Data Dissemination in Vehicular Ad Hoc Networks

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Data Dissemination in Vehicular Ad Hoc Networks

• Guohong Cao
• Department of Computer Science and Engineering
• The Pennsylvania State University
• http//www.cse.psu.edu/gcao

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Vehicular Ad Hoc Network

• VANET has been envisioned to be useful in road
  safety and many commercial applications
• A VANET can be used to alert drivers to potential
  traffic jams, providing increased convenience and
  efficiency.
• It can also be used to propagate emergency
  warning to drivers behind a vehicle (or incident)
  to avoid multi-car collisions
• A moving vehicle may want to know the sale
  information or remaining stocks at a department
  store or gas station the available parking lot
  at a parking place.

Vehicle-Vehicle Communication
Vehicle-Infrastructure Communication

• In this work, we focus on non-safety related data
  dissemination issues.

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Data Dissemination in VANET

• Push-based data dissemination, where the messages
  can be efficiently delivered from moving vehicles
  or fixed stations to other vehicles.
• Examples include traffic condition monitoring,
  road-side e-advertisements, etc.
• Pull-based data dissemination/access, where a
  vehicle is enabled to query information about
  specific targets.
• For example, a vehicle can query the average
  vehicle speed in a region, the parking lot, hotel
  availability.
• Push-based approach is used for disseminating
  data that is useful for many people, whereas
  pull-based approach is used for querying data
  specific for some user. In practice, a hybrid of
  push/pull is used

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Data Dissemination in VANET

• Although data dissemination has been studied in
  database and network area, many unique
  characteristics of VANET bring out new research
  challenges.
• Due to fast vehicle movement, network topology
  and channel conditions change rapidly.
• The network density is highly dynamic. Different
  scenarios at day, night, traffic jam.
• The vehicle mobility is partially predictable
  since it is limited by the traffic pattern and
  the road layout.
• Data dissemination techniques should address
  these unique characteristics

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Our Techniques

• Pull-based approach (IEEE infocom06)
• The predictable vehicle mobility is exploited to
  get the data quickly
• Push-based approach (IEEE TVT07)
• Data are disseminated to several major directions
  by moving vehicles and then disseminated to other
  vehicles
• Scheduling (ACM VANET07)
• Techniques used in roadside data units to reduce
  the data access delay

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Pull-Based Data Access

• Task to deliver a message from mobile vehicle to
  the fixed site several miles away.
• A driver may want to find out the sale
  information in a store, the room availability in
  a hotel.
• Challenges
• There may be network partitions due to mobility
  or traffic condition
• End-to-end connection through multi-hop is hard
  to set up. Most existing ad hoc routing protocols
  such as DSR/AODV may not work well.
• Mobility creates opportunities
• Buffer and carry the packet when no routes
• Forward the packet to the nodes moves into the
  vicinity which can help packet delivery
• Possible to deliver the packet without an
  end-to-end connection
• Different from existing store-carry forward
  protocols, we make use of the predictable traffic
  pattern and vehicle mobility to assist data
  delivery.

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Vehicle-Assisted Data Delivery (VADD)

• Key issue
• Select a forwarding path with smallest packet
  delivery delay
• Why not GPSR?
• Guidelines
• Make the best use of the wireless transmission
• Forward the packet via high density area
• Use intersection as a opportunity to switch the
  forwarding direction and optimize the forwarding
  path

Geographically shortest path
Fast speed wireless communication
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VADD Model

• Find out the next forwarding direction with
  probabilistically the shortest delay
• 1. Estimate the packet forwarding delay (dmn)
  between two adjacent intersections based on
  traffic statistics
• 2. Use the probabilistic method to estimate
  the expected delivery delay from current
  intersection to the destination.
• 3. Generate a linear equation system, and
  solve it by Gaussian Elimination

Output Priority list of the outgoing directions
for the packet forwarding
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Intersection Forwarding Protocol

• Knowing the priority list of outgoing directions,
  probe the available contact to ensure the packet
  is forwarded to the preferred directions
• Not trivial, need to consider
• Location
• Mobility
• VADD Intersection Protocols
• Location First VADD (L-VADD)
• Direction First VADD (D-VADD)
• Multi-Path D-VADD (M-VADD)
• Hybrid VADD (H-VADD)

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Push-based Data Dissemination in VANET

• Task
• Deliver the data to all the vehicles within a
  given area
• Applications
• Transportation control
• E-advertisement
• Emergency announcement
• Metrics
• Dissemination capacity
• The maximum number of data items can be
  disseminated by the system
• Delivery ratio
• The percentage of the data items can be received
  by the vehicles
• Network traffic

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Opportunistic Dissemination (OD)

• Idea opportunistic data exchange
• Vehicles store and carry the data
• Propagate data to the encountered vehicles and
  obtain new data in exchange
• Dissemination capacity is low. Performance
  suffers when vehicle density is high
• Excessive interference
• Hard to schedule the transmission
• Too many redundant exchanges

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Data Pouring Scheme (DP)

• Basic idea explore road layout and partially
  predictable vehicle moving pattern
• Pour the data along several selected Primary Road
  (P-Road) by periodic broadcasting
• All the roads intersected with the P-Roads are
  called Crossing Roads (C-Roads)
• Vehicles on the C-Road passively receive the data
  when moving through the intersections on the
  P-Roads.

• Techniques to make it reliable
• Invalidation
• High overhead

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DP with Intersection Buffering (DP-IB)

• Basic idea of DP-IB
• Each intersection buffers the data, and
  rebroadcasts periodically
• Data center only broadcasts for data invalidation
  or refresh the lost data copies.
• Objective of DP-IB
• Reduce the amount of data poured from the source
• Increase the broadcast throughput
• Find the broadcast cycle time through analysis

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Roadside Data Unit Scheduling

• Download (query) requests and upload (update)
  requests to the roadside data unit
• Which request to serve at which time is critical
  to the system performance
• Most existing scheduling work does not consider
  uplink updates

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Our Contribution

• We first propose a basic low complexity
  scheduling scheme which considers both data size
  and request deadline
• DS Deadline Size, always serve the request
  with the minimum DS
• Implementation is tricky, to reduce the
  computation complexity by pruning the search
  space.
• We improve the performance of the basic
  scheduling algorithm by using broadcasting
  techniques to serve more requests
• DS/N Number of pending requests
• We study the tradeoffs between service ratio and
  data quality, and propose a two step scheme to
  address the tradeoffs between updates and
  downloads
• Data become stale if an upload is missed.
• Schedule upload and download with separate queues
  and different priorities

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Publications

• J. Zhao, Y. Zhang, and G. Cao, "Data Pouring and
  Buffering on The Road A New Data Dissemination
  Paradigm for Vehicular Ad Hoc Networks," IEEE
  Transactions on Vehicular Technology, to appear
• J. Zhao and G. Cao, "VADD Vehicle-Assisted Data
  Delivery in Vehicular Ad Hoc Networks," IEEE
  Transactions on Vehicular Technology (A
  preliminary version appeared in IEEE INFOCOM06)
• Y. Zhang, J. Zhao, and G. Cao "On Scheduling
  Vehicle-Roadside Data Access," The Fourth ACM
  International Workshop on Vehicular Ad Hoc
  Networks (VANET), 2007
• Papers are available in http//mcn.cse.psu.edu