REAL TIME COMMUNICATION IN WIRELESS SENSOR NETWORKS

1
REAL TIME COMMUNICATION IN WIRELESS SENSOR
NETWORKS

• BY
• ZILLE HUMA KAMAL

2
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 St

3
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. St
• 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. St

4
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

5
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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REAL TIME COMMUNICATIONS IN WIRELESS SENSOR
NETWORK

• NOW PRESENTING SPEED
• BY
• Zille Huma Kamal

22
UNFAVORABLE

• Despite the simplicity of RAP and the high miss
  deadline ratio it serves, RAP does not guarantee
  for soft or hard real time communication systems.
• Therefore, our search for a Real Time
  Communication protocol is unsatisfied.

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TO END THE SEARCH

• SPEED is a real time communication protocol which
  guarantees end to end soft real time
  communication
• We will discuss the components of SPEED and then
  relate SPEED to other existing protocols for
  MANETS, ad-hoc networks and real-time
  communication systems.

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COMPONENTS OF SPEED

• API
• Neighbor Beacon Exchange
• Delay Estimation
• Stateless Non-deterministic Geographic
  Forwarding(SNGF)
• Neighborhood Feedback Loop(NFL)
• Backpressure Rerouting
• Void Avoidance
• Last mile processing

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API PACKET FORMAT

• UnicatSend(Global_ID, packet)
• AreaMulticastSend(position, radius, packet)
• AreaAnycastSend(position, radius, packet)
• SpeedReceive( )
• SPEED packet format

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Neighbor Beacon Exchange

• Periodic beacons exchange location information
• In static or slow moving sensor networks very
  low beaconing rate
• Further reduce overhead piggybacking, include
  ID on data packets, so that you are using the
  existing packets and not introducing more traffic

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NEIGHBOR TABLE

• Through beaconing each node is capable of
  maintaining a Neighbor Table (NT)
• In addition to location beacons, you have delay
  estimation beacons and backpressure rerouting
  beacons

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DELAY ESTIMATION

• Single Hop Delay delay across one router
• Sender - timestamps when packet leaves node and
  then waits for acknowledgement from receiver.
• Receiver in acknowledgment packet sends the
  time taken to process the acknowledgment
• Sender after receiving the acknowledgment,
  calculates round trip time as
• timestamp ACK time ACK processing time

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DELAY ESTIMATION

• This round trip delay time is aggregate with
  previous delay times via EWMA
• Since delay estimation expensive SPEED only
  invokes delay estimation when round trip delay
  for an individual case exceeds a predetermined
  threshold value

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BACK-PRESSURE REROUTING

• Routing layer adaptation to congestion
• Beacon format
• When congestion occurs, node sends back-pressure
  beacon to sender with AvgSendToDelay equal to
  infinity

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SNGF - TERMINOLOGY

• Nsi all nodes within radio range of nodei
• FSi(destination) x x ? Nsi and it is closer
  to the destination than nodei
• Relay Speed

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SNGF FORWARDING CONDITIONS

• Only if node belongs to FSi(destination)
• FSi(destination) into 2 categories
• FS1i(destination)of nodes with
• relay speed gt Ssetpoint
• FS2i(destination) of nodes with
• relay speed lt Ssetpoint
• Forwarding node is always from FS1i(destination)
• If no node in FS1i(destination) then call
  Neighborhood Feedback Loop (NFL) and decide
  whether to drop packet or not

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NFL - TERMINOLOGY

• Miss when packet delivered at neighbor with
  relay speed lt Ssetpoint or any packet loss due to
  collision
• Miss ratio calculation

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NFL

• MAC layer adaptation to avoid congestion

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VOID AVOIDANCE

• By using backpressure rerouting
• Only guarantees to find a path if a greedy path
  exists

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LAST MILE PROCESS

• For AreaMulticast and AreaAnycast TTL
  manipulation
• For Unicast

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EXPERIMENTATION - CONGESTION
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EXPERIMENTATION - CONGESTION
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EXPERIMENTATION E2E DEADLINE MISS RATIO
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EXPERIMENTATION E2E DEADLINE MISS RATIO
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EXPERIMENTATION - COST
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EXPERIMENTATION - COST
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EXPERIMENTATION ENERGY CONSUMPTION
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EXPERIMENTATION TRAFFIC BALANCING
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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.
• http//www.cs.wustl.edu/venkita/publications/clas
  s/implicitedf.pdf

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REFERENCES

• HSLA T He, J.A Stankovic, C Lu, T Abdelzaher,
  SPEED A Stateless Protocol for Real-Time
  Communication in Sensor Networks, Department of
  Computer Science, University of Virginia and
  Department of Computer Science and Engineering,
  Washington University in St Louis
• http//www.cs.virginia.edu/stankovic/psfiles/SPEE
  D_ICDCS.pdf

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REFERENCES

• 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
• P T. F Piatkowski, Citation and acknowledgment
  guide, Department of Computer Science, Western
  Michigan University, Aug, 2000
• www.cs.wmich.edu/piat/citationAckGuide.pdf

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REFERENCES

• Sp Delay Analysis, Sprint, 2003
• http//ipmon.sprintlabs.com/delaystat/
• St D.B Stewart, Introduction to Real Time,
  Embedded.com, Nov 1, 2001
  www.embedded.com/story/OEG20011016S0120