Wireless Sensor Networks

1. Wireless Sensor Networks Aν. Καθηγητής Συμεών Παπαβασιλείου Εθνικό Μετσόβιο Πολυτεχνείο Τμήμα Ηλεκτρολόγων Μηχανικώνκαι Μηχανικών Υπολογιστών papavass@mail.ntua.gr Τηλ: 210 772-2550

2. Sensors What is a sensor: Device that measures a physical quantity and converts it into a signal which can be read by an observer or by an instrument. • What is a sensor node: • Node in a wireless sensor network that is capable of performing some processing, gathering sensory information and communicating with other connected nodes in the network. • small size • energy constrained • limited capabilities • Consists of:

3. Wireless Sensor Networks • A Wireless Sensor Network(WSN) consists of spatially distributed autonomous sensors to monitor physical or environmental conditions, such as temperature, sound, vibration, pressure etc. and to cooperatively pass their data through the network to a main location • Operation: • Every sensor node collects data (acoustic, seismic etc.) from its environment • Data is sent to the collection center (aka sink) for further processing • Data gathering is realized through multi-hop routing

4. Base Station Internet and/or Satellite Wireless Sensor Network Architecture B A D E C Control & Management Station Sensor Network Sensor Nodes End User 4

5. Sensor Network Applications • Environment detection and monitoring –Great Duck Island, Maine, SEA-LABS • Disaster Prevention • Medical Care – Mercury project, Harvard • Home Intelligence • Scientific exploration • Surveillance

6. WSNs vs Ad hoc networks I Wireless Sensor Networks differ from traditional ad hoc networks: • The number of sensor nodes in a sensor network can be several orders of magnitude higher than the nodes in an ad hoc network. • Sensor nodes are densely deployed. • Sensor nodes are prone to failures. • The topology of a sensor network changes very frequently. • Sensor nodes mainly use a broadcast communication paradigm, whereas most ad hoc networks are based on point-to-point communications. • Sensor nodes are limited in power, computational capacities, and memory. • Sensor nodes may not have global identification (ID) because of the large amount of overhead and large number of sensor

7. WSNs vs Ad hoc networks II Why not port ad hoc protocols? • Ad Hoc networks require significant amount of routing data storage and computation • Sensor nodes are limited in memory and CPU • Topology changes due to node mobility are infrequent as in most applications sensor nodes are stationary • Topology changes when nodes die in the network due to energy dissipation • Scalability with several hundred to a few thousand nodes not well established Therefore: Need for development of new protocols specific for WSN

8. WSNgeneralrequirements Wireless ad hoc sensor network requirements include the following 1.Large number of (mostly stationary) sensors:  Aside from the deployment of sensors on the ocean surface or the use of mobile, unmanned, robotic sensors in military operations, most nodes in a smart sensor network are stationary. 2. Low energy consumption:  Since in many applications the sensor nodes will be placed in a remote area, service of a node may not be possible. In this case, the lifetime of a node may be determined by the battery life, thereby requiring the minimization of energy expenditure. 3. Ease of installation and maintenance. In case of a malfunction, it is difficult to visit in-situ and check the problem. 4. Network dynamic self-organization:  Given the large number of nodes and their potential placement in hostile locations, it is essential that the network be able to self-organize; manual configuration is not feasible.   5. Querying ability:  A user may want to query an individual node or a group of nodes for information collected in the region.  

