The Chinese Univ. of Hong Kong
Node Scheduling Schemes for Coverage Preservation and Fault Tolerance in Wireless Sensor Networks
Chen XinyuGroup Meeting
2004-10-25
Dept. of Computer Science and Engineering
Outline
MotivationK-coverage sleeping candidate conditionNode scheduling schemes
Round-basedAdaptive sleeping
Performance evaluationsConclusions
Dept. of Computer Science and Engineering
Wireless Sensor Networks
Composed of a large number of sensor nodes
Sensors communicate with each other through short-range radio transmission
Sensors react to environmental events and relay collected data through the dynamically formed network
Dept. of Computer Science and Engineering
Applications
Military reconnaissance Physical security Traffic surveillance Industrial and
manufacturing automation
Distributed robotics Environment monitoring …
Dept. of Computer Science and Engineering
Problems
The energy source is usually battery power
Battery recharging or replacement is undesirable or impossible due to the unattended nature of sensors and hostile sensing environments
Sensors may fail or be blocked due to physical damage or environmental interference
Dept. of Computer Science and Engineering
Concerns
A good coverage-preserved and fault-tolerant node scheduling scheme should have the following characteristics: It should allow as many nodes as possible to turn
their radio transceivers and sensing functionalities off to reduce energy consumption, thus extending network lifetime
Enough nodes must stay awake to form a connected network backbone and to preserve area coverage
Void areas produced by sensor failures and energy depletions should be recovered as soon as possible
Dept. of Computer Science and Engineering
Outline
MotivationK-coverage sleeping candidate conditionNode scheduling schemes
Round-basedAdaptive sleeping
Performance evaluationsConclusions
Dept. of Computer Science and Engineering
Problem Formulation
Each sensor node Ni knows its location (xi, yi), sensing radius ri, communication radius R
Sensing region SRi = { p | dip < ri }
The neighbor set of Ni, N(i) = { Nj S | dij ≤ R, j i }
Assuming that Nj N(i), R ≥ ri + rj
Ensures that coverage implies connectivity
Dept. of Computer Science and Engineering
Some Definitions
Ni
Nj
Sponsored Sensing Arc (SSA) ij
Sponsored Sensing Region (SSR)
Sponsored Sensing Angle (SSG) ij
Covered Sensing Angle (CSG) ij
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Special Cases of SSR and SSA
dij ≤ ri - rj
Ni
Nj
SSG ij =2CSG ij is not defined
Completely Covered Node (CCN) of Ni
Dept. of Computer Science and Engineering
Special Cases of SSR and SSA
dij ≤ rj - ri
Ni
Nj
Complete-Coverage Sponsor (CCS) of Ni
Degree of Complete Coverage DCC i = | CCS(i) |
SSG ij is not defined
CSG ij =2
CCS(i)
Dept. of Computer Science and Engineering
Minimum Partial Arc-Coverage (MPAC)
The minimum partial arc-coverage (MPAC) sponsored by node Nj to node Ni, denoted as ij, the number of Nj's non-CCSs covering the
point on the SSA ij that has the fewest nodes covering it.
Dept. of Computer Science and Engineering
Derivation of MPAC ij
0 2ij
jljm
ij = 2ij = 1
Covered Sensing Angle (CSG)
Sponsored Sensing Angle (SSG) ij
Dept. of Computer Science and Engineering
MPAC and DCC Based k-Coverage Sleeping Candidate Condition
K-coverageEvery point in the deployed area is covered
by at least k nodesTheorem
A sensor node Ni is a sleeping candidate while preserving k-coverage, iff i ≥ k or Nj N(i) - CCS(i), ij > k - i .
Dept. of Computer Science and Engineering
Extended Sleeping Candidate Condition
Constrained deployed area
Dept. of Computer Science and Engineering
Outline
MotivationK-coverage sleeping candidate conditionNode scheduling schemes
Round-basedAdaptive sleeping
Performance evaluationsConclusions
Dept. of Computer Science and Engineering
Round-based Node Scheduling Scheme
onsleeping
ready-to-sleeping
ready-to-on
uncertain
Tround
eligible / STATUSineligible
Tround
TwaitTwait
eligible / STATUS
ineligible / STATUS
on-sleeping decision phase1. Set a backoff timer Thello, a window timer Twin,
a wait timer Twait, and a round timer Tround
2. Collect HELLO messages from neighbors3. After Thello times out, broadcast a HELLO
message to all neighbors4. After Twin expires, evaluate the sleeping
eligibility according to sleeping candidate conditions
Approximately synchronized
Dept. of Computer Science and Engineering
Adaptive Sleeping Node Scheduling Scheme
A node may suffer failures or deplete its energy loss of area coverage
Round-based: timer Tround is a global parameter and not adaptive to recover a local area loss
Letting each node calculate its sleeping time locally and adaptively
Dept. of Computer Science and Engineering
Adaptive Sleeping Node Scheduling Scheme
1. Set a timer Tsleeping 2. When Tsleeping times out, broadcast a PROBE
message3. Each neighbor receiving the PROBE message will
return a STATUS message to the sender4. Evaluate sleeping eligibility. If eligible, set Tsleeping
according to the energy information collected from neighbors
Dept. of Computer Science and Engineering
Performance Evaluation
ESS: extended sponsored sector Proposed by Tian et. al. of Univ. of Ottawa, 2002 Consider only the nodes inside the SR of the
evaluated node Mpac: round-based scheme with elementary
MPAC condition MpacB: round-based scheme with extended
MPAC condition in constrained area MpacBAs: adaptive sleeping scheme with
MpacB
Dept. of Computer Science and Engineering
Performance Evaluation (1)
Sensor number vs. sensing radius
Dept. of Computer Science and Engineering
Performance Evaluation (2)
Standard deviation of sensing radius
Dept. of Computer Science and Engineering
Performance Evaluation (4)
Fault tolerance approachesAdaptive sleeping scheduling (k+1)-coverage scheduling
Provide one more coverage degree than the design requirement k
-coverage accumulated timeThe total time during which percentage of
the deployed area satisfies the coverage requirement
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Performance Evaluation (7)
System lifetime vs. live sensor
Dept. of Computer Science and Engineering
Conclusions
Develop MPAC-based node sleeping eligibility conditionsachieve k-coverage degreecan be applied with different sensing radii
Propose two fault tolerant approaches:Adaptive sleeping scheduling (k+1)-coverage scheduling
Identify that a tradeoff exists between sensing coverage and network lifetime