15-03-2011, 10:10 AM
Introduction to Wireless Sensor Networks
Sensors integrated into structures, machinery, and the environment, coupled with the effi cient deliveryof sensed information, could provide tremendous benefi ts to society. Potential benefi ts include: fewercatastrophic failures, conservation of natural resources, improved manufacturing productivity, improvedemergency response, and enhanced homeland security [1]. However, barriers to the widespreaduse of sensors in structures and machines remain. Bundles of lead wires and fi ber optic “tails” aresubject to breakage and connector failures. Long wire bundles represent a signifi cant installation andlong term maintenance cost, limiting the number of sensors that may be deployed, and therefore reducingthe overall quality of the data reported. Wireless sensing networks can eliminate these costs, easinginstallation and eliminating connectors.The ideal wireless sensor is networked and scaleable, consumes very little power, is smart and softwareprogrammable, capable of fast data acquisition, reliable and accurate over the long term, costs little topurchase and install, and requires no real maintenance.Selecting the optimum sensors and wireless communications page link requires knowledge of theapplication and problem defi nition. Battery life, sensor update rates, and size are all major design considerations.Examples of low data rate sensors include temperature, humidity, and peak strain capturedpassively. Examples of high data rate sensors include strain, acceleration, and vibration.Recent advances have resulted in the ability to integrate sensors, radio communications, and digitalelectronics into a single integrated circuit (IC) package. This capability is enabling networks of verylow cost sensors that are able to communicate with each other using low power wireless data routingprotocols. A wireless sensor network (WSN) generally consists of a basestation (or “gateway”) thatcan communicate with a number of wireless sensors via a radio link. Data is collected at the wirelesssensor node, compressed, and transmitted to the gateway directly or, if required, uses other wirelesssensor nodes to forward data to the gateway. The transmitted data is then presented to the system by thegateway connection. The purpose of this chapter is to provide a brief technical introduction to wirelesssensor networks and present a few applications in which wireless sensor networks are enabling.
Individual Wireless Sensor Node Architecture
A functional block diagram of a versatile wireless sensing node is provided in Figure 22.2.1. Amodular design approach provides a fl exible and versatile platform to address the needs of a widevariety of applications [2]. For example, depending on the sensors to be deployed, the signal conditioningblock can be re-programmed or replaced. This allows for a wide variety of different sensors to beused with the wireless sensing node. Similarly, the radio page link may be swapped out as required for agiven applications’ wireless range requirement and the need for bidirectional communications. Theuse of fl ash memory allows the remote nodes to acquire data on command from a basestation, or by anevent sensed by one or more inputs to the node. Furthermore, the embedded fi rmware can be upgradedthrough the wireless network in the fi eld.The microprocessor has a number of functions including:1) managing data collection from the sensors2) performing power management functions3) interfacing the sensor data to the physical radio layer4) managing the radio network protocolA key feature of any wireless sensing node is to minimize the power consumed by the system. Generally,the radio subsystem requires the largest amount of power. Therefore, it is advantageous to send dataover the radio network only when required. This sensor event-driven data collection model requires analgorithm to be loaded into the node to determine when to send data based on the sensed event. Additionally,it is important to minimize the power consumed by the sensor itself. Therefore, the hardwareshould be designed to allow the microprocessor to judiciously control power to the radio, sensor, andsensor signal conditioner.
Wireless Sensor Networks Architecture
There are a number of different topologies for radio communications networks. A brief discussion ofthe network topologies that apply to wireless sensor networks are outlined below.
Star Network (Single Point-to-Multipoint)
A star network (Figure 22.3.1) is a communications topology where a single basestation can sendand/or receive a message to a number of remote nodes. The remote nodes can only send or receive amessage from the single basestation, they are not permittedto send messages to each other. The advantage ofthis type of network for wireless sensor networks is inits simplicity and the ability to keep the remote node’spower consumption to a minimum. It also allows for lowlatency communications between the remote node andthe basestation. The disadvantage of such a network isthat the basestation must be within radio transmissionrange of all the individual nodes and is not as robust asother networks due to its dependency on a single node tomanage the network.
