MC Lecture 9234455566667777777777777.pptx
- 1. 7. Wireless Sensor Networks 7.1. Wireless sensor networks (WSNs) A class of ad hoc networks A collection of hundreds or thousands of tiny, disposable (bec. low- cost) & low-power (bec. battery-operated) sensor nodes Communicating together to achieve an assigned task: monitoring & analysis of an area Sensor node in WSN Converts a sensed physical attribute (e.g., temperature) into data Includes : Sensing module Communications module Computing module Memory module Power source (usually a battery)
- 2. Wired sensor networks Example: wired sensornet within a plane Known for years in many applications Monitor critical physical parameters Alert when anomalies perceived Sensor locations carefully predesigned Sensors distributed in strategic locations # of sensor nodes can be huge As many as needed to cover the monitored area Connected via wires Fault tolerance requirements Avoid single points of failure If work unattended => can not repair if failed E.g., in inhospitable or inaccessible milieus
- 3. Advances in technology enabled wireless sensornets (WSNs) Especially advances in miniaturization, low-cost sensors (& multisensors), wireless communications, batteries Low costs (due to advances in technology) enabled massive deployment of WSN nodes No need for predesigned locations Can drop them from a plane, a speeding car, etc. => no installation costs (saves more)
- 4. Advantages of WSNs over wired sensornets 1) Ease of deployment Deployed without careful design E.g., dropped from a plane 2) Extended range At the same cost can cover a larger area No need for infrastructure One large wired sensornet replaced by many smaller wireless sensornets at the same cost Can easily move from area to area 3) Fault tolerance: design requirement! Must tolerate node failures (maybe reduces monitoring accuracy) Some fault tolerance is natural Bec. failure of a sensor node is masked by other nodes collecting similar data in the same area 4) Mobility No wires inhibiting mobility not even for power (batteries) Still WSNs less mobile than ad hoc networks Inherent limitations of WSNs 1) Limited energy (batteries, etc.) 2) Low-bandwidth transmission 3) Error-prone transmission
- 5. Design requirements for WSNs & their protocols 1) Maximize WSN lifetime Discussed below 2) Accommodate dynamic, fast-changing physical parameters affecting WSNs, such as: (1) Power availability for nodes (2) Positions of nodes (3) Reachability For a given node, which other nodes can it reach (4) Types of tasks executed by nodes I.e. what attributes (such as temperature) the node monitors & reports Dealing with limited energy: Design WSN & its protocols carefully to maximize WSNs lifetime One approach: balance energy use in such a way that all nodes die at approximately the same time Once this happens, can replace the whole sensornet Better than replacing individual nodes (impossible or inconvenient)
- 6. Data exchange in WSN is fundamentally different than in other wireless networks WSNs are data-centric networks The interest is in what is the data? rather than where is the data? E.g., WSNs focus on attributes (e.g., temperature, velocity) WSNs must efficiently respond to application/user queries asking for data => WSNs require different routing protocols then MANETs Routing protocols for WSNs must be application-data-specific
- 7. Challenges in design of (application-data-specific) routing protocols for WSNs 1) No unique node ID to be used for routing Which is typical in traditional wired/wireless networks Bec. (#1) routing to/from a specific node is not required in data-centric WSNs Recall: It does not matter where is the data? Bec. (#2) with the large # of nodes in WSNs, ID would be large ID might be larger than amount of actual data being xmitted 2) Nodes often send aggregated data, not raw data Adjacent nodes may have similar data, so aggregation cuts traffic 3) Routing protocols must be application-specific and data-centric Bec. WSNs are application-specific and data-centric E.g., WSN may require protocol customized to very efficient delivery of data on a single attribute (e.g., temperature) 損 Could be very inefficient for delivery of data on other single attribute, or delivery of multi-attribute data 4) Minimizing energy consumption
- 8. Features of an ideal WSN 1) Attribute-based addresses Composed of a series of attribute-value pairs Specify physical parameters to be sensed Example attribute address: temperature>100C, location=? All nodes that sense temperature>100C must report their locations 2) Location-awareness of nodes A node often (as in example in (1) above) needs to know its location Otherwise cannot provide sensed data. 3) Must react immediately to drastic environment changes Necessary for time-critical monitoring applications Can react slowly to non-critical changes/events Saving bandwidth & energy at the cost of increased latency 4) Efficient handling of queries Efficient xmission of queries from usesr/applications to appropriate nodes (=> need efficient routing!) Efficient xmission of answers to queries from nodes to users/applications (=> need efficient routing again!) Can reply with larger latency for noncritical changes/events E.g., can increase interval for reporting periodic data
