ExOR:Opportunistic Multi-Hop Routing For Wireless Networks

1
ExOROpportunistic Multi-Hop Routing For Wireless
Networks

• Sanjit Biswas and Robert Morris
• MIT CSAIL
• http//pdos.csail.mit.edu/roofnet

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What is Ex Opportunistic Routing ?

• A Link/Network Layer diversity routing technique
  that uses standard radio hardware
• Achieves substantial increase in throughput for
  large unicast transfers in mesh network.
• Thoughts
• Does opportunistic mean Greedy ?
• Do we need Dijkstras algorithm?
• How is it different?

3
Traditional Routing
Packet
Packet
B
A
Packet
D
S
C

• Packets get forwarded on fixed path
• Retried on failures
• Looks like a circuit switched network

4
Cooperative Diversity / Probabilistic Broadcast
B
A
D
S
C

• Packets are broadcasted
• Selects each hop, after the transmission for that
  hop.
• Unlike Coop Diversity only a single node forwards
  each packet

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Why ExOR promises high throughput?
N5
N1
N3
N7
N6
N2
N4
N8
S
D
Traditional Path

• Gradual falloff of probability with distance
  (80, 40, 20..)
• Lucky longer path can reduce transmission count
• Shorter path ensures some forward progress

6
Why ExOR promises high throughput?
S
25
100
S
25
100
S
D
25
100
S
100
S
25

• Reception at different node is independent, no
  interference
• Traditional Routing 1/ 0.25 1 5tx
• ExOR 1/ (1-(1-0.25) ) 1 2.5tx

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7
Design Challenges Order in Anarchy

1. The nodes must agree on which subset of them
   received each packet Protocol ?
2. A metric to measure the probable cost of moving
   packet from any node to destination
3. Choosing most useful participants
4. Avoid simultaneous transmission to minimize
   collisions.

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Agreement using Gossip and Batch
N7
N8
F
F
F
N1
N2
N5
S
F
N4
D
Batch
N3
1st round
N6
2nd round
F
3rd round

• A complete schedule, undelivered packet are
  retried in subsequent one
• A subset within a transmission batch is called
  Fragment (F)
• After each batch destination sends packet just
  containing batch map
• Okay, where is the agreement ?

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Gossip Protocol Very similar to P2P

• Only one kind of packet/message.
• Batch Map (Summary) - Map of each packet against
  the received highest priority node.
• Forwarder List A list of nodes ordered in
  priority i.e. proximity towards the destination.
• Knowledge is shared by means of Gossip. Each data
  packet contains summary.
• Summary is updated, retrofitted and transmitted
  by each node.

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Cost Metric and Selection of Participants

• ETX (Expected Transmission Count) metric based on
  Forward Delivery Probability
• Periodically link state flooding
• Path is weighted shortest path (Dijkstra)
• The source runs ExOR simulation and selects only
  the nodes which transmit at least 10 of total
  transmissions in a batch.
• Need to know about entire Mesh before hand? Is
  that okay?

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Forwarding Timer and Transmission Tracker
N7
N8
F
F
F
N1
N2
N5
S
F
N4
D
Batch
N3
1st round
N6
2nd round
F
3rd round

• Header contains information to predict source
  transmission rate
• Transmission schedule allows high priority node
  to send first
• Uses EMWA to set Forwarding Timer

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Batch? Whats going on with TCP Window?
13
25 Highest Throughput Pairs

• ACK might get dropped even for single hop.

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25 Lowest Throughput Pairs

• Asymmetric long links affect ACK handling

15
Transmission Range

• ExOR requires less packet transmissions to travel
  far.

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Going Forward

• Leverage 802.11 bit rate for throughput
  improvement.
• Cooperation between simultaneous flows.
• Transport level protocol development ?
• Security ?
• More organic improvements based on cooperative
  diversity theory ?