教育部資通訊科技人才培育先導型計畫 Sensor Network
1.IntroductionGoal • Wireless Sensor Network • Ubiquitous Computing • Ubiquitous Network Society • Human-centric
1.IntroductionUbiquitous • Ubiquitous • 7A • Anytime • Anyone • Anywhere • Any Device • Affordable • All Security • Any Information/Service
1.IntroductionGeneral Purpose • A wireless sensor network (WSN) is a wireless network using sensors to cooperatively monitor physical or environmental conditions • The development of wireless sensor networks was originally motivated by military applications. • Wireless sensor networks are now used in many wide-range application areas.
1.IntroductionSensors Ultrasonic Magnetic Sensor (22.5×22.5×39mm) WII sensor (240×35×15mm) Image Sensor Modules (8×8×5.7mm)
1.IntroductionTypical Sensor Network Data gathering sensor sensor Data transmitting Center sensor Relay node Relay node sensor processing sensor sensor
1.Introductionsensor characteristics • Wireless sensors are small devices that gather information. • Pressure, Humidity, Temperature • Speed, Location • Wireless sensors have some characteristics: • Low power • Small size • Low cost
1.Introductionsensor network characteristics • Primary Function • Sample the environment for sensory information • Propagate data back to the infrastructure • Traffic pattern in sensor network • Low activity in a long period • Bursting data in short time • Highly correlated traffic
1.Introductionsensors categories • Sensors can be classified into two categories: • Ordinary Sensors • Data gathering • Ordinary Sensors require external circuitry to perform some dedicated tasks like data analyzing. • Smart Sensors • Data gathering and processing • Smart Sensors have internal circuitry to perform dedicated tasks.
1.IntroductionRelated Work • Related work • CSMA • To improve the energy consumption by avoiding overhearing among neighboring nodes • TDMA • No contention-introduced overhead and collisions • Not easy to manage the inter-cluster communication and interference • Not easy to dynamically change its frame length and time slot assignment
1.IntroductionRelated Work • PAMAS • Power off radio when not actively transmitting and receiving packet. • Zigbee • Combined with IEEE 802.15.4 (Low-Rate Wireless Personal Area Network, LR-WPAN) • Low rate: 250kbps • Short distance: 50-300m • Low power consumption • frequency band: • Global: 2.4GHz ,16 channels • America: 915MHz, 10 channels • Europe: 868MHz, 1 channel.
1.IntroductionZigbee stack • Zigbee Platform Stack and IEEE802.15.4 Application/Profiles ZigBee or OEM Application Framework Network / Security Layers ZigBee Alliance Platform MAC Layer IEEE 802.15.4 PHY Layer
1.IntroductionZigbee Application Reference: NTP無線感測網路與ZigBee 協定簡介
2.MAC for Sensor NetworkSensor Network MAC Protocol • Carrier Sensing • Only during low traffic load. • Backoff • Backoff in application layer is desired other than in MAC layer. • Contention • RTS-CTS only during high traffic load.
2.MAC for Sensor NetworkSources of Energy Wastage • The major sources of energy wastage are: • Collisions • Overhearing • Control packet overhead • Idle listening • Achieving good scalability and collision avoidance capability is necessary.
2.MAC for Sensor NetworkS-MAC • Sensor-MAC (S-MAC): Medium Access Control for Wireless Sensor Networks • S-MAC is a medium-access control (MAC) protocol designed for wireless sensor networks. • Sensor networks are deployed in an ad hoc fashion, with individual nodes remaining largely inactive for long periods of time, but then becoming suddenly active when something is detected.
2.MAC for Sensor NetworkS-MAC • These characteristics of sensor networks and applications motivate a MAC that is different from traditional wireless MACs such as IEEE 802.11 in almost every way • Energy conservation and self-configuration are primary goals. • Per-node fairness and latency are less important.
2.MAC for Sensor NetworkThree techniques in S-MAC • S-MAC uses three techniques to reduce energy consumption. • Nodes go to sleep periodically. • Nearby nodes form virtual clusters to synchronize their wake-up and sleep periods to keep the control packet overhead of the network low. • Message passing is used to reduce the contention latency and control overhead.
