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. Why ExOR promises high throughput? (1) S I1 I2 D I4 I3 25% 100%  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

4. Why ExOR promises high throughput? (2) N1N3 N5 N7N6N2N4N8SD  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 Traditional Path

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… S N2 N3 N4 N1 N5 N6 D N7 N8 Batch F F F F 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?