Urban Multi-Hop Broadcast Protocol for Inter-Vehicle Communication Systems

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Urban Multi-Hop Broadcast Protocol for
Inter-Vehicle Communication Systems
Data Engineering Laboratory, Aristotle
University of Thessaloniki

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VANET

• Key features
• Mobility rate is high
• Movement direction and speeds are predictable
• Vehicle enter and leave the network frequently
• Broadcast is a frequently used method
• VANET applications relying on broadcast
• Traffic, Accident warnings
• Weather (warning packets generated when the road
  is slippery)
• Delivery of advertisements and announcements from
  hotels, restaurants etc

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Broadcast

• Disadvantages of multi-hop broadcast
• Packet collisions, hidden nodes, interference
• It is difficult to disseminate the packets to
  different road segments due to tall buildings
  around intersections
• IEEE 802.11
• RTS/CTS handshake and acknowledgement mechanisms
  decreases the hidden terminal problem and makes
  the protocol reliable
• May cause packet storms around the source
• UMB is designed to address
• Broadcast storm
• Hidden node
• Reliability problems

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UMB

• Key Idea
• Directional broadcast. Sender node try to select
  the furthest node in the broadcast direction to
  assign the duty of forwarding without any apriori
  topology information
• Intersection broadcast. Repeaters at the
  intersections, forward the packet to all road
  segments.
• Assumption
• Each node knows the location of itself,
  intersections and repeaters
• Goals
• Avoiding collisions due to hidden nodes.
• Using the channel efficiently
• Making the broadcast communication as reliable as
  possible
• Disseminating messages in all directions at an
  intersection

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Directional Broadcast (1/3)

• Divide the road portion inside the transmission
  range into segments
• If there is more than one node in the furthest
  segment, then it is divided
• If segment based iterations are not sufficient to
  pick only one node, then they enter to a random
  phase
• UMB uses RTB and CTB
• An RTB packet includes source node position and
  broadcast direction
• A node receiving RTB packet
• Compute the distance to source node
• Based on the distance, it sends a channel jamming
  signal, called black-burst
• Sends its black-burst in the shortest possible
  time
• Listens to the channel. If it is empty, then its
  black-burst was the longest and replies with CTB
  after CTBTIME.

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Directional Broadcast (2/3)

• Receivers send black-burst signals proportional
  to their distance to the source
• When there are more than one vehicle in the
  furthest non empty segment
• They all find the channel empty and continue to
  send CTB packets
• Source node detects the collisions and repeat RTB

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Directional Broadcast (3/3)

• The furthest segment is divided into Nmax
  sub-segments
• Only nodes that sent the longest black-burst in
  the previous iteration can join to the current
  iteration
• Random collision resolution phase after the Dmax
  iteration
• Source node goes back to the first segment
  iteration after a random amount of time

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Intersection Broadcast (1/2)

• If the forward node is inside the transmission
  range of a repeater, the node sends the packet to
  the repeater using point-to-point IEEE802.11
• Packet Loops
• All cars in the network record
• the packet IDs when they hear packets
• Repeaters record the packet IDs

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Intersection Broadcast (2/2)

• RTB/CTB/DATA/ACK handshake is repeated several
  times in intersections
• Disadvantage
• Waste bandwidth
• Degrade the overall performance of the network,
  since packets from all directions will wait the
  for the repeater to be idle
• Solution
• Repeaters do not repeat the information in the
  DATA packet if the forward node has already
  received the message