ExOR: Opportunistic Multi-hop routing for Wireless Networks

1. ExOR: Opportunistic Multi-hop routing for Wireless Networks by; Sanjit Biswas and Robert Morris, MIT Presented by; Mahanth K Gowda Some pictures/graphs adopted from authors’ slