Wireless Ad Hoc Network Routing Protocols

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Wireless Ad Hoc Network Routing Protocols

• CSE 802.11
• Maya Rodrig

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Ad hoc networking

• Infrastructureless networking mobile nodes
  dynamically establish routing among themselves to
  form their own network on the fly.
• Mobile nodes operate as routers
• Mobile nodes participate in an ad hoc routing
  protocol

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Why not reuse existing protocols?

• Highly dynamic interconnection topology
• LS generates loads of link status change msgs
• DV suffers from out-of-date state or generates
  loads of triggered updates
• Heavy computational burden on mobile nodes
• Wireless medium differs in important ways from
  wired media

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The Protocols

• DSDV, TORA, DSR, AODV
• Proactive vs. reactive (on-demand)

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Destination-Sequenced Distance Vector (DSDV)

• Preserve the simplicity of RIP while avoiding the
  routing loop problem
• Hop-by-hop distance vector
• Routing table contains entries for every
  reachable node
• Each route is tagged with a sequence number
  originated by destination (even numbers)
• Routing info is transmitted by broadcast
• Updates are transmitted periodically and when
  there is a significant topology change

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DSDV cont.

• Route R is more favorable than R if R has a
  greater sequence number or if the two routes have
  equal sequence numbers but R has a lower metric
  (hop count)
• Broken links are indicated by ? metric and the
  sequence number of destination is incremented to
  odd number before broadcast

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No count to infinity
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Temporally-Ordered Routing Algorithm (TORA)

• Based on a link-reversal algorithm
• Node broadcasts a QUERY packet which propagates
  to destination or to node having a route to the
  destination
• Recipient of the QUERY broadcasts an UPDATE
  packet listing its height with respect to the
  destination
• Each node that receives the UPDATE sets its
  height to be greater than the height of the
  neighbor from which the UPDATE came ? creates a
  series of directed links from the QUERY
  originator to the node initiating the UPDATE

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TORA cont.

• When a node discovers a route is no longer valid,
  it adjusts its height so that it is a local
  maximum and transmits an UPDATE
• When a network partition is detected, a node
  generates a CLEAR packet to reset routing state
  and remove invalid routes

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Dynamic Source Routing (DSR)

• Packet headers contain the route the packet must
  follow
• Route Discovery
• Source node S broadcasts Route Request packet
  that is forwarded through the network
• Destination node D or another node that knows a
  route to D answers with a Route Reply
• Route Maintenance
• When the network topology has changed s.t. the
  route to D can no longer be used, a Route Error
  packet is sent to S
• S can try another route to D from its cache or
  invoke Route Discovery again
• Network interfaces in promiscuous mode ? nodes
  cache overheard route information

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DSR Example
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Ad Hoc On-Demand Distance Vector (AODV)

• Combination of DSR (on demand) and DSDV
  (hop-by-hop routing, sequ nums)
• Node S broadcasts a Route Request message for
  destination D, including the last known sequence
  number for D
• Node with a route to D generates a Route Reply
  with its sequence number for D
• Nodes that forward Route Request store reverse
  route back to S nodes that forward Route Reply
  store forward route to D

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AODV cont.

• No HELLO messages from neighbor indicate link is
  down
• Nodes that recently forwarded packets using the
  failed link are notified via an UNSOLICITED ROUTE
  REPLY with infinite metric for the destination ?
  reinitiate Route Discovery

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Simulation Environment

• Model attenuation of radio waves between antennas
  
• Link layer implements 802.11 standard MAC
  protocol DCF
• Broadcast packets sent only when virtual and
  physical carrier sense indicate the medium is
  clear (no RTS/CTS and no ACKs)

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Methodology

• Network simulation
• 50 wireless nodes moving in 1500m300m flat space
• Over 200 different scenarios
• Movement model
• Random waypoint model (pause times 0, 30, 60,
  120, 300, 600, 900 seconds)
• Avg speed 10 meters/second
• Communication model
• Sending rates 1, 4, 8 packets/second
• 10, 20, 30 CBR sources
• Packet size of 64 bytes

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Metrics

• Packet delivery ratio- ratio between num packets
  originated by sources and num packets received at
  their destination
• Routing overhead- num routing packets transmitted
  during the simulation
• Path optimality- difference between the num hops
  a packet took to reach its destination and the
  length of the shortest path

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Packet Delivery Ratio

• DSR and AODV deliver over 95 of data packet
• TORA does well with 20 sources
• DSDV fails to converge at pause time lt 300

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Routing Overhead

• TORA, DSR, AODV are on demand
• DSDV is largely periodic
• DSR limits overhead of Route Requests through
  caching

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Path Optimality

• Internal mechanism knows the length of the
  shortest path between all nodes at any time
• DSDV and DSR use routes close to optimal
• AODV and TORA have a tail

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Another Protocol Greedy Perimeter Stateless
Routing (GPSR)

• Geography to achieve scalability in wireless
  routing protocols
• Assume bidirectional radio reachability
• Assume a location registration and lookup service
  that maps node addresses to locations
• Position of a packets destination and positions
  of candidate next hops sufficient to make correct
  decisions

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Greedy Forwarding

• Beaconing algorithm provides all nodes with their
  neighbors positions
• Packets are marked with their destinations
  locations
• A forwarding node makes a locally optimal greedy
  choice next hop is the neighbor geographically
  closest to the destination

Problem topologies in which the only route to
the destination requires temporarily moving
farther in geometric distance from the destination
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Planar Perimeters

• Right-hand rule when arriving at node x from
  node y, the next edge traversed is the next one
  sequentially counterclock-wise about x from edge
  (x,y) ? navigating around the void
• Construct planarized graphs to eliminate crossing
  links from the network without partitioning the
  network

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GPSR versus DSR
Routing Overhead
Packet Delivery Success Rate
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Comparison cont.
Network Diameter
Path Length
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Choosing Routes

• Shortest path is not a good metric ? choose
  routes with less capacity than best existing
  paths
• Minimum hop-count routes include links with high
  loss ratios ? retransmissions consume bandwidth

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Link Behavior in Experimental Networks

• Link quality distribution is spread out
• 30 of link pairs are unusable
• Best 40 of link pairs deliver 90 of their
  packets
• 30 link pairs have asymmetric delivery rate
• Delivery rates sometimes change very quickly
  (averaging not applicable)
• No good correlation between delivery rate and
  radios signal strength
• We need practical estimates for link quality and
  ways to combine link metrics into path metrics

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Expected Transmission Count (ETX)

• Find paths with fewest expected number of
  transmissions required to deliver a packet to its
  destination
• Use per-link measurements of delivery ratios in
  both directions
• Modified DSDV and DSR
• ETX outperforms minimum hop-count
• ETX incurs more overhead due to loss-ratio probes

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• Early protocols assume cooperating nodes that are
  willing to forward packets for others
• The role of power in routing protocols