REAL TIME COMMUNICATION IN WIRELESS SENSOR NETWORKS

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REAL TIME COMMUNICATION IN WIRELESS SENSOR
NETWORKS

• BY
• ZILLE HUMA KAMAL

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WHAT IS A REAL TIME SYSTEM (RTS)

• A real time system is one in which the
  correctness of the computations not only depends
  on their logical correctness, but also on the
  time at which the result is produced S

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CLASSIFICATION OF RTS

• 2 Categories of RTS
• A Hard RTS is one in which one or more activities
  must never miss a deadline or timing constraints,
  otherwise the system fails or results in
  catastrophe. S
• A Soft RTS is one that has timing constraints,
  but occasionally missing them has negligible
  effects, as application requirements as a whole
  continue to be met. S

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TERM AND DEFINITIONS

• Task executable entity
• Job instance of a task
• Release Time time at which task becomes ready
  to run and job is released
• Period time between releases of two instances
  of the same task
• Deadline relative time at which a job should
  complete execution
• Execution Time/ Run Time time taken to complete
  execution without interruption
• Frame discrete unit of time CZSB

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WIRELESS SENSOR NETWORKS

• CHARACHTERISTICS
• An instance of MANET
• Resource constraint energy and storage capacity
• Limited range for communication and sensing
• Frequent network topology changes
• Individual entities are not critical, aggregation
  of results is necessary for effectiveness and
  accuracy

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RTS IN WSN

• Two types of communication groups are inherently
  formed
• Local Coordination to aggregate results
• Sensor-Base Communication to send results to
  base station
• This introduces contention on the communication
  channel, thus the main schedulable resource is
  the communication channel

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RAP

• A Real-Time communication architecture

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APIs

• Issue Query
• - query name
• - attribute list
• - area
• - timing constraints, e.g. period, deadline
• - querier location

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APIs

• Event Registration
• - event name
• - area
• - query

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Example

• register_event
• virus_found(0,0,100,100),
• query
• virus.count,
• area(Xevent-1 ,Yevent-1,Xevent1,Yevent1),
• period1.5, deadline5,
• base(100,100)

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LAP

• Location Addressed Protocol
• - transport layer
• - connectionless
• - no IP/ID addressing, location based
  addressing
• - three types of communication
• unicast
• area multicast
• area anycast

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LAP

• Unicast
• Message is delivered to node closest to
  destination, e.g when sensors send query results
  back to base station
• Area Multicast
• Message is delivered to every node in a specified
  area, e.g when base station sends query to an
  area, or for local coordination
• Area Anycast
• Message is delivered to at least one node in the
  specified area, e.g when base station wants to
  send a query to an area, the node which receives
  it can start the initiation process

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GF

• Greedy algorithm
• A packet is forwarded to a neighbor only if
• (1) the neighbor node has the shortest distance
  to the packets destination among all immediate
  neighbors AND
• (2) the neighbor node is closer to the
  destination than the forwarding node
• If these conditions not satisfied, GPSR is used
  instead of GF

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VMS Deadline aware Distance aware

• Deadline aware
• Distance aware
• Packet scheduling policy
• 2 types of packet scheduling policies
• Static Velocity Monotonic
• Dynamic Velocity Monotonic

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VMS

• SVM
• Requested velocity is fixed at each hop
• V dis(x0, y0, xd, yd)/D
• DVM
• Requested velocity changes at each hop and
  reflects the time the packet has spent in the
  network
• vi dis(x0, y0, xd, yd)/(D-Ti)
• v0 dis(x0, y0, xd, yd)/D

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Priority Queues

• various FIFO queues, one for each priority
• Advantage per packet overhead decreases,
  ordering of each packet is not required
• Disadvantage more storage capacity required
• single FIFO queue, with priority ordering
• Advantage reflects order of packets requested
• Disadvantage greater number of packets lost

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MAC PRIORITIZATION

• Extensions to 802.11
• Initial wait time after idle
• Backoff Increase Function
• Initial wait time after idle
• DIFS BASE_DIFS PRIORITY
• Backoff Increase Function
• CW CW (2(PRIORITY-1)/MAX_PRIORITY)

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EXPERIMENTATION
Overall deadline miss ratio of DSR and GF with
deadlines (5,10)
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EXPERMENTATION
Overall deadline miss ratio
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EXPERIMENTATION
Miss ratio vs distance between source and
destination (Deadline (510) s Rates (0.8,
0.36)/s)
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REFERENCES

• CZSB M Caccamo, L.Y Zhang, L Sha, G Buttazzo,
  An Implicit Access Protocol for Wireless Sensor
  Networks,Proceedings of IEEE Real-Time Systems
  Symposium, Austin, TX , Dec 2002.
• LBASH C Lu, B.M Blum, T.F Abdelzaher, J.A
  Stankovic, T He, RAP A Real-Time Communication
  Architecture For Large-Scale Wireless Sensor
  Networks, Department of Computer Science,
  University of Virginia
• www.cs.virginia.edu/stankovic/psfiles/rtas02-rap.
  pdf

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REFERENCES

• P T. F Piatkowski, Citation and acknowledgment
  guide, Department of Computer Science, Western
  Michigan University, Aug, 2000
• www.cs.wmich.edu/piat/citationAckGuide.pdf
• S D.B Stewart, Introduction to Real Time,
  Embedded.com, Nov 1, 2001.
  www.embedded.com/story/OEG20011016S0120