Mesh Network
A mesh network allows for any node in the network totransmit to any other node in the network that is withinits radio transmission range. This allows for what isknown as multihop communications; that is, if a nodewants to send a message to another node that is out of radiocommunications range, it can use an intermediate nodeto forward the message to the desired node. This networktopology has the advantage of redundancy and scalability.If an individual node fails, a remote node still can communicateto any other node in its range, which in turn, canforward the message to the desired location. In addition,the range of the network is not necessarily limited by therange in between single nodes, it can simply be extendedby adding more nodes to the system. The disadvantageof this type of network is in power consumption for thenodes that implement the multihop communications aregenerally higher than for the nodes that don’t have this capability,often limiting the battery life. Additionally, as thenumber of communication hops to a destination increases,the time to deliver the message also increases, especially iflow power operation of the nodes is a requirement
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http://googleurl?sa=t&source=web&cd=4&ve...ter-22.pdf&ei=vex-Td3oHMrirAfwtvmzBw&usg=AFQjCNHKimcgrjQosbKP6cO7UeufLsQQ3w
Sensors integrated into structures, machinery, and the environment, coupled with the effi cient deliveryof sensed information, could provide tremendous benefi ts to society. Potential benefi ts include: fewercatastrophic failures, conservation of natural resources, improved manufacturing productivity, improvedemergency response, and enhanced homeland security [1]. However, barriers to the widespreaduse of sensors in structures and machines remain. Bundles of lead wires and fi ber optic “tails” aresubject to breakage and connector failures. Long wire bundles represent a signifi cant installation andlong term maintenance cost, limiting the number of sensors that may be deployed, and therefore reducingthe overall quality of the data reported. Wireless sensing networks can eliminate these costs, easinginstallation and eliminating connectors.The ideal wireless sensor is networked and scaleable, consumes very little power, is smart and softwareprogrammable, capable of fast data acquisition, reliable and accurate over the long term, costs little topurchase and install, and requires no real maintenance.Selecting the optimum sensors and wireless communications page link requires knowledge of theapplication and problem defi nition. Battery life, sensor update rates, and size are all major design considerations.Examples of low data rate sensors include temperature, humidity, and peak strain capturedpassively. Examples of high data rate sensors include strain, acceleration, and vibration.Recent advances have resulted in the ability to integrate sensors, radio communications, and digitalelectronics into a single integrated circuit (IC) package. This capability is enabling networks of verylow cost sensors that are able to communicate with each other using low power wireless data routingprotocols. A wireless sensor network (WSN) generally consists of a basestation (or “gateway”) thatcan communicate with a number of wireless sensors via a radio link. Data is collected at the wirelesssensor node, compressed, and transmitted to the gateway directly or, if required, uses other wirelesssensor nodes to forward data to the gateway. The transmitted data is then presented to the system by thegateway connection. The purpose of this chapter is to provide a brief technical introduction to wirelesssensor networks and present a few applications in which wireless sensor networks are enabling.
Individual Wireless Sensor Node Architecture
A functional block diagram of a versatile wireless sensing node is provided in Figure 22.2.1. Amodular design approach provides a fl exible and versatile platform to address the needs of a widevariety of applications [2]. For example, depending on the sensors to be deployed, the signal conditioningblock can be re-programmed or replaced. This allows for a wide variety of different sensors to beused with the wireless sensing node. Similarly, the radio page link may be swapped out as required for agiven applications’ wireless range requirement and the need for bidirectional communications. Theuse of fl ash memory allows the remote nodes to acquire data on command from a basestation, or by anevent sensed by one or more inputs to the node. Furthermore, the embedded fi rmware can be upgradedthrough the wireless network in the fi eld.The microprocessor has a number of functions including:1) managing data collection from the sensors2) performing power management functions3) interfacing the sensor data to the physical radio layer4) managing the radio network protocolA key feature of any wireless sensing node is to minimize the power consumed by the system. Generally,the radio subsystem requires the largest amount of power. Therefore, it is advantageous to send dataover the radio network only when required. This sensor event-driven data collection model requires analgorithm to be loaded into the node to determine when to send data based on the sensed event. Additionally,it is important to minimize the power consumed by the sensor itself. Therefore, the hardwareshould be designed to allow the microprocessor to judiciously control power to the radio, sensor, andsensor signal conditioner.
Wireless Sensor Networks Architecture
There are a number of different topologies for radio communications networks. A brief discussion ofthe network topologies that apply to wireless sensor networks are outlined below.
Star Network (Single Point-to-Multipoint)
A star network (Figure 22.3.1) is a communications topology where a single basestation can sendand/or receive a message to a number of remote nodes. The remote nodes can only send or receive amessage from the single basestation, they are not permittedto send messages to each other. The advantage ofthis type of network for wireless sensor networks is inits simplicity and the ability to keep the remote node’spower consumption to a minimum. It also allows for lowlatency communications between the remote node andthe basestation. The disadvantage of such a network isthat the basestation must be within radio transmissionrange of all the individual nodes and is not as robust asother networks due to its dependency on a single node tomanage the network.
Mesh Network
A mesh network allows for any node in the network totransmit to any other node in the network that is withinits radio transmission range. This allows for what isknown as multihop communications; that is, if a nodewants to send a message to another node that is out of radiocommunications range, it can use an intermediate nodeto forward the message to the desired node. This networktopology has the advantage of redundancy and scalability.If an individual node fails, a remote node still can communicateto any other node in its range, which in turn, canforward the message to the desired location. In addition,the range of the network is not necessarily limited by therange in between single nodes, it can simply be extendedby adding more nodes to the system. The disadvantageof this type of network is in power consumption for thenodes that implement the multihop communications aregenerally higher than for the nodes that don’t have this capability,often limiting the battery life. Additionally, as thenumber of communication hops to a destination increases,the time to deliver the message also increases, especially iflow power operation of the nodes is a requirement
download full report
http://googleurl?sa=t&source=web&cd=4&ve...ter-22.pdf&ei=vex-Td3oHMrirAfwtvmzBw&usg=AFQjCNHKimcgrjQosbKP6cO7UeufLsQQ3w
