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Differentiated Surveillance for Sensor Networks

Differentiated Surveillance for Sensor Networks. Ting Yan, Tian He, John A. Stankovic. CS294-1 Jonathan Hui November 20, 2003. Idea. Exploit node density/redundancy to maximize effective network lifetime. Degree of coverage matters! Sensing constraints Fault tolerance. Assumptions.

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Differentiated Surveillance for Sensor Networks

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  1. Differentiated Surveillance for Sensor Networks Ting Yan, Tian He, John A. Stankovic CS294-1 Jonathan Hui November 20, 2003

  2. Idea • Exploit node density/redundancy to maximize effective network lifetime. • Degree of coverage matters! • Sensing constraints • Fault tolerance

  3. Assumptions • Static placement • Localization • Time Synchronization • Uniform expected node lifetime • For simplicity of describing protocol? • Nodes on 2D plane • Circular sensing radius r • Communication range > 2r

  4. Basic Protocol • Initialization Phase • Localization, Time Sync, Determine Working Schedule (T, Ref, Tfront, Tend) • Sensing Phase • Nodes power on and off based on working schedule Round 0 (T) Round 1 (T) Round i (T) Ref Ref Ref t Tfront Tend Tfront Tend Tfront Tend Sensing Phase Init Phase

  5. Basic ProtocolDetermining Working Schedule • Goal: Each node determines its own working schedule such that all points within sensor coverage are covered for all time. • Approach: Represent sensor coverage with grid of points

  6. Basic ProtocolDetermining Working Schedule • Reference Point Scheduling Algorithm • Randomly choose Ref from [0, T) and broadcast to all nodes within 2r. • For each discrete point • Order neighboring Ref times and calculate • Tfront = [Ref(i)-Ref(i-1)]/2 • Tend= [Ref(i+1)-Ref(i)]/2 • Final schedule = union of schedules for all points RefC RefA RefB Point 1 RefC RefA RefB Node A: RefD RefA RefE RefD RefE Point 2

  7. Example (a = 2) Example (a = 3) RefB RefC RefB RefC RefA RefA A A B B Uh-Oh! C C Enhanced Protocolwith Differentiation Example (a = 1) • Working schedule for a desired coverage of degree a. • (T, Ref, Tfront, Tend, a) • Working period defined as: • Power On: • Power Off: RefB RefC RefA A B C

  8. Design Issues • Possible blind spots with discrete points • Choose points within sensing range conservatively • Offset in time synchronization • Power on (off) slightly earlier (later) • Irregular sensing regions • Okay, as long as sensing regions of neighboring nodes are known • But also requires knowledge of orientation • Fault Tolerance • Awake nodes use heartbeat messages to detect failed nodes • If a node fails, wakeup all nodes within 2r and reschedule. • What if communication range < 2r?

  9. Extensions and Optimizations • Second Pass Optimization • After determining working schedule, broadcast schedule to all nodes within 2r. • The node which has the longest schedule: • Minimize Tfront and Tend while maintaining sensing guarantee • Rebroadcasts new schedule • Done when every node has recalculated schedule or when no more can be done.

  10. Extensions and Optimizations • Multi-Round Extension for Energy Balance • Calculate M schedules each with different Ref values during Init Phase. • Rotate schedules during Sensing Phase. RefB RefC RefA RefB RefC RefA RefA RefB RefC RefA RefC RefB A B C Schedule 1 Schedule 2 Schedule 3 Schedule 4

  11. Evaluation • Simulation parameters • Nodes distributed randomly with uniform distribution in 160mX160m field. • Results taken from center 140mX140m to avoid edge effects • Sensing range = 10m • Communication range = 25m • Ideal conditions • Fault tolerance included? • Compare against sponsored approach

  12. Evaluation • Total energy consumption nearly constant with changes in density. • Variation in total energy consumed decreases with greater densities. • What’s happening with the sponsored approach?

  13. Evaluation • Half-life increases linearly as density increases. • Coverage provided for longer period of time.

  14. Evaluation • Energy consumption increases linearly with different degrees. • Energy consumption constant with different densities. • Degree of coverage provided >= a. • a only guarantees a lower bound.

  15. Comments • Localized algorithm? • But still requires time synchronization and doesn’t support mobility • Inflexible • mobility not supported, schedules are fixed • No notion of the “goodness” of a node • Nodes that have more energy should take up a larger portion of the working schedule • Difficult to reliably broadcast Ref values to all 2r neighbors in a dense network • Only have one chance to get it right! • Worse in cases where communication range < 2r (i.e. acoustic sensors)

  16. Comments • Working schedules determined without taking other schedules and protocols into account • How does it affect other protocols (i.e. TDMA)? • Comparison to Sponsored Coverage unfair • Sponsored Coverage supports fault tolerance, limited mobility, and is more adaptable • Ability to specify degree of coverage • But current algorithm doesn’t correctly guarantee with a > 2! • Fault tolerance relies on communication range > 2r for heartbeat messages

  17. Conclusion • Pros • Improved performance in lifetime and workload balance • Specify a degree of coverage • Cons • No upper bound on degree estimation • Inflexible • Static working schedule, static nodes, time synchronization, reliable communication

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