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CS 851 Presentation:Differentiated
Surveillance for Sensor Network
Presented by Liqian LuoReference:1. T. Yan, T. He, and J. A. Stankovic, “Differentiated Surveillance for Sensor networks”, First ACM Conference on Embedded Networked Sensor Systems (SenSys 2003), Los Angeles, CA 2003
Assessment of the Paper
Pros The first algorithm to guarantee different degrees
of coverage for different requirements Good performance in power conservation and
balancing Cons
Pessimistic degree of coverage estimation Lack of flexibility
Require clock synchronization; Do not support mobility; work/sleep schedule never changes after decided; Expensive fault tolerance
Outline
Problem Statement Differentiated Surveillance solution
Introduction Design goals Assumptions Basic design without differentiation Enhanced design with differentiation
Extensions and Optimizations Related Work Evaluation Conclusion and Discussion
Roadmap
Problem Statement Differentiated Surveillance solution
Introduction Design goals Assumptions Basic design without differentiation Enhanced design with differentiation
Extensions and Optimizations Related Work Evaluation Conclusion and Discussion
Problem statement
How to provide sensing coverage for a sensor network in a power-efficient way?
Problem statement – Sensing Coverage
Problem statement – Sensing Coverage
Problem statement – Sensing Coverage
Problem statement – Degree of Sensing Coverage Current solutions regard the sensing coverage to a
certain geographic area as a binary. This paper argues that higher degree of sensing
coverage is desired to obtain high detection confidence since individual nodes are not reliable.
0
1
2
F
T
T
Roadmap
Problem Statement Differentiated Surveillance solution
Introduction Design goals Assumptions Basic design without differentiation Enhanced design with differentiation
Extensions and Optimizations Related Work Evaluation Conclusion and Discussion
Differentiated surveillance solution – Introduction Degree of coverage (DOC) Differentiated surveillance
Providing different degrees of sensing coverage for a sensor network according to different requirements
0
1
2
Differentiated surveillance solution – Introduction
DOC = 1 DOC = 2
Roadmap
Problem Statement Differentiated Surveillance solution
Introduction Design goals Assumptions Basic design without differentiation Enhanced design with differentiation
Extensions and Optimizations Related Work Evaluation Conclusion and Discussion
Differentiated surveillance solution – Design Goals Provide energy efficient sensing coverage for
a geographic area covered by sensor nodes extend system life
Reduce total energy consumption Reduce energy consumption variation among nodes
provide differentiated surveillance
Roadmap
Problem Statement Differentiated Surveillance solution
Introduction Design goals Assumptions Basic design without differentiation Enhanced design with differentiation
Extensions and Optimizations Related Work Evaluation Conclusion and Discussion
r
Differentiated surveillance solution – Assumptions Each node knows its own location and nodes are
not moving. Neighboring nodes are roughly time synchronized. The sensing area of a node is a circle with radius r
centered at the location of this node. Radio radius is larger than 2r
< 2r
Roadmap
Problem Statement Differentiated Surveillance solution
Introduction Design goals Assumptions Basic design without differentiation Enhanced design with differentiation
Extensions and Optimizations Related Work Evaluation Conclusion and Discussion
Basic design without differentiation – Goal Goal: find a work-sleep schedule for each
node which achieves 100% Sensing coverage guarantee.
Ideally we should consider each point in the area when do scheduling, but it is impossible because the number of points is infinite. What can we do?
For EACH POINT p in a certain geographic area, Guarantee that at ANY TIME, p is covered by at least one node’s sensing range.
Basic design without differentiation –100% sensing coverage Solution – 100% Grid point sensing coverage
Divide whole network into grids For each grid point x, guarantee that x is covered by at
least one node’s sensing range at ANY time
r
Basic design without differentiation –100% sensing coverage 100% Grid point sensing coverage = 100% sensing
coverage guarantee? No.
r
r
r
r
Basic design without differentiation –100% sensing coverage Solution – Conservative sensing radius (Rc)
Rc = r – d/ For each grid point x, guarantee that x is covered by at
least one node’s conservative sensing range at ANY time.
d
Rc
Rc
Rc
Rc
r
Basic design without differentiation - decide working schedule A schedule example
If we want to provide sensing coverage for point x, we can have either A or B or C awaken.
