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ExOR: Opportunistic Multi-Hop Routing For Wireless Networks

ExOR: Opportunistic Multi-Hop Routing For Wireless Networks. Sanjit Biswas & Robert Morris. Contributions. This paper contributes the first complete design and implementation of a link/network-layer diversity routing technique that uses standard radio hardware.

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ExOR: Opportunistic Multi-Hop Routing For Wireless Networks

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  1. ExOR: Opportunistic Multi-Hop Routing For Wireless Networks Sanjit Biswas & Robert Morris

  2. 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.

  3. I1 25% 100% I2 25% 100% S D 25% 100% I3 100% 25% I4 • Reception at different node is independent, no interference • Traditional Routing: 1/ 0.25 + 1 = 5tx • ExOR: 1/ (1-(1-0.25) ) + 1 = 2.5tx 4 Why ExOR promises high throughput? (1)

  4. Why ExOR promises high throughput? (2) N5 S N1 N2 N3 N4 N6 N7 N8 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 • ExOR works better with local interference than global

  5. 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

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

  7. Working… N7 N8 F F F N1 N2 N5 S F Batch N4 D N3 N6 F 1st round 2nd round 3rd round

  8. Cost Metric: similar to ETX • ETX: Expected Transmission Count • ExOR uses forward delivery probability • Knowledge of inter-node loss rate

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

  10. Evaluation – 1A • Bars higher than 1000 indicates that nodes had to transmit the packet more than once.

  11. Evaluation – 1B • ExOR exhibits better throughput

  12. Evaluation - 2

  13. 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?

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