ACQUIRE

1. ACQUIRE ACTIVE QUERY FORWARDING IN SENSOR NETWORKS

2. OVER VIEW • ABSTRACT • INTRODUCTION • DESCRIPTION OF ACQUIRE • ANALYSIS OF ACQUIRE • ANALYSIS OF ALTERNATIVE APPROACHES • COMPARISION OF ACQUIRE,ERS and FBQ • DISCUSSION AND FUTURE WORK • CONCLUSION

3. ABSTRACT • A novel and efficient mechanism for obtaining information in sensor networks. • Considering the query as an active entity that is forwarded through the network in search of the solution. • ACQUIRE incorporates a look-ahead parameter d in which the intermediate nodes that handle the active query use information from all nodes within d hops inorder to partially resolve the query.

4. When the active query is fully resolved, a completed response is sent directly back to the querying node. • We take a mathematical modeling approach to calculate the energy cost associated with ACQUIRE. • The performance of ACQUIRE is compared with respect to other schemes such as Flooding-Based querying (FBQ) and Expanding Ring Search. • ACQUIRE obtains order of magnitude reduction over FBQ and potentially 60 to 85% energy reduction over ERS.

5. INTRODUCTION • Wireless sensor networks are envisioned to consist of large number of devices each capable of some limited computation, communication and sensing, operating in an unattended mode. • The key challenging in these networks is dealing with limite energy resources on the nodes. • With a small set of independent sensors it is possible to collect all measurements from each device to a central warehouse and perform data-processing centrally. • However for large networks this may not applicable.

6. There may be a central queries/data sink to respond the queries which led to the concept of data-centric information routing. • Depending upon the application, there are likely to be different queries in these sensor networks. The types of queries are • Continuous queries. • Aggregate queries. • Complex queries. • Queries for replicated data.

7. Basic Principle • Inject an active query packet into the network that follows a random trajectory through the network. • At each step, the node which receives the active query performs a triggered, on-demand, update to obtain the information from all neighbours within a look-ahead of d hops. • As this active query progress through the network it gets progressively resolved into smaller and smaller components until it is completely solved and is returned back to the querying node as a completed response.

8. A categorization of queries in sensor networks: the shaded boxes represents the query categories for which the ACQUIRE mechanism is well suited

9. Basic Description of ACQUIRE • In FBQ there is a clear distinction between the query dissemination and response gathering stages. The querier/sink first floods several copies of the query. Nodes with relevant data respond, if it is not a continuous, then the flooding can dominate the costs associated with querying. When the data aggregation is employed ,duplicate responses can result in suboptimal data collection in terms of energy costs. • By contrast in ACQUIRE there are no distinct query/response stages • The querier issues an active query which can be a complex query,i.e. can consists of several sub-queries, each corresponding to a different variable/interest.

10. The active query is forwarded step by step through a sequence of nodes. • At each intermediate step, the node which is currently carrying the active query utilizes updates received from all nodes within a look ahead of d hops in order to resolve the query partially . • New updates are triggered reactively by the active node upon reception of the query only if the current information it has is obsolete

11. After the active node has resolved the active query partially, it chooses a next node to forward this active query to. • The choice may be random or directed intelligently based on other information. • Thus the active query proceeds through the network, keeps getting smaller as pieces of it becomes resolved, until eventually it reaches an active node which is able to completely resolve the query. • At this point the active query becomes a completed response and is routed back directly to the originating querier.

14. Illustration of ACQUIRE with a one-hop look ahead (d=1)

15. Basic Model And Notation • A sensor network consists of X sensors • Let V={v1,v2,……..vn} be the N variables tracked by the network • Let Q={q1,q2,……….qn} be the queries that we have to find the answers, consisting of M sub-queries • Let SM be the average number of steps taken to resolve a query consisting of M sub-queries • d- be as the look-ahead parameter • The neighborhood of a sensor consists of all sensors within d hops away from it. • The following assumptions are made in the sensor placements.

16. The sensors are laid out uniformly in the region. • All the sensors have the same transmission range. • The nodes are stationary We model the size of a sensor’s neighborhood as a function of d, f (d), Which is assumed to be independent of the particular node. We also assume that all possible queries Q are resolved by this network.

17. Mechanism of Query Forwarding • The average energy associated to answer a query of size M is given by Eavg=((cEupdate+d)SM+α Eavg- Average energy Eupdate- Updated energy c- average amortization factor SM-average no. of steps to answer a query of size M

18. LOCAL UPDATE: If current information is not up-to-date, X sends a request to all sensors within d hops away. The request is forwarded hop by hop.The sensor who get request will then forward their information to x.This will be Eupdate FORWARD: After answering the query based on the information obtained, x then forwards the remaining query to a node that is chosen randomly from those d hops away.

