Reliable Reporting of DelaySensitive Events in Wireless SensorActuator Networks

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Reliable Reporting of Delay-Sensitive Events in
Wireless Sensor-Actuator Networks

• Edith C.-H. Ngai, Yangfan Zhou, Michael R.
  Lyu, and Jiangchuan Liu
• Department of Computer Science Engineering,
  Chinese University of Hong Kong
• School of Computing Science, Simon Fraser
  University

The Third IEEE International Conference on Mobile
Ad-hoc and Sensor Systems (MASS'06), Vancouver,
Canada, 9-12 Oct 2006.
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Outline

• Introduction
• Related Work
• Network Model and Objective
• The Reliable Event Reporting Framework
• Grid-Based Data Aggregation
• Priority-Based Event Reporting
• Actuator Allocation
• Simulation Results
• Conclusion

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WSAN

• Collection of sensors and actuators
• Sensors
• small and low-cost devices with limited energy,
  sensing, computation, and transmission capability
• passive devices for collecting data only and not
  interactive to the environments
• Actuators
• resource-rich devices equipped with more energy,
  stronger computation power, longer transmission
  range, and usually mobile
• make decisions and perform appropriate actions in
  response to the sensor measurements

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WSAN

• Sensors and actuators collaborate
• sensors perform sensing and report the sensed
  data to the actuators
• actuators then carry out appropriate actions in
  response
• Applications
• environmental monitoring
• sensing and maintenance in large industrial
  plants
• military surveillance, medical sensing, attack
  detection, and target tracking, etc.

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Our Focus

• Design of a generic framework for reliable event
  reporting in WSANs
• Suggest a delay- and importance-aware reliability
  index for the WSANs
• Non-uniform importance of the events can be
  explored in the optimization

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Our Framework

• Seamlessly integrates three key modules to
  maximize the reliability index
• A multi-level data aggregation scheme, which is
  fault-tolerant with error-prone sensors
• A priority-based transmission protocol, which
  accounts for both the importance and delay
  requirements of the events
• An actuator allocation algorithm, which smartly
  distributes the actuators to match the demands
  from the sensors.

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Related Work

• Real-time communication protocol in WSN
• SPEED Hu et al. 2003
• Combines feedback control and non-deterministic
  QoS-aware geographic forwarding
• Velocity Monotonic Scheduling Lu et al. 2002
• Packet scheduling policy that accounts for both
  time and distance constraints
• MMSPEED Felemban et al. 2005
• Multi-path and multi-speed routing protocol for
  probabilistic QoS guarantee in WSN

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Related Work

• Reliable transmission with error-prone sensors
• Node-level fault tolerance (NLFT) Aidemark et
  al. 2005
• Masks transient faults locally by using
  time-redundant task scheduling in nodes
• Bi-criteria scheduling heuristic Assayad et al.
  2004
• Uses heuristic in data-flow graph to maximize
  reliability and minimize runtime
• Routing in DTN Jain et al. 2005
• Applies erasure code and data replication

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Related Work

• Heterogeneous sensor networks
• Anycast communication paradigm Hu et al. 2004
• Constructs an anycast tree rooted at each event
  source and updates the tree dynamically
• Power-aware many-to-many routing Cayirci et al.
  2005
• Actuator broadcasts registration messages, while
  sensors build their own routing tables
• Distributed coordination frameworkMelodia et al.
  2005
• Sensors forward readings to the appropriate
  actuators by the data aggregation trees

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Network Model

• Compose of sensors and actuators
• Nodes aware of their locations
• Divide the network into a number of grids cell
  for data aggregation
• A subset of nodes, referred as reporting nodes,
  send data to the actuators
• Anycast routing

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Objective

• Reliability index
• Measures the probability that event data are
  aggregated and received accurately within
  pre-defined latency bounds

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Workflow of Framework

• Data aggregation
• Delay- and importance-aware prioritized routing
• Actuator allocation

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Grid-Based Data Aggregation
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Priority-Based Event Reporting

• Priority queues
• prioritized scheduling to speed up important
  event data transmission
• queue utilization as an index for route selection
  to meet the latency bounds
• first-in-first-out (FIFO) discipline

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Queueing Delay

• The queueing delay of the highest priority queue
• The queueing delay of kth priority queue

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Next Hop Selection

• Consider node i receives new type of event data
  datae
• It broadcasts a control message to its immediate
  neighbors
• Every neighbors j replies with the message

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Next Hop Selection

• The end-to-end delay to actuator should be less
  than the latency bound Be
• Node i first estimates the advancement hi,j
  towards the actuator a from i to j, and then the
  maximum delay from i to j, delayi,j.

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Next Hop Selection

• Only neighbors with dq_maxgt0 will be considered
  as next hop
• Node i starts inspecting the neighbors with
  ?high0 and ?low0
• ?low 0 means it will not affect the transmission
  time for the existing packets in that node
• ?high 0 means it can be served with the highest
  priority
• Node i calculates the maximum data rate?i,j that
  it can forward while satisfying the latency
  bound

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Data Transmission

• Data packets are forwarded to the neighbor with
  the highest hi,j and ?i,j
• The intermediate nodes will update the latency
  bound Be before forwarding to next hop
• Be Be ( tdepart tarrive) dtran
  dprop
• Sensor will update its and the routes
  regularly
• If the latency bound is not met, another route
  will be selected
• In the worst case, if no alternative route,
  sensor may inform the previous node to select
  another route

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Actuator Allocation

• The actuators may record the event frequency and
  re-arrange their standby positions periodically
• Actuator allocation algorithm
• The event frequency freqg of every grid g will be
  summed up
• The field A will be equally divided in to two,
  denoted by A1 and A2, according to the frequency
  distribution
• Each area is allocated with half of the actuators
• The process repeats recursively for A1 and A2,
  until each subfield contains only one actuator

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Actuator Allocation
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Simulations

• Simulator NS-2
• Metrics
• On-time Reachability
• Average Delay
• Overall Reliability
• 4 events
• 2 with high importance
• 2 with low importance
• Located in left bottom corner

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On-Time Reachability
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Average Delay
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Overall Reliability
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Actuator Allocation

• Divide whole field into tree, with event
  occurrence probability 0.6, 0.333, and 0.067
• Data rate 60pkt/s

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Actuator Allocation

• No. of concurrent events 10

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Conclusion

• We provide a distributed and comprehensive
  solution for reliable event reporting and
  actuator coordination in WSAN
• We formulate the event reporting problem and
  define reliability index
• We provide a distributed data aggregation
  mechanism
• We propose a reliable priority-based event
  reporting algorithm
• We propose an actuator allocation algorithm
• Simulation results are provided to demonstrate
  the effectiveness of our solutions

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