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Sensor Networks

Sensor Networks. Lecture 8. LEACH Clustering. LEACH: (Low Energy Adaptive Clustering Hierarchy) rotates cluster heads to balance energy consumption Each cluster head performs its duty for a period of time

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Sensor Networks

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  1. Sensor Networks Lecture 8

  2. LEACH Clustering • LEACH: (Low Energy Adaptive Clustering Hierarchy) rotates cluster heads to balance energy consumption • Each cluster head performs its duty for a period of time • Each sensor makes an independent decision on whether to become a cluster head and if yes broadcasts advertisement packets

  3. LEACH Clustering (cont.) • Each sensor that is not a cluster head listens to advertisements and selects the closest cluster head • Once a cluster head knows the membership, a schedule is created for the transmission from sensors in the cluster to the cluster head to avoid collision (e.g., based on TDMA) • The cluster head can send a single packet to the base station (directly) over long distance to save energy consumption • No assurance of optimal cluster distributions

  4. HEED Clustering • HEED (Hybrid Energy-Efficient Distributed clustering) uses the residual energy info for cluster head election to prolong sensor network lifetime • Probability of a sensor becoming a cluster head is: • Clusters are elected in iterations: • A sensor announces its intention to become a cluster head, along with a cost measure indicating communication cost if it were elected a cluster head • A non-CH sensor picks a candidate with the lowest cost • A non-CH sensor not covered doubles its CHprob in iterations until CHprob is 1, in which case the sensor elects itself to the cluster head

  5. PEGASIS: Power-Efficient Gathering in Sensor Information Systems • A chain of sensors is formed for data transmission (could be formulated by the base station) • Finding the optimal chain is NP-complete • Sensor readings are aggregated hop by hop until a single packet is delivered to the base station: effective when aggregation is possible • Advantages: No long-distance data transmission; no overhead of maintaining cluster heads • Disadvantages: • Significant overhead: Can use tree instead • Disproportionate energy depletion (for sensors near the base station): Can rotate parent nodes in the tree

  6. Aggregation/Duplicate Suppression • Aggregation of information in a tree structure • In-network information processing such as max, min, avg • Duplication Suppression: • On forwarding messages, sensor nodes whose values match those of other sensor nodes can simply annotate the message • Or just remain silent, on overhearing identical (or “similar enough”) values

  7. Querying a Sensor Network • Can have sensor nodes periodically transmit sensor readings • More likely: Ask the sensor network a question and receive an answer • Issues: • Getting the request out to the nodes • Getting responses back from sensor nodes who have answers • Routing: • Directed Diffusion Routing • Geographic Forwarding (such as Geocasting)

  8. Query-Oriented Routing • For query-oriented routing: Queries are disseminated from the base station to the sensor nodes in a feature zone • Sensor readings are sent by sensors to the base station in a reverse flooding order • Sensor nodes that receive multiple copies of the same message suppress forwarding

  9. Query: Asking a Question

  10. Response to Base Station:Initial

  11. Directed Diffusion Routing • Direction: From source (sensors) to sink (base station) • Positive/negative feedback is used to encourage/discourage sensor nodes for forwarding messages toward the base station • Feedback can be based on delay in receiving data • Positive is sent to the first and negative is sent to others • A node will forward with low frequency unless it receives positive feedback • This feedback propagates throughout the sensor network to suppress multiple transmissions • Eventually message forwarding converges to the use of a single path with data aggregation for energy saving from the source to the base station

  12. Responses, After Some Guidance • Use directed diffusion based on positive/negative feedback to guide response message forwarding

  13. Directed Diffusion Routing Cont. • Pros • On demand route setup • Each node does aggregation and caching, thus good energy efficiency and low delay • Cons • Query-driven, not a good choice for continuous data delivery • Extra overhead for data matching and queries

  14. Geographic Routing [Ref. 11] • For dense sensor networks such that a sensor is available in the direction of routing • Location of destination is sufficient to determine the routing orientation • Research issue: • selecting paths with a long lifetime for delivering messages between sensors, or from sensors to a base station without excessively consuming energy • Determining paths that avoid “holes” – determining the boundary or perimeter of a hole through local information exchanges periodically to trade energy consumption (for hole detection) vs. routing efficiency

  15. Geographic Forwarding

  16. References • Chapters 8-11, F. Adelstein, S.K.S. Gupta, G.G. Richard III and L. Schwiebert, Fundamentals of Mobile and Pervasive Computing, McGraw Hill, 2005. • Other References: • 10. X. Yu, “Distributed cache updating for the dynamic source routing protocol,” IEEE Transactions on Mobile Computing, Vol. 5, No. 6, pp. 2006, pp. 609-626. • 11. S. Wu and K.S. Candan, “Power-Aware Single and Multipath Geographic Routing in Sensor Networks,” Ad Hoc Networks, Vol. 5, 2007, pp. 974–997.

