maturity. An overview of available sensor node platforms can be found in [58] and in [111]. BTnodes The BTnode is an autonomous wireless communication and computing platform based on a Bluetooth radio module and a microcontroller. The design has undergone two major revisions from the initial prototype. The latest generation uses a new Bluetooth subsystem and adds a second low-power radio, which is identical to those used in Berkeley Motes (see below). For a detailed description of all generations of BTnodes refer to [20].
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A State-Based Programming Model for Wireless Sensor Networks

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