2.MAC for Sensor NetworkThree techniques in S-MAC • Periodic Listen and Sleep: • Nodes do not waste energy by listening to an empty channel or when a neighboring node is transmitting to another node. • Nodes use RTS and CTS to talk to each other and contend for the medium.
2.MAC for Sensor NetworkThree techniques in S-MAC • Collision and Overhearing Avoidance: • S-MAC adopts a contention-based scheme to avoid collisions. • A duration field is introduced in each transmitted packet which indicates how much longer the transmission will last. • When a node receives a packet, it will not transmit any packets for at least the time that is specified in the duration field.
2.MAC for Sensor NetworkThree techniques in S-MAC • Collision and Overhearing Avoidance: • Overhearing is avoided by letting the nodes, which get RTS and CTS packets which are not meant for them, go to sleep. • All immediate neighbors also go to sleep till the current transmission is completed after a sender or receiver receives the RTS or CTS packet.
2.MAC for Sensor NetworkThree techniques in S-MAC • Message Passing: • Long messages are fragmented into smaller messages and transmitted in a burst. • To avoid the high overhead and delay encountered for retransmitting when message is lost. • ACK messages are used to indicate if a fragment is lost at any time. • The sender can resend the fragment again. • The ACK message also have the duration field to reduce overhearing and collisions.
3. Challenges • Challenges: • 1. Energy Efficiency: • Power consumptions are crucial to wireless sensor network applications because sensor nodes are not connected to any energy source. • Energy efficiency is a dominant consideration no matter what the problem is. • Sensor nodes only have a small and finite source of energy. Many solutions, both hardware and software related, have been proposed to optimize energy usage.
3. Challenges • 2. Ad hoc deployment: • Most sensor nodes are deployed in regions which have no infrastructure. • We must cope with the changes of connectivity and distribution. • 3. Unattended operation: • Generally, once sensors are deployed, there is no human intervention for a long time. • Sensor network must reconfigure by itself when certain errors occur.
3. Challenges • 4. Dynamic changes: • As changes of connectivity due to addition of more nodes or failure of nodes, Sensor network must be able to adapt itself to changing connectivity.
4.Coverage • Coverage can be classified into three types: • Area coverage • deployment of sensors to cover a given area • Point coverage • deployment of sensors to cover a set of points • Barrier coverage • The goal is to minimize the probability of undetected penetration through the barrier. • To find a path in a region • For any point on the path, the distance to the closest sensor is minimized.
4.CoverageArea coverage • Area coverage • deployment of sensors to cover a given area
4.CoveragePoint coverage • Point coverage • deployment of sensors to cover a set of points
4.CoveragePoint coverage A • Barrier coverage • To find a path from A to B • For any point on the path, the distance to the closest sensor is minimized. B
5.Localization • In sensor networks, nodes are deployed without priori knowledge about their locations. • Estimating spatial-coordinates of the node is referred to as localization.
5.LocalizationGPS • Global Positioning System (GPS) is an immediate solution. • There some factors against the usage of GPS: • GPS can work only outdoors. • GPS receivers are too expensive to unsuitable for wide-range deployment. • It cannot work in the presence of obstructions.
5.LocalizationCategories • Localization can be classified into two categories: • Fine-grained • Based on timing / signal strength • Coarse-grained • Based on proximity
5.LocalizationProximity base localization • Trilateration / Multilateration technique • Proximity based localization: • Some nodes which can know their position through some technique (ex. GPS) broadcast their position information. • Other nodes listen to these broadcast messages and calculate their own position. • A simple method would be to calculate its position as the centroid of all the positions it has obtained. • This method leads to accumulation of localization error.
A 5.LocalizationTrilateration Example • Trilateration • A is 5m from B • A is 10m from C • A is 8m from D C B D
5.LocalizationTrilateration • Trilateration is a geometric principle which allows us to find a location if its distance from other nodes are known. • The same principle can be extended to three-dimensional space. • Four spheres would be needed to locate certain point in 3D space.