B
A
C
Point x
Node A
Node B
Node C
Waking Sleeping
0 10030 70
10 60
5 45
time
A scheduling example of A, B and C
Basic design without differentiation –decide working schedule Challenge: For each node, how to coordinate
with other nodes and decide its own schedule? Solution - Random Reference Point Scheduling
Algorithm
Basic design without differentiation –decide working schedule Concepts
Initialization Phase In this phase, nodes find their own positions,
synchronize time with neighboring nodes and decide their own working schedule.
Sensing Phase Nodes enter this phase after initialization phase and
choose to sense or sleep according to their schedules. Sensing Round - T
Sensing phase is divided into sensing rounds with equal duration T. A node has the same schedule for each round. Decide working schedule for sensing round T
Basic design without differentiation –decide working schedule Concepts
A node’s working schedule is determined by Four parameter tuple – (T, Ref, Tfront, Tend) Ref: a random time reference point chosen by a node
within [0, T) Tfront: the duration of time prior to Ref Tend: the duration of time after Ref. By this tuple, A node’s working period is determined as
follows: [T*j + Ref – Tfront , T*j + Ref + Tend)
And all the other time the node is sleeping.
Basic design without differentiation –decide working schedule Solution – Random Reference Point Scheduling Algorithm
1) Each node N chooses a “Reference Point (Ref)” randomly from [0, T) and broadcasts its Ref and position.
e.g. T = 100, RefA = 40, RefB= 90, RefC = 20
2) For each grid point P in its own sensing area, N sorts all the Refs from nodes (including N) which can also sense P in ascending order.
For A according to point P1, we have:
Ref(1) = RefC = 20, Ref(2) = RefA = 40, Ref(3) = RefB = 90
B
A
C
Point P10 refC refA refB
20 40 90
100
Basic design without differentiation –decide working schedule3) Assuming RefN is the (i)th Ref, N’s four parameter tuple is
computed as follows: TfronN = (Ref(i)- Ref(i-1))/2, 1<i<M TendN = (Ref(i+1)-Ref(i))/2, 1<i<MTfrontA = (Ref(2)-Ref(1))/2 = (40-20)/2 = 10TendA = (Ref(3)-Ref(2))/2 = (90-40)/2 = 25(T, RefA, TfrontA, TendA) = (100, 40, 10, 25)
4) N’s working period for point P (TwN(P)) is decided by:[T*j + RefN – TfrontN , T*j + RefN + TendN), j = 0, 1, 2, …
TwA(P1) = [100*j+40–10, 100*j+40+25) = [100*j+30, 100*j+65)
0
refC refA refB refCt
t20 40 90
30 65
Basic design without differentiation –decide working schedule5) Calculate the union of TwN(Px) for all grid points within N’s
sensing area, choose this union as the final working period of N (TwN).
TwA(P1)TwA(P2)TwA(P3)
TwA(Pn)
TwA
.
.
.
