3 Introduction- Definition Sensor: measures a physical phenomenon (motion, heat, light …) and converts it into an electrical signal.
4 Introduction- Definition Wireless Sensor Networks (WSN): A wireless sensor network is a special network with large numbers of nodes. The nodes are equipped with embedded processors, sensors and radios. These nodes collaborate to accomplish a common task such as environment monitoring or asset tracking.
6 Ad Hoc Wireless Networks In many applications, the nodes are deployed in Ad Hoc fashion.
8 Sensor network topology The sensor nodes are usually scattered in a sensor field. Nodes collect data and route data back to the end users by a multi-hop infrastructure-less architecture through the sink. The sink may communicate with the task manager node via Internet or Satellite.
9 Applications Environmental monitoring Seismic activity detection; Industrial monitoring and control High-precision agriculture Structural health monitoring healthcare and medical research Homeland security. military applications. Smart Buildings to improve living conditions and reduce energy consumption Inventory Management Fire Monitoring
10 Generic Node Architecture A sensor node is made up of four basic components: 1)Sensing Unit. 2) Processing Unit. 3) Transceiver Unit4) Power Unit. Additional units location finding system--power generator--mobilizer.
11 Constraints for Sensor Nodes Required small size: Can be placed in more locations and used in more scenarios (applications) more flexibility. Collect more data deployed densely.
12 Constraints for Sensor Nodes Consume extremely low power (µAmps.): use low-power hardware components. Transmit and receive only if necessary. Power consumption in each node: sensing, data processing and communication. Radio communication will consume a significant fraction of total energy.
13 Constraints for Sensor Nodes Strategies to reduce the average supply current of the radio: Reduce the amount of data transmitted through data compression and reduction. Reduce the frame overhead. Implement strict power management mechanisms (power-down and sleep modes). only transmit data when a sensor event occurs
14 Constraints for Sensor Nodes Have low production cost. In some application response time is a critical (security system) quick response time is required. WSN need privacy & also be able to authenticate data communication. Scalability: Some nodes may die or new nodes may join
15 Hardware Overview Node (1/2) Examples of nodes
16 Hardware Overview Node (2/2) (( Mica Z Mote )) Sensors: light, temperature, pressure, acceleration, acoustic, magnetic… Characteristics: Microcontroller (ATMega128L): 7.4 MHz, 8 bit. Memory: 4KB data, 128 KB program. Radio: < 40 Kbps, 2.4GHz, DS-SS (ZigBee). Special connector for Crossbow sensor boards. Special Operating System: TinyOS. Power Alkaline/Lithium batteries. Lifetime of 450 days requires 1% duty cycle.
17 Protocol Stack The protocol stack used by the sink and all sensor nodes: Combines power and routing awareness, integrates data with networking protocols, communicates power efficiently through the wireless medium. promotes cooperative efforts of nodes.
18 Protocol Stack The power, mobility, and task management planes: monitor the power, mobility, and task distribution among the sensor nodes.
19 Physical Layer (1/3) Responsible of: –Frequency selection : 916 MHz, 2.4 GHz –carrier frequency generation, –signal detection, –modulation and data encryption.
20 Physical Layer (2/3)-Propagation Aspects Energy minimization has significant importance more than: scattering, shadowing, reflection, diffraction, multi-path and fading effects. Multi-hop communication can effectively overcome shadowing and path-loss effects, if the node density is high enough.
21 Physical Layer (3/3)-Modulation Scheme M-ary scheme increased radio power consumption. Binary modulation scheme is more energy efficient BFSK used.
22 Data Link Layer Responsible for: –the multiplexing of data streams, –data frame detection, –medium access and error control.
23 Data Link Layer-MAC Protocol Sources of energy inefficiency: Collision. Overhearing. Control packet overhead. Idle listening. So, there is a need for a MAC protocol that solve these problems.
24 Data Link Layer-MAC Protocol Several Protocols used in the Link Layer: 1.Self-Organizing Medium Access Control for Sensor Networks (S-MACS) 2. CSMA. 3. Hybrid TDMA/FDMA based.