  17. Fault Tolerance and Reliability • Sensor nodes are more susceptible to failure because of direct exposure to the environment and energy depletion • Failure and fault recovery are basic assumptions: incorporate redundancy to cope with failure • Performing consensus in a cluster for high reliability of measurement • Clustering based on sensing responsibility • Static vs. dynamic grouping • Dynamic grouping does not need to maintain state information and is more accurate (near the event) but incurs overhead in forming the group and reaching consensus

  18. Searching for Agreement: StaticGrouping

  19. Searching for Agreement: DynamicGrouping

  20. MAC Layer Protocols • IEEE 802.11 scheduling protocols are not suitable for wireless sensor networks because: • With RTS/CTS (Request to Send / Clear to Send) , collision can still occur because of hidden/expose terminal problems • Listening to traffic to avoid collision requires the nodes to stay on • TDMA is more suitable (requiring clock synchronization) • A number of reservation mini-slots can be used to reserve each of the transmission slots • Sensors can indicate whether or not they wish to transmit a message during the scheduling time segment • Nodes that are not planning to send or receive a packet need to stay on only during the reservation time slot to see if other sensors are sending a packet to them • Collisions are avoided, except for small reservation packets

  21. Tradeoff between Energy Efficiencyand Reliability/Performance • An important design issue • Improved reliability vs. energy consumption • Aggregating sensor readings vs. loss of information • Energy-efficient protocols often involve increased delay, loss of accuracy, reduced reliability and/or other performance penalty • Direct sensor-BS transmission vs. sensor-CH-BS • Sensor readings with redundancy • Achieving application requirements while prolonging lifetime is a major challenge

  22. Fault Tolerant Data Propagation • Reference: [12] listed at the end • Use path redundancy to cope with sensor “reading” faults • One path (no redundancy) • Multiple paths to return sensor readings and a majority voting of the first three readings returned is performed to cope with faults • For example, use Time To Live (TTL) to indicate how many hops a sensor reading message is to be propagated, thereby creating multiple paths to propagate the sensor reading message from source to sink

  23. Fault Tolerant Data Propagation • Source: node A • Sink: node I • When TTL = 3 hops, there are 7 paths from A to When TTL=4 hops, there are 21 paths

  24. Fault Tolerant Data Propagation • An example • Source: node E • Sink: node I • p: link fault probability (causing • reading error) • q: node fault probability (causing • reading error) • TTL=1: Reliability is 1-p • TTL=2: what is the reliability? • Three possible paths: E->I, E->H->I, E->F->I, with fault probability of p A A • System fails when two out of three paths fail, so reliability is 1-pA2-2pA(1-A)-(1-p)A2 where A=1- (1-q)(1-p)2 =2p+q-2pq-p2+p2q • The more the path redundancy, the higher the reliability at the expense of more energy consumption

  25. Energy Efficiency • Metric: Mean Time to Failure (MTTF) • Time till the first node dies (not useful) • Time half of the sensor nodes die (too arbitrary) • Time when the sensor network can no longer perform its intended function (yeah!) • Difficult to define precisely • Designing protocols so that • All the sensors die at roughly the same time • Sensors die in random locations instead of in specific locations

  26. Balancing Energy Consumption • Clustering – is it always good? • Triangular routing: sensors -> cluster head -> base station • Overhead in selecting and rotating among sensors to be cluster heads • Good only if message aggregation is feasible; otherwise directly sending sensing readings to the base station may end up saving energy more

  27. Energy-Efficient Clustering • Reference: [13] listed at the end • Two key parameters: • p: probability of a sensor becoming a cluster head • k: number of hops covered by a cluster • Find optimal (p, k) that would minimize the energy consumed

  28. Energy-Efficient Clustering:Formulation • Sensors are distributed following a homogeneous spatial Poisson process with intensity → in a square area of size 4a2 • Per-hop distance is r • Energy model: each sensor uses 1 unit of energy to transmit or receive 1 unit of data • The information processing center is in the middle of the area • Idea: Define a function for the energy used and find (p, k) that would minimize the energy used

  29. On Optimal Path and SourceRedundancy in Sensor Networks • Reference: [14] listed at the end • Analyze the effect of redundancy on MTTF and determine the optimal path and source redundancy level to maximize MTTF while satisfying reliability (Rreq) and timeliness (Treq) QoS requirements in WSNs. • Develop a hop-by-hop data delivery mechanism utilizing source and path redundancy with the goal to satisfy QoS requirements while maximizing the lifetime of the sensor system • Query: must return a sensor reading to the PC within the real-time deadline.

  30. Cluster based WSN architecture

  31. Hop-by-Hop Data DeliveryProtocol

  32. Hop-by-hop Data DeliveryProtocol • Based on localized geographic routing • Path redundancy: Form m paths from a source CH to the PC: • m SNs in hop one relay the data through broadcasting • only one SN relays the data in each of the subsequent hops in each path • Source redundancy: Each of the ms SNs to communicate with the source CH through a distinct path: • only one SN relays the data through broadcast in each of the subsequent hops in each path

  33. Probability Model • System MTTF - Total number of queries the system can answer before it fails due to energy depletion, sensor faults, or channel error • Rq - Reliability of a query as a result of applying the hop-by-hop data delivery mechanism with m paths for path level redundancy and ms sensors for source level redundancy

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