- 9. Classification of sensor networks Proactive networks Nodes periodically switch on their sensors & transmitters, sense the environment & transmit the data of interest Reactive networks Nodes react immediately to sudden or drastic changes in the value of the sensed attribute Once proactive/reactive network type is chosen, efficient routing protocols must be designed Preferably using a suitable MAC sublayer protocol to avoid collisions 7.2. Classification of Sensor Networks
- 10. A wireless sensor and actuator network (WSAN) is a group of sensors that gather information about their environment and actuators, such as servos or motors, that interact with them. All elements communicate wirelessly; interaction can be autonomous or human-controlled. For eg. A sensor and actuator network in smart homes for supporting elderly and handicapped people. The primary goal was to monitor domestic systems such as air conditioning, lights, and heating, as well as to control the basic functions of the home entertainment and security systems. The sensor network consisted of three BTnodes, an autonomous wireless communication and computing platform based on a Bluetooth radio, and a microcontroller. Each is equipped with sensors for light, motion, and temperature detection. 7.3 A Wireless sensor and actuator network
- 11. Some of Wireless Sensor and actuator network is used to monitor environments which is known as fire detection system. A group of sensor nodes are placed in a building or an area of interest. In the event of a fire in the monitoring region, the sensor nodes that are close to the origin of the fire report the location and intensity of the fire to water sprinkler actuators. On receiving alarm messages from sensor nodes, the water sprinkler actuators analyze the intensity of the fire and take appropriate actions before the fire becomes uncontrollable. Each sensor sends their measurements of the plant state to the their controllers. When the controller receives the plant states, actuation signals computed by the control algorithm are forwarded to actuators through the same WSAN. Wireless sensors transmit data in each assigned time slot dependent on the transmission scheduling scheme.
- 12. Both the controller and actuator only respond to newly received data over unreliable wireless links. Hence, both the controller and actuator operate in an event-driven fashion, but each sensor operates in a time-driven fashion. Assuming sensors sample the plant state right before the transmission slot and transmit it during their allocated transmission time to the controller in order to minimize the delay. In wireless sensor actuator networks (WSANs), both sensor-actuator and actuator- actuator coordination are required. After sensors detect an event that has occurred in the environment, the event data is processed (e.g., aggregated with reports from nearby sensors) and transmitted to the actuators, which gather, process, and eventually reconstruct the characteristics of the event. The process of establishing data paths between sensors and actuators is referred to as sensor-actuator coordination. Sensor-actuator coordination provides the transmission of event features from sensors to actuators. Sensors and actuators coordinate also for some other tasks, such as sensor placement or improving connectivity.
- 13. A wireless sensor and actuator network is a networked system of geographically distributed sensor and actuator nodes. These nodes are interconnected via wireless links. The scale of the network depends highly on the target application. In general, both sensor and actuator nodes are equipped with some data processing and wireless communication capabilities, as well as power supply. Sensors gather information about the state of physical world and transmit the collected data to actuators through single-hop or multi-hop communications over the radio channel. Upon receipt of the required information, the actuators make the decision about how to react to this information and perform corresponding actions to change the behavior of the physical environment.
- 14. The sensors and actuators are usually used to sense the operation of the physical system, compare it against the desired behavior, compute control commands, and perform actions onto the system to effect the desired change.
- 15. Network of sensors Leads towards the realization of the vision of Internet of things!