9. WSNrequirements summary

10. Sensor Network Architectures Layered Architecture Clustered Architecture

11. Sensor Localization

12. Sensor Localization It is essential, in some applications, for each node to know its location • Sensed data coupled with loc. data and sent • We need a cheap, low-power, low-weight, low form-factor, and reasonably accurate mechanism • Global Positioning Sys (GPS) is not always feasible • GPS cannot work indoors, in dense foliage, etc. • GPS power consumption is very high • Size of GPS receiver and antenna will increase node form factor

13. Indoor Localization • Use a fixed infrastructure • Beacon nodes are strategically placed • Nodes receive beacon signals and measure: • Signal Strength • Signal Pattern • Time of arrival; Time difference of arrival • Angle of arrival • Nodes use measurements from multiple beacons and use different multi-lateration techniques to estimate locations • Accuracy of estimate depends on correlation between measured entity and distance • Examples of Indoor Loc. Systems • RADAR (MSR), Cricket (MIT), BAT (AT&T), SPA

14. Sensor Network Localization I • No fixed infrastructure available • Prior measurements are not always possible • Basic idea: • Have a few sensor nodes who have known location information • These nodes sent periodic beacon signals • Other nodes use beacon measurements and triangulation, multi-lateration, etc. to estimate distance

15. Sensor Network Localization II • Receiver Signal Strength Indicator (RSSI) was used to determine correlation to distance • Suitable for RF signals only • Very sensitive to obstacles, multi-path fading, environment factors (rain, etc.) • Was not found to have good experimental correlation • RF signal had good range, few 10metres • RF and Ultrasound signals • The beacon node transmits an RF and an ultrasound signal to receiver • The time difference of arrival between 2 signals is used to measure distance • Range of up to 3 m, with 2cm accuracy

16. Localization Algorithms • Based on the time diff. of arrival • Atomic Multi-lateration: • If a node receives 3 beacons, it can determine its location (similar to GPS) • Iterative ML: • Some nodes not in direct range of beacons • Once an unknown node estimates its location, will send out a beacon • Multi-hop approach; Errors propagated • Collaborative ML: • When 2+ nodes cannot receive 3 beacons (but can receive say 2), they collaborate

17. Sensor MAC Protocols

18. Multiple Access Control (MAC) Protocols • MAC allows multiple users to share a common channel. • Conflict-free protocols ensure successful transmission. Channel can be allocated to users statically or dynamically. • Only static conflict-free protocols are used in cellular mobile communications- Frequency Division Multiple Access (FDMA): provides a fraction of the frequency range to each user for all the time- Time Division Multiple Access(TDMA) : The entire frequency band is allocated to a single user for a fraction of time- Code Division Multiple Access (CDMA) : provides every user a portion of bandwidth for a fraction of time • Contention based protocols must prescribe ways to resolve conflicts- Static Conflict Resolution: Carrier Sense Multiple Access (CSMA) - Dynamic Conflict Resolution: keeps track of various system parameters, ordering the users accordingly

19. Media Access in Sensor Networks • Why STUDY MAC protocols in sensor networks? • Application behavior in sensor networks leads to very different traffic characteristics from that found in conventional computer networks • Highly constrained resources and functionality • Small packet size • Deep multi-hop dynamic topologies • The network tends to operate as a collective structure, rather than supporting many independent point-to-point flows • Traffic tends to be variable and highly correlated • Little or no activity/traffic for longer periods and intense traffic over shorter periods

20. New techniques need to be found for decreasing the energy consumption within the sensor network Energy Consumption in Sensor Networks • Transmission and reception of data require the highest energy consumption • FACT: • Energy required for the transmission of 1 bit in100 m = Energy required for the performance of 300 operation (Pottie & Kaiser, 2000) Increase the network lifetime

21. Energy Consumption in Sensor Networks 4 main reasons for expenditure of energy • Collisions: Packets form neighboring nodes conflict and require retransmission • Overhearing: Sensor nodes listen and receive packets not destined to them • Control Packet Overhead: Many protocols require the exchange of control packets • Idle listening: Sensor nodes wait and listen for packets that may not arrive eventually

22. MAC Protocols – Properties Wireless Sensor Networks due to their unique nature and characteristics call for development of new MAC protocols Desired Properties • Energy efficient • Scalable • Adaptive • Mean delay & throughput can be of secondary importance

