The Suitability of Intervehicle Ad hoc Networks for SafetyCritical Applications

1
The Suitability of Inter-vehicle Ad hoc Networks
for Safety-Critical Applications

• Jeremy Blum
• Center for Intelligent Systems Research
• The George Washington University
• October 27, 2006

2
Early Electronic Toll Systems

• Vendors of early electronic toll systems, using
  spectrum near 915 MHz, developed proprietary
  systems
• Predictably, the systems did not work well with
  each other

• The FCC has allocated 5.8505.925 GHz for future
  Intelligent Transportation Systems
• Standards have been developed to promote
  interoperability and enable a large new
  generation of applications

3
New Applications

• In addition to e-tolls, this network is
  envisioned to support a wide range of
  Vehicle-Infrastructure Integration (VII)
  applications, including
• Safety Applications
• Intersection Collision Avoidance
• Emergency Response
• Incident Notification
• Curve Speed Warnings
• Signal Pre-emption
• Efficiency Applications
• Traffic Signal Timing
• Ramp Metering
• Consumer Applications
• Music/Movie Downloads
• Location-based Advertising

4
Equipped Vehicle
From ASTM. (2002) Standard Specification for
Telecommunications and Information Exchange
Between Roadside and Vehicle Systems 5 GHz Band
Dedicated Short Range Communications (DSRC)
Medium Access Control (MAC) and Physical Layer
(PHY) Specifications (modified to include
additional likely components)

• Interface Devices
• Sensors
• GPS receiver
• Forward facing radar
• Networking Equipment
• In-vehicle Network
• On-Board Unit
• Cellular Communications Equipment (optional)
• On-board Computer

5
Inter-Vehicle Communication Networks

• These on-board units will enable the development
  of applications that rely on mobile ad hoc
  vehicular networks.

• A wide range of safety applications are
  envisioned
• Initially, used to extend the perception horizon
  of users
• Ultimately, could support Automated Highway
  Systems

6
Virtual Mirror

• Virtual mirrors present computer-generated images
  of vehicles in a drivers blind spot.

• Could use either sensor data or IVC
• Required data would be in periodic IVC messages

Picture of Vehicle Interior from University of
Minnesotas Intelligent Vehicles Laboratory
Demonstration
7
Roadway Hazard Notification via IVC

• IVC will transmit warning messages of roadway
  hazards, including obstacles in the roadway,
  accidents, and hard-braking incidents
• These messages are sporadic and may require
  multi-hop message propagation

8
Proposed IVC Safety Applications

• Emergency Brake Lights
• Vehicle-Based Road Condition Warning
• Vehicle-To-Vehicle Road Feature Notification
• Visibility Enhancer
• Cooperative Collision Warning
• Pre-crash sensing
• Lane Change Warning/Highway Merge Assistant
• Cooperative Adaptive Cruise Control
• Vehicle Platoons
• Do design choices made in the development of the
  IVC standards support these applications?

9
The OSI Reference Model
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Data Link Layer

• This layer specifies how messages are transmitted
  in an error free fashion between two adjacent
  entities on the network.
• These entities are adjacent in the sense that
  they can communicate directly with each other.
• Must manage contention for shared physical media
• May provide error checking and acknowledgement
  processing

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IVC Link Layer Protocol

• IVC networks must manage contention for media

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802.11 Point Coordination Function

• The access point manages contention for resources

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802.11 Distributed Coordination Function (DCF)

• Before transmitting a message, a sender chooses a
  random backoff time in the interval from 0 to the
  size of the Contention Window.
• If the sender senses that the channel is busy,
  the backoff timer is frozen.
• It is restarted once the channel has been
  detected as being idle.
• If the sender detects that its transmission
  experienced a collision, the Contention Window is
  doubled until it reaches a maximum value.
• The sender resets its Contention Window size upon
  successful transmission.

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DCF
CW
CW
CW
CW
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Impact on Safety Applications

• This exposure to Denial of Service attacks is
  important even for safety devices that simply
  provide information to drivers, as drivers have
  been shown to quickly adapt their driving
  behavior
• Safety-related systems relying on IVC must
  include fail-safes that allow for safe operation
  when network is under attack
• Impact on system effectiveness?
• Impact on consumer acceptance?
• We may need a new Data Link Layer to support
  robust IVC networks

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Conclusions

• There are a number of other challenges that the
  IVC Network must address including problems of
  minimum required equipment deployment,
  scalability, privacy,
• The IVC network is well-suited for a range of
  applications, including applications for
  improving traffic flow and consumer applications
• However, given the exposure of the Data Link
  Layer to Denial of Service Attacks, there are
  significant challenges that must be addressed to
  utilize this network is problematic for
  safety-related applications

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Questions???
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Link Layer

• Adaptive Space Division Multiplexing (ASDM)
  extends existing Space Division Multiple Access
  (SDMA) protocols

• New Assignment Rules

• New Mapping Function
• Faster multi-hop message propagation
• Even inter-message departure time

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ASDM Timeslot Mapping Function

• ASDM introduces significant flexibility in its
  mapping function
• To produce even inter-message departure times
  with to the introduction of the new assignment
  rules in ASDM
• To speed multi-hop message propagation

Lc is the maximum number of lanes in this section
of roadway Li is the lane offset assigned to each
lane, such that n is the ASDM Period Length
Lc
Example with Li I, Lc 8, p 3, n 40
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Properties of the Mapping Function

• We prove that the ASDM mapping function is
  well-formed (no repeated timeslots within the
  ASDM Period Length) as long as p is a number that
  is relatively prime to the ASDM Period Length
• Given uniform traffic distribution
• For any sequence of n consecutive cells, the
  inter-departure time is invariant under the
  location and direction of the vehicle
• All vehicles have the same Quality of Service for
  periodic beacon messages
• Given highway at capacity
• The message propagation speed is invariant under
  vehicle location and direction
• All vehicles have the same QoS for a-periodic
  multi-hop messages

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Parameterization of the Mapping Function

• We can choose a parameterization of the ASDM
  mapping function that
• Minimizes unused timeslots
• Creates even inter-message departure time
  (important for periodic beacon messages)
• Promotes faster multi-hop message propagation
  (important for a-periodic warning messages)

2006-01-1427
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Even Inter-message Departure Times
Uneven Timeslot Allocation in an SDMA Mapping
Function for an ASDM Period Length of 24
A More Even Timeslot Allocation for an ASDM
Period Length of 24

• To promote even inter-message departure times,
  can look at most likely number of cells that will
  be allocated to each vehicle
• Examine possible combinations of p and ASDM
  buffer size to find the one that has the lowest
  deviation in the inter-message departure times

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Ideal Multi-hop Message Propagation

• With current SDMA protocols, multi-hop message
  delivery is slow.

• Ideally, vehicle allocated next hop would be
  located at about the transmission range from the
  current vehicle

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Actual Message Propagation

• Assume next timeslot allocated to the current
  lane is k slots back
• The length of the expected hop length is a
  function of vehicle spacing
• May be less than or greater than k
• For a uniform distribution of vehicles, the Ideal
  Average Hop Length, which maximizes the value of
  the Expected Actual Hop Length, lies between c
  B and c
• c is the communications range
• B is the ASDM Buffer size.

Hop length with the ASDM Buffer greater than 0.
Cell f0 is allocated the next timeslot in this
lane after cell s0. The B cells, where B is the
ASDM Buffer size, at the left of the figure
represent the possible locations for a vehicle to
be allocated the timeslot for cell f0.