Coordination: Topology control for sensor area and communication coverage

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CoordinationTopology control for sensor area
and communication coverage
Tutorial
Ivan Stojmenovic Ivan_at_site.uottawa.ca www.site.uot
tawa.ca/ivan
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Why sensors should mostly sleep ?

• Active state Transmitting, Receiving, Idle
• Sensors in active state spend considerably more
  energy than node in sleep state
• Differences between idle and transmitting /
  receiving energy consumption not major
• T R I S 13 9 7 1 CJBM, MIT, 2001
• Topology control aims at keeping active minimal
  number of sensors while preserving proper
  functioning

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Top view

• Select sensors for area coverage
• Select sensors for communication coverage
  backbone
• Sensors are sleeping, idle (sensing only) or
  active (sensing and communicating)
• (sensors may specialize)
• Data communication is performed on backbone
  (active) nodes, and thus not on
• Very sparse or very dense networks

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Backbone creation

• Clustering
• Grid partitioning
• Energy based activity decisions
• Connected dominating sets
• Sensor area coverage

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To cluster or not?

• Sometimes natural organization military
• Used in most existing sensornets

Clustering by self-organization Local changes
may trigger global updates Better organizations
exist !
Sensed objects
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LEACH Low Energy Adaptive Clustering Hierarchy

• W. R. Heinzelman, A. Chandrakasan, and H.
  Balakrishnan (MIT),
• Hawaii Int. Conf. on System Sciences, 1-10,
  January, 2000.
• Each node randomly decides whether or not to
  become clusterhead (CH) (parameter percentage of
  desired clusterheads) if so, sends a packet with
  the decision
• Each node reports to CH with highest signal
  strength (Voronoi diagrams are clusters)
• CHs assign time slots for reporting
• CHs aggregate data and send directly to sink
• Repeat CH selection periodically
• Problems optimal number of clusters? Sink very
  far for direct transmission? Overhead for cluster
  creation?

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Grid partitioning

• Xu, Heidemann, Estrin Mobicom 2001 partition
  into squares, choose 1 node from each square ?
  suboptimal, needs parameter, can disconnect
  graph

TR
Example with minimal transmission radius (TR)
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Example arbitrary TR/length ratio

• partition into squares, choose 1 node from each
  square ? can disconnect graph

TR
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Energy based activity decision

• Pearlman, Deng, Liang and Haas 2002 probability
  of a node to be awake is proportional to the
  ratio of the remaining energy over its initial
  energy ? too many active nodes at beginning,
  too little at end
• Feeney 2002 each station is awake a bit over
  half the time ? energy savings limited 50

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Connected dominating sets
Each node either in dominating set or has a
neighbor from dominating set Flooding reduced if
only nodes in connected dominating set nodes
retransmit
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Dominating sets by covering
Jie Wu and students 1999-02
K
Keys AltBltClt.
C
DS
covered
J
E
L
A
G
I
F
F covered by I and L H covered by I
B
D
H
Not intermediate (no two unconnected neighbors)
F covered by I, L, ? I, L, connected and any
neighbor of F is neighbor of one of I, L,.. and
key(F) lt min (key(I), key(L), )
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Dominating sets by covering
Jie Wu and students 1999-02
Key (degree, ID)
D
I
B
No two unconnected neighbors
J
covered
A
C
H
G
F
M
K
L
E
I covered by A, H, ? A, H, connected and any
neighbor of I is neighbor of one of A, H,.. and
key(I) lt min (key(A), key(H), )
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Generalized covering rule

• Dai and Wu 2002
• Covering A by few connected neighbors
• Construct subgraph G of higher Id neighbors
• If G empty or disconnected then A in DS
• If G connected but exists neighbor of A which is
  not neighbor of any node from G then A in DS
• Otherwise A is covered and not in DS
• Dijkstras shortest path to test connectivity
• No message exchange to decide DS status !!
• Localized maintenance

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SPAN

• MIT CJBM Mobicom 2001, SPAN
• A node becomes coordinator if it discovers that
  two of its neighbors cannot communicate with each
  other directly or through one or two existing
  coordinators.
• Coordinators are not necessarily neighbors, ?
  3-hop neighboring topology knowledge is required.
• Blough, Santi Mobicom 2002 overhead explodes
• Variant of Wu/Li, with worse performance

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Sensor area coverage
Select sensors that are needed for connected area
coverage, other sensors to sleep mode
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Area coverage threshold distance

• Ye, Zhong,Chen, Lu, Zhang 2003 PEAS
• Asynchronous, no prior neighbor knowledge
• A sensor sleeps for a while, then sends probing
  packet
• It decides to be active if and only if there is
  no active sensor closer than a threshold distance
• Once active, it remains active until life ends
• Non-active periodically reevaluates decision
• High probability of full coverage if threshold lt
  0.3 sensing radius

