Sensor Networks Lecture 8 LEACH Clustering LEACH Low

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 • Each sensor makes an independent decision on whether to become a cluster head and if yes broadcasts advertisement packets

(. 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

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

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

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

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)

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

Query: Asking a Question

Response to Base Station: Initial

Directed Diffusion Routing • Direction: From source (sensors) to sink (base station) • Positive/negative feedback is used to encourage/discourage sensor nodes 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

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

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

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

Geographic Forwarding

References • Chapters 8 -11, F. Adelstein, S. K. S. Gupta, G. G. Richard III and L. Schwiebert, Fundamentals of Mobile and Pervasive Computing, Mc. Graw 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.

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

Searching for Agreement: Static Grouping

Searching for Agreement: Dynamic Grouping

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

Tradeoff between Energy Efficiency and 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

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

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

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 -p. A 2 -2 p. A(1 -A)-(1 -p)A 2 where A=1 - • (1 -q)(1 -p)2 =2 p+q-2 pq-p 2+p 2 q The more the path redundancy, the higher the reliability at the expense of more energy consumption

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

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

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

Energy-Efficient Clustering: Formulation • Sensors are distributed following a homogeneous spatial Poisson process with intensity → in a square area of size 4 a 2 • 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

On Optimal Path and Source Redundancy 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) Qo. S requirements in WSNs. • Develop a hop-by-hop data delivery mechanism utilizing source and path redundancy with the goal to satisfy Qo. S requirements while maximizing the lifetime of the sensor system • Query: must return a sensor reading to the PC within the real-time deadline.

Cluster based WSN architecture

Hop-by-Hop Data Delivery Protocol

Hop-by-hop Data Delivery Protocol • 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

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