ExOR: Opportunistic Multi-Hop Routing For Wireless Networks

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ExOR Opportunistic Multi-Hop Routing For
Wireless Networks

• Sanjit Biswas Robert Morris

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Contributions

• This paper contributes the first complete design
  and implementation of a link/network-layer
  diversity routing technique that uses standard
  radio hardware.
• It demonstrates a substantial throughput
  improvement over traditional routing.

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Why ExOR promises high throughput? (1)
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Why ExOR promises high throughput? (2)
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Traditional Path

• Gradual falloff of probability with distance
  (80, 40, 20..)
• Lucky longer path can reduce transmission count
• Shorter path ensures some forward progress
• ExOR works better with local interference than
  global

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Four Design Challenges

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

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Node State

• Packet buffer
• Forwarding timer
• Transmission tracker
• Used to adjust forwarding tracker.
• Batch map
• Highest priority node that received the pkt

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Working
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Batch
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1st round
2nd round
3rd round
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Cost Metric similar to ETX

• ETX Expected Transmission Count
• ExOR uses forward delivery probability
• Knowledge of inter-node loss rate

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Evaluation Setup

• 38 Roofnet nodes participated using 802.11b
• One hop at a time for fair comparison in
  traditional routing.

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Evaluation 1A

• Bars higher than 1000 indicates that nodes had
  to transmit the packet more than once.

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Evaluation 1B

• ExOR exhibits better throughput

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Evaluation - 2
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Summary

• Pros
• ExOR achieves 2x to 4x throughput improvement for
  more distant pairs
• ExOR implemented on Roofnet and evaluated in
  detail
• Exploits radio properties, instead of hiding them
• Does not require changes in the MAC layer
• Cons
• Not scalable to large network as traditional
  routing
• Overhead in packet header (batch info)
• Batches affect the TCP performance
• What if not enough packets to make the batch?