19. Steps To Query Completion • We present a simple analysis of the average number of steps to qyery completion as a function of M,N, and f(d). • FIRST ORDER ANALYSIS: Probability of success in each trial is p=M/N Probability of Failure is q=(N-M)/N The number of trials till the first success i.e. the number of sensors from which information has to be fetched till one of the queries is given by p=(M-1)/N q=N/(M-1)

20. Let us define the following

22. Local Update Cost The energy spent in updating the information at each active node that is processing the active query Eupdate is calculated as follows Where N(i) is the number of nodes at hop i

23. Total Energy Cost Hence the average energy spent in answering a query of size M is given as follows Where is the expected no.of hops from the node where the query is completely resolved to the querier xThis is the cost of returning the completed response back to the querier node. This response can be returned along the reverse path in which case can be atmost dSM. Thus we have

24. Optimal Look-ahead If we ignore the boundary effects, we have f(d)=(2d(d+1))+1 for grid also N(i) =f(i)-f(i-1) =2i(i+1)-2(i-1)i =4i i.e. the number of nodes exactly i hops away from a node x on a grid is 4i

25. Thus And upon solving the above equation we have the final energy as follows If d==0.Since no look-ahead is involved,Eavg is independent of c and d

26. Effect of c on ACQUIRE The above figure shows the energy consumption of the ACQUIRE scheme for different amortization factors and look-ahead values. Let d* be the look-ahead value which produces the minimum average energy consumption It appears d* significantly depends on the amortization factor

27. This figure shows that as the amortization factor c decreases, d* increases. i.e. as the query rate increases and the network dynamics decreases it is more energy-efficient to have a higher look-ahead.

28. Analysis of alternative Approaches Expanding Ring Search (ERS) The querier x* will request information from all sensor exactly one hop away. If the querier is not completely resolved in the first stage , x* will send a request to all sensor two hops away in the second stage.Thus in general at stage i, x* will request information from sensors exactly I hops away .The average no.of stages tmin taken to completely resolve a query of size M can be a approximately determined by the first order analysis.

29. Thus the total update cost is given as follows:

30. Total energy is given by

31. Flooding Based Query: In FBQ, the querier x* sends out a request to all its intermediate neighbors. These nodes in turn, resolve the query as much as possible based on their information and then forward the request to all their neighbors and so on. Thus the request reaches all the nodes in the network. IN FBQ 1. The request for triggered updates will have to be sent as far as R hops away from the querier x* where R is the radius of the network i.e. the maximum no.of hops from the centre of the grid 2. d=0, as the query is not forwarded. 3 a=0, as the query is completely resolved at the origin of the query itself 4. SM=1 Let Navg be the expected no. of nodes at hop I,that can resolve some part of the query.

32. Let Navg be the expected no. of nodes at hop I,that can resolve some part of the query. Thus for FBQ, Eavg is given as follows:

33. Comparison of ACQUIRE,ERS and FBQ Effect of c: The above figure shows the comparison of ACQUIRE*,ERS,ACQUIRE with d=0 and FBQ with energy on a log scale (left) and a linear scale (right). (For N=100 and M=20)

34. Effect of M/N: Both c and M/N seem to have a significant impact on the performance of ACQUIRE and ERS. As c increases and M/N increases, ACQUIRE achieves significant energy saving over ERS (and FBQ).

35. Discussion and Future work • One of our major next step is to convert ACQUIRE in to a functional protocol that can be validated on an experimental sensor network test-bed • Our analytical model of ACQUIRE assumes that query packet is always of a fixed size consisting of all the individual sub-queries and their responses. • The efficiency of ACQUIRE can also be improved if the neighborhoods of the successive active nodes in the query trajectory have minimal overlap.

36. One interesting result of our analysis is that the performance of ACQUIRE and the optimal choice of the look-ahead parameter d* are functions of the amortization factor c and independent of M,N, and the total number of nodes X. • ACQUIRE is meant to be used in situations where there is replicated data. • At the very least there should be one node in the network that can resolve each component sub-query. • Exploring the behavior of ACQUIRE on such topologies is a focus of our ongoing effort

37. We should mention that ,however that our results do already have some generality in this regard. • So as long as a reasonable model for f (d) can be developed for the network topology, the analysis presented here can be extended in a straightforward manner • In our modeling we have only counted the number of transmission for energy costs, although it is true that reception can also influence energy consumption. This is the case especially for broadcast message where there is no channel reservation and all the direct neighbors receive the message.

38. Conclusion • In conclusion we believe that ACQUIRE is a highly scalable technique energy-efficient at solving complex one-shot queries for replicated data. • We argue that ACQUIRE deserves to be incorporated into a portfolio of query mechanism for use in real world sensor networks. • We found that ACQUIRE with optimal parameter settings outperforms the other schemes for complex, one-shot queries in terms of energy consumption.

39. Specifically optimal ACQUIRE performs many orders of magnitude better than flooding-based schemes for such queries in large networks. • We also observe that optimal ACQUIRE can reduce the energy consumption by more than 60-75% as compared to other techniques.

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