0 1005 65
6545
5 50…
Roadmap
Problem Statement Differentiated Surveillance solution
Introduction Design goals Assumptions Basic design without differentiation Enhanced design with differentiation
Extensions and Optimizations Related Work Evaluation Conclusion and Discussion
Enhanced design with differentiation
Provide different DOC according to different requirements
DoC = 3
DoC = 2
DoC = 1
Enhanced design with differentiation Goal
provide sensing coverage with DOC = a Solution
Extend 4-parameter tuple to 5-parameter tuple (T, Ref, Tfront, Tend, a)
Determine a node’s working period as follows: [T*j + Ref – Tfront*a , T*j + Ref + Tend*a)
Enhanced design with differentiation – An exampleSchedule for Grid Point P1 (a=1)
B
A
C
Point P1
(T, RefA, TfrontA, TendA) = (100, 40, 10, 25)
(T, RefB, TfrontB, TendB) = (100, 90, 25, 15)
(T, RefC, TfrontC, TendC) = (100, 20, 15, 10)
TwA = [T*j + Ref – Tfront , T*j + Ref + Tend)
= [100*j + 30, 100*j + 65)
TwB = [100*j + 65, 100*j + 105)
TwC = [100*j + 5, 100*j + 30)
A
refC refA refB
20 40 90
C
B
0
30 655
Enhanced design with differentiation – An exampleSchedule for Grid Point P1 (a=2)
(T, RefA, TfrontA, TendA, a) = (100, 40, 10, 25, 2)
(T, RefB, TfrontB, TendB, a) = (100, 90, 25, 15, 2)
(T, RefC, TfrontC, TendC, a) = (100, 20, 15, 10, 2)
TwA = [T*j + Ref – Tfront*2,T*j + Ref + Tend*2)
= [100*j + 20, 100*j + 90)
TwB = [100*j + 40, 100*j + 120)
TwC = [100*j -10, 100*j + 40)
A
refC refA refB
20 40 90
C
B
0
30 655
Question - Can the algorithm guarantee 100% DOC>=2 sensing coverage by setting a=2?Answer - Yes
Enhanced design with differentiation – An exampleSchedule for Grid Point P1 (a=3)
(T, RefA, TfrontA, TendA, a) = (100, 40, 10, 25, 3)
(T, RefB, TfrontB, TendB, a) = (100, 90, 25, 15, 3)
(T, RefC, TfrontC, TendC, a) = (100, 20, 15, 10, 3)
TwA = [T*j + Ref – Tfront*3,T*j + Ref + Tend*3)
= [100*j + 10, 100*j + 115) = T
TwB = [100*j + 15, 100*j + 135) = T
TwC = [100*j -25, 100*j + 50)
A
refC refA refB
20 40 90
C
B
0
30 655
Question - Can the algorithm guarantee 100% DOC>=3 sensing coverage by setting a=3?Answer - No
Enhanced design with differentiation – An extension to guarantee 100% DOC>=a
A
refC refA refB
20 40 90
C
B
0
30 655
My Extension to guarantee 100% DOC>=a sensing coverage Separate the time line into segments by using Refs and the
middle points between Refs Instead of expanding Tw by its own Tfront or Tend, expand one
segment on both sides when a is increased by 1.
Roadmap
Problem Statement Differentiated Surveillance solution
Introduction Design goals Assumptions Basic design without differentiation Enhanced design with differentiation
Extensions and Optimizations Related Work Evaluation Conclusion and Discussion
Optimizations and Extensions – Second Pass Optimization Existing Problem
Taking the union of Tw for all grid points within sensing range as final Tw will be more than efficient to provide coverage guarantee
Solution make a second pass
optimization to reduce the redundancy
B
A
1
2
TwA(1)
TwA
TwB(1)
TwB(2)
TwB
Optimizations and Extensions – Second Pass Optimization
Second Pass Optimization1)After getting the final Tw, each
node sends it to neighbors within the distance of 2r
2)Within 2r neighbors that have not recalculated their Tw, the one with the longest Tw recalculates its Tw and sends it to 2r neighbors
3) Repeat 2) until everyone has recalculated its Tw.
Why not the one with the shortest Tw?
B
A
1
2
TwA(1)
TwA
TwB(1)
TwB(2)
TwB
Optimizations and Extensions – Multi-Round Extension for Energy Balance Existing Problem Reference points are
selected randomly instead of uniformly, which results in big variation in Tw among nodes and big variation in power consumption.
Solution Multi-Round Extension
TwA
TwC
TwB
refCrefBrefA
Optimizations and Extensions – Multi-Round Extension for Energy Balance Multi-Round Extension
Instead of calculating a single schedule, calculate M schedules according to M independently selected random Refs for each node.
At each round T in sensing phase, the nodes choose one schedule consecutively.