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PEAS Example
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Coverage may not be complete
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Synchronous unit graph model
Equal communication range CR, equal sensing range
SR Time clocks at nodes are synchronized
Zigbee requirement Nodes in sleep state wake
at predetermined time Sleep scheduling in rounds
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Area coverage withdrawal messages

• Tian, Georganas 2002 - Synchronous
• Each sensor knows position of all neighbors
• If neighbors cover its sensing area then sensor
  sends withdrawal message after timeout negative
  acknowledgement
• Repeats periodically
• Neighbor sensors may disappear without notice
• Covering sensors may not be connected
  reporting to MS may fail
• Sensing radius communication radius
• Same or different sensing radii

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Jiang, Dou - improvements

• Neighbor discovery phase
• Each node broadcasts one hello message
• Perimeter coverage criterion
• All ratios of sensing and transmission radii
• Random backoff by each sensor
• sensor active if its area is not covered at end
  of timeout
• Withdrawal messages sent negative
  acknowledgements by sleeping sensors

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Area coverage criterion

• Theorem If there are at least two covering
  circles and any intersection point of two
  covering circles inside sensing area is covered
  by a third covering circle then
• Sensing area is covered !

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Intersection based coverage evaluation scheme
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Connectivity criterion

• Communication range ? 2 (Sensing range) ?
  coverage implies connectivity
• Otherwise neighbors that cover node area must be
  tested for connectivity before node can decide to
  sleep.

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Localized Sensor Area Coverage withLow
Communication Overhead

• Gallais, Carle, Simplot-Ryl, Stojmenovic IEEE
  PerCom 2006
• Neighbor knowledge not required !!
• Each node selects random timeout
• Cases sr, sltrlt2s, 2sltr, in rounds
• Transmission contains the position of node
• At end of timeout, if all area is not covered,
  transmit and active otherwise sleep (with or
  without transmission)
• If active but later area covered by other active
  ?retreat
• Covered active neighbors are connected and
  together cover its sensing area

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Connected coverage ? sleep
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disconnected coverage ? active
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Negative ack helps

• Nodes 1,2,3,4 are active, Node 5 decides to be
  inactive
• If node 5 does not announce its deactivation,
• Node 6 decides to be active
• Else, node 5 announce its status
• Node 6 decides to be inactive

Node 5 informs node 6 by Neg ack that shaded area
is covered
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Positive-only acknowledgements
1
?
5
?
active
1
2
7
active
?
2
?
6
6
active
3
4
?
3
sleep
5
active
?
7
4
?
active
active
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Positive and Negative acks
1
5
active
1
2
7
active
Im OFF
2
?
6
6
active
3
4
3
sleep
5
active
?
7
4
sleep
active
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Retreat messages
Red node made decision before all black node
decisions gt It decides to be active
Afterwards, it learns that all black nodes are
active and cover it
Red node then changes its mind and sends retreat
message
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Proposed variants

• Positive only acks PO
• Positive and negative acks PN
• Positive and retreat acks PR
• Positive, negative and retreat acks PNR
• Retreat After node already sent a positive ack,
  it may discover that it becomes covered by active
  neighbors after hearing more positive acks
  afterwards it then sends retreat ack

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Analysis

• N the number of deployed sensors, NAS
• A the number of active nodes
• S the number of sleeping nodes
• R the number of retreat nodes
• Jiang/Du NS messages
• PO A messages
• PN AN messages
• PR A R messages
• PNR ANR messages

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Message overhead, coverage, active nodes
Ideal MAC, no message collisions TGDJ, PN, PRN
increase message overhead with density Active
nodes (one scenario) TGDJ 19, PNR 20, PR 24,
PN 30, PO 35 Contention window MAC
Integer timeouts between 1 and 32 What is the
impact on the performance ?
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Why message overhead is to be minimized
Many withdrawal messages are not received, and
number of active nodes increases in TGDJ, PN,
PRN! Also many neighbors are not discovered in
TGDJ! When neighbor message in TGDJ received, but
withdrawal message from it not received, node may
decide to sleep leaving coverage holes! PO and PN
preserve 100 coverage but have higher active
nodes PR 99, PRN 95 TGDJ 60 coverage (one
scenario) PR nearly preserves coverage with ?10
more active nodes than TGDJ
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Future work

• Random backoff could be replaced by timeout that
  depends on portion of area being covered
• To increase reliability active neighbors
  connected and together cover its sensing area
  twice or k times
• How many times an area can be covered with given
  sensors (layering) ?
• Realistic physical layer of sensing coverage
  instead of unit disk graph model
• Avoiding simultaneous or chain retreats