5.LocalizationFine-grained method • Signal strength method • Attenuation happens when signals are propagated. We can use the degree of attenuation to calculate the distance. • Timing method • The distance between two nodes is determined by the time of flight of the signal.
6.RoutingCategories • Routing protocols can be divided into two types. • Proactive routing protocol • Proactive routing protocol maintain consistent updated routing information between all nodes. • To update routing table periodically. • Reactive routing protocol • Routes are created only when they are needed.
6.RoutingThree types in sensor network • Because of the energy constrained nature of sensor networks, conventional routing protocols have many limitations when being applied to sensor networks. • Three types of routing protocol in sensor network: • Data-centric • Hierarchical • Location-based
Data A Data A Data A 6.RoutingData-centric • Data-centric: • Managers broadcast a Query message to the network. • If a sensor observes some events related to the Query message, it sends the data to the data center. • Data aggregation: sensor1 Data Center Relay node1 sensor2
6.RoutingData-centric • Data centric: Flooding • Flooding is one of basic data transmitting methods. • If any sensor receives or generates some packets, it will broadcast these packets to all its neighbors. • Nodes may receive duplicate data. • More power consumption.
6.RoutingData-centric • Data centric: Sensor Protocols for Information via Negotiation (SPIN) • There are three messages in SPIN: • Advertisement (ADV): When a node has some data to send, it sends an ADV message to its neighbors containing data descriptor (meta-data). • Request (REQ): When a node wants to receive some data. It sends an REQ message first. • DATA: Actual data message with a meta-data header.
ADV (meta data A) ADV (meta data A) ADV (meta data A) ADV (meta data A) ADV (meta data A) ADV (meta data A) DATA (meta data A) DATA (meta data A) REQ (meta data A) REQ (meta data A) 6.RoutingData-centric SPIN: Node3 Node1 Node6 Node4 Node7 Node2 Node5
6.RoutingData centric • Data centric: Directed Diffusion • This is a destination-initiated reactive routing technique. • Routes are established when requested. • A interest is propagated throughout the network for named data by a node and data which matches this interest is then sent toward this nodes. • Interests are described by a list of attribute-value pairs. • Example: type=birds & response=20 ms
6.RoutingData centric • Directed Diffusion • The propagation of data and its aggregation at intermediate nodes on the way to the request originating node are determined by the messages which are exchanged between neighboring nodes within some distance.
Node1 Node1 interest interest Gradient Node4 Gradient interest interest Node4 interest Gradient Node2 interest Node2 Node5 Gradient interest Node5 Node3 interest Node7 Gradient interest Node3 Node7 Gradient Node6 Gradient return path (Gradient) Gradient interest Gradient Gradient 6.RoutingData centric Directed Diffusion: Node6 Forward interest
Node1 Node4 Node2 Node5 Node3 Node7 Node6 6.RoutingData centric • Directed Diffusion: • Sender can choose the best return path. • EX: minimum response time, least hops
6.RoutingHierarchical • Hierarchical: Low Energy Adaptive Clustering Hierarchy (LEACH) • LEACH is a two-tier protocol. • Cluster head • Cluster member • Every node runs a random algorithm periodically to decide its identity. (cluster head or not)
6.RoutingHierarchical • LEACH • All cluster heads broadcast Advertisement (ADV) message and other nodes decide which cluster they belong to according the strength of ADV message. • Cluster members only send data to their cluster head. Then, cluster heads reply data to Sinks.
6.RoutingHierarchical Cluster 2 Node6 LEACH Example Node5 Node7 Cluster Head2 Node8 Node4 Node9 Node3 Node10 Sink Cluster Head1 Cluster Head3 Node2 Node11 Node12 Node13 Node1 Cluster 3 Cluster 1
6.RoutingLocation-based • Location-based: Geographic Adaptive Fidelity (GAF) • GAF divides the network into several virtual grids. • For adjacent virtual grids A and B, every node in A can directly connect with every node in B. • In GAF, every node has three types of status: • Active • Discovery • Sleep