TwA1 TwA1TwA2 TwA3 TwA2 TwA3
Roadmap
Problem Statement Differentiated Surveillance solution
Introduction Design goals Assumptions Basic design without differentiation Enhanced design with differentiation
Extensions and Optimizations Related Work Evaluation Conclusion and Discussion
Related Work – Communication Coverage SPAN, ASCENT: providing a communication
coverage within an energy conservation context
Related Work – Sensing Coverage 1 Energy Efficient Robust Sensing
Coverage: a probing-based mechanism After a sleeping node wakes up,
use a probing message to see whether there is another node working within its sensing area. If no, it takes the responsibility of sensing until it dies.
Drawbacks Overestimate neighbor’s
contribution, so no guarantee on sensing coverage
a
b
Related Work – Sensing Coverage 2 A Node Scheduling Scheme for
Energy Conservation: sponsored coverage scheme At the beginning of each
round, each node advertises its position to neighbors
After receiving neighbors’ position advertises, each node calculates its eligibility for going to sleep. Here, a back-off scheme is used to avoid simultaneous actions of multiple nodes.
Related Work – Sensing Coverage 2 Drawbacks
Require broadcasting at the beginning of each round Underestimate the area that the neighbor nodes can
cover
Roadmap
Problem Statement Differentiated Surveillance solution
Introduction Design goals Assumptions Basic design without differentiation Enhanced design with differentiation
Extensions and Optimizations Related Work Evaluation Conclusion and Discussion
Evaluation - Introduction
Nodes are distributed with a uniform random distribution in a 160 X 160 rectangle
Guarantee sensing coverage in the inner 140 X 140 rectangle to eliminate the edge effect
sensing radius = 10, communication radius = 25
160
160
140
140
Evaluation 1 – Energy Conservation
Total Energy Consumption per Unit of Time
Sponsored Coverage
Basic Design
2nd Pass Optimization
Evaluation 1 – Energy Conservation
Single Node Energy Consumption: Standard Deviation
Sponsored Coverage
Basic Design
Multiple Round Extension
?
Evaluation 1 – Energy Conservation
Half-life of the network
Sponsored Coverage
Basic Design
2nd Pass Optimization
Evaluation 2 – Sensing Coverage
Actual Degree of Coverage for Differentiated Surveillance
Roadmap
Problem Statement Differentiated Surveillance solution
Introduction Design goals Assumptions Basic design without differentiation Enhanced design with differentiation
Extensions and Optimizations Related Work Evaluation Conclusion and Discussion
Conclusion and Discussion
Conclusion Novelty - guarantee not only full sensing coverage
to a certain geographic area, but also sensing coverage with specific degree of coverage.
Scalability - localized distributed algorithm Power management - Good job in energy
conservation and balancing Robustness - fixed schedule throughout the life
time, expensive fault tolerant extension, can not work without clock synchronization, can not support mobility
?
Conclusion and Discussion
Discussion 1 This solution can not guarantee certain degree of
coverage more than 2. Discussion 2
Each node chooses its Ref randomly. What if multiple neighbors have the same Refs?
A simple solution is to order the same Refs by node ID.
Conclusion and Discussion
Discussion 3 In initialization phase, each node should send out
Ref broadcast and should receive all Refs from 2r neighbors. It is very hard in high density sensor network. So there must be some nodes which are ignored and have not attended the scheduling algorithm in initialization phase.
An extension, which allows these nodes to attend the scheduling later, is necessary.
Conclusion and Discussion
Discussion 4 Each node decides its working schedule only
based on sensing coverage. Some other layer protocols or applications may need a different working schedule. How to integrate with other working schedule will be a big problem.
Conclusion and Discussion
Discussion 5 The baseline - Sponsored coverage scheme can
provide fault-tolerance and support certain mobility since it updates neighbor hood information every round
DS without the expensive fault tolerance scheme can not provide fault-tolerance at all
So it is unfair to compare the power consumption between DS without fault-tolerance and the baseline with fault-tolerance.
Thanks!