23. Contention based MAC protocols I IEEE 802.11 (DCF) was the 1ο standard protocols for the communication of wireless devices • Based on CSMA/CA (Carrier Sense Multiple Access / Collision Avoidance) • Use of RTS/CTS packets for avoiding thehidden terminal problem • Use of DATA/ACK packets

24. Contention based MAC protocols II Pros • Simple • Scalable – insertion / deletion of nodes easy • Robust • No synchronization required • Knowledge of the topology not needed Cons • Multiple conflicts – το carrier sense does not work for more than one hop • Great amount of control packets (RTS/CTS) – 40%-75% of channel utilization • Long idle listening (~75% of total time)

25. WiseMAC • Use of np-CSMA with preamble sampling • Τhepreamble proceeds the data packet in order to notify the receiver node (no RTS/CTS) • Method to dynamically determine the length of the preamble packet • Every node has a sleep-wake program Cons:Collisions because of the hidden terminal problem

26. sleep listen listen sleep Periodic Listening Its main use is for decreasing the energy consumption caused by idle listening Nodes “sleep” periodically and turn off their radio Less energy consumption but increased delay in data gathering

27. SMACI Basic features: • Periodic listen and sleep • Collision and overhearing avoidance • Message passing

28. Schedule 1 Schedule 2 SMACII Periodic listen and sleep • Listen + Sleep = Frame • Nodes synchronize with each other by sending their schedule • Neighboring nodes follow the same schedule • Border nodes follow 2 or more schedules At the beginning of each listenperiod, nodes synchronize by sending SYNC

29. SMAC III Collision & Overhearing Avoidance • Use of RTS/CTS • Use of physical & virtualcarrier sense • Use of NAV (Neighbor Allocation Vector) • When a nodes listens the transmission of its neighbor, it can determine how long it will last and become “silent” • This value is saved into NAVand then decreases • For a node to transmit, it has to succeed in CS but also holds NAV=0 • When a node listens to RTS/CTS, then by knowing how long the transmission will last, it can be put to sleep

30. SMAC IV Transmission of long packets • Break the packet into smaller junks • Transmission of only one pair of RTS/CTS • Neighboring nodes sleep for the whole duration of the transmission

31. SMAC V Main drawback • Increased delay because of the periodic sleep of nodes • Partial solution by adaptive listening method

32. SMAC variations I TMAC: • Decrease in idle listeningby transmitting the packets in burstand then sleep • The Listenperiod is adapted based on the network load • Use of RTS/CTS/ACK& FRTS (Future Request to Send)control packets for dealing with the delay caused of the sleep period

33. SMAC variations II DMAC: • Adapt the listen period when a node has many packets to send • Inform the receiver nodes in order to adjust their schedule too • No use of RTS/CTS • Use of Data prediction method – a node expects data form its children • Use of MTS (More to Send)control packet - sent by the children of a node to it in order to adjust its schedule

34. SMAC variations III ΖMAC: • It is adaptive to the level of contention and the load of the network • Under low contention it behaves as CSMA • Under heavy load behaves as TDMA • Every node picks its slots and decides the length of its frame • Use of control packets (no RTS/CTS) but ECN (Explicit Contention Notification) • ECN is used to notify for two-hop contention

35. TDMA based MAC protocols • These protocols use the notion of timeslots • Each nodes transmits during its own slot • Solve the hidden terminal problem without the use of control packets Drawbacks • Require tight synchronization • It is hard to find a conflict-freeprogram (NP hard when channel reuse is wanted) • Difficult to scale

36. Spatial TDMA and CSMA • Use of 2 separate channels • Use of TDMA for transmission of data packets • Use of CSMA (low power- preamble) for signaling (transmission of control packets)

37. TRAMA I • Time is divided into “Scheduled Access” and “Random Access” • Random Access: for signaling • Scheduled Access: for regular traffic

38. TRAMA II • Nodes have knowledge for all their two-hop neighbors • This information is exchanged during the signaling which is contention based • Each node announces the slots in which it will transmit as well as its receivers • When a node is not transmitting or receiving, it is put to sleep