25 Data Link Layer/S-MAC S-MAC: –MAC protocol specifically designed for WSN. –Building on random access - based protocols. S-MAC solve the problem of all the major sources of energy waste: –idle listening, collision, overhearing and control overhead. Not suitable for time-critical applications because latency in end-to-end communication. Design goals: Reduce energy consumption Support good scalability Self-configurable
26 Data Link Layer/S-MAC Uses a sleep/wakeup cycle to allow nodes to spend most of their time sleep: Listen period: for nodes that have data to send to coordinate. A sleep period : nodes sleep if they have no data to send or receive, and nodes remain awake and exchange data if they are involved in communication. In a sleep mode when the radio is switched off, the node sets a timer to awake later. When the timer expires, it wakes up. Selection of sleep and listen duration is based on the application scenarios.
27 Data Link Layer/S-MAC Each node maintains a schedule table. Nodes exchange schedules by broadcast. Multiple neighbors contend for the medium A communication link : a pair of time slots operating at a randomly chosen but fixed frequency (or frequency hopping sequence). Once transmission starts, it does not stop until completed.
28 Data Link Layer/S-MAC Nodes a and b follow different schedules. If a wants to send to b, it just wait until b is listening.
29 Data Link Layer/S-MAC Neighboring nodes are synchronized together. Maintaining Synchronization: –Needed to prevent clock drift –Periodic updating using a SYNC packet –Receivers adjust their timer counters Sender Node ID Next-Sleep Time SYNC Packet
30 Data Link Layer/S-MAC Collision avoidance: Perform virtual and physical carrier sense before transmission. RTS/CTS solves the hidden terminal problem. Interfering nodes go to sleep after they hear the RTS or CTS packet Overhearing Avoidance: NAV. indicates how long the remaining transmission will be. The medium is busy when the NAV value is not zero All immediate neighbors of sender and receiver should go to sleep avoiding energy waste on overhearing.
31 Data Link Layer/S-MAC
32 Network Layer (1/6) Special multi-hop wireless routing protocols between sink node and sensors are needed. –Traditional ad hoc routing techniques do not usually fit. When we design network layer protocols for sensor networks, we need to consider: –Power efficiency. –Sensor networks are data-centric. –addressing and location awareness.
33 Network Layer (2/6) Routing Techniques: Maximum PA route: Max. total PA without including routes that add extra hops. Minimum Energy route: Route that consumes min. energy. Energy-efficient routes: –can be found based on the available power (PA) and the energy required α for transmission in the links. Minimum hop route: Min. hop to reach the sink. Maximum minimum PA node route: Use the route in which the min. PA is larger than the min. PAs of the other routes. This scheme prevents the risk of using up a sensor node with low PA much earlier than the others just because it is on the route with nodes that have very high PAs.
35 Flooding is an old technique for routing. Duplicate messages. Overlap. Resource blindness. Sensor Protocols for Information via Negotiations (SPIN): Send sensor data instead of all the data. 3 types of messages: Advertise, Request & Data. Network Layer (4/6)
36 Network Layer (5/6) The Directed diffusion: Sink send out “interest”. Each S-node stores the interest entry in its cache. Interest entry contains a timestamp and several gradient fields. As the interest propagates, the gradients back to sink are set up. When the source has data for the interest, the source sends data along the interest’s gradient path. Based on data-centric routing.
37 Network Layer (6/6) (Low-Energy Adaptive Clustering Hierarchy) LEACH The characteristics of LEACH: Randomly rotating the cluster-head among sensors. Low energy consumption.
38 Transport Layer Transport layer is especially needed when the system is planned to be accessed through the Internet or other external networks. TCP: transmission window mechanisms is not suitable, TCP splitting will be used: Between User Sink (TCP or UDP) Between Sink nodes (UDP)
39 Application Layer Sensor Management protocol: Exchanging the data. Time synchronization Moving the nodes, turning them on and off etc. Sensor Query and Data distribution protocol. User applications with interfaces to issue queries, respond to queries and collect incoming replies.
40 Conclusion The Protocols used are not well defined and they are open research issues. The advantages of WSN’s create many new and exciting application areas for remote sensing, so they will be an integral part of our lives.