1 On Improving Data Accessibility in Storage Based Sensor Networks Tan Apaydin, Serdar Vural and Prasun Sinha IEEE International Conference on Mobile Adhoc and Sensor Systems, 2007 [ MASS 2007 ]

2 Outline Introduction Preliminaries Methodology Experimental Results Conclusion

3 Introduction Limited amount and fast depletion of resources restrict information storage capability It is critical to ensure that the data can be accessed from other nodes for a long period of time Data must be moved to other locations to avoid loss of critical information

4 Motivation Determining more suitable locations to improve data accessibility To design efficient and scalable solutions to the problem

5 Preliminaries Emergency Incidence A Measure of Node Ranking: Accessibility

6 Emergency Incidence Node’s energy is below a certain amount Critically low energy levels of the neighbors of a particular node

7 A Measure of Node Ranking: Accessibility Data should be available to a large number of nodes Nodes that need the data should have long periods of connection  There is at least one path between two connected nodes  The node with the least energy on a multi-hop path constitutes the bottleneck node of that path  The lifetime of the connection between two nodes is proportional to the bottleneck energy between them

8 Availability β A,A = 30 β C,A = 15 β D,A = 20 β B,A = 20 Availability (β m,n ) of a node m to another node n is the maximum bottleneck energy over all paths between m and n

9 Accessibility(1/2) Accessibility (Ψ(n)) of a node n is the sum of availabilities of all nodes in the network to n Ψ(A) =β A,A + β B,A + β C,A + β D,A = = 85 (1)

10 Accessibility(2/2) Ψ(A) = 85 Ψ(B) = 115 Ψ(C) = 60 Ψ(D) = 75

11 Methodology Accessibility calculation within a segment Diffusion of accessibility through segments Incorporating all segments into the calculation

12 Accessibility Calculation Within a Segment Centralized accessibility calculation within a segment Distributed accessibility calculation within a segment

13 Centralized Accessibility Calculation Within a Segment To reduce the packet exchange traffic for frequent accessibility calculations by the segment nodes

14 Accessibility Tree Accessibility Tree of a node n is a tree such that the path on the tree from a node to n is a widest path in the given topology A greedy algorithm similar to the Dijkstra’s algorithm A (30) D (20) C (15) B (60) Cost(B, D) =1/min{E(B), E(D)} =1/20 Cost(A, D)=1/20 Cost(A, C)=1/15 Cost(B, C)=1/15

15 Distributed Accessibility Calculation Within a Segment The distributed algorithm allows the nodes to compute their accessibility information periodically B e : 25 B e : 20 B e : Bottleneck energy

16 Pseudo Code

17 Diffusion of Accessibility Through Segments

18 Accessibility of Node n Ψ(B 1 ) = 40, n 1 = 4 Ψ(B 2 ) = 50, n 2 = 5 E(B 1 ’) = 40/4 = 10 E(B 2 ’) = 50/5 = 10 Ψ(n) has to add n 1 * β B1’,n = 4 * 15 = 60 n 2 * β B2’,n = 5 * 10 = 50 Into accessibility

19 Incorporating All Segments into The Calculation

20 Calculate a Single Summarization Value from Segment II to I

21 Ψ(B 1 ) = 40, n 1 = 4 Ψ(B 2 ) = 50, n 2 = 5 E(B 1 ’) = 40/4 = 10 E(B 2 ’) = 50/5 = 10 E(A 1 ) = 8 E(A 2 ) = 10 Scheme 1: Mean = (8+10)/2 = 9 Scheme 2: Maximum = 10 Scheme 3: Weighted =(4*8 + 5*10)/9 =9.11

22 Calculation of Inter-segment Availabilities The availability of segment D to A is, β D,A =

23 Optimum Path Inside Minimum Bounding Rectangle (MBR)

24 Finalizing Accessibility Calculation Method 1: Summation of Segment Availabilities Method 2: Segment Grouping and Average Availabilities

25 Experimental Results Experimental Setup Global vs. Estimated Accessibility

26 Experimental Setup Uniform distribution of N sensors with density σ in a 400 x 400 unit 2 field Each topology is divided into 16 segments Sensors have disk-shaped communication ranges with radius R

27 Global vs. Estimated Accessibility

28 The Effects of R and σ on the Accessibility Calculation

29 Scheme 1: MeanScheme 2: Max Scheme 3: Weighted

30 Average Percent Error in Accessibility Calculations

31 Conclusion Relocate the data of sensors with critically low energy resources to other nodes in the network with higher accessibility values Partitioning the network into segments Provide both centralized and distributed approaches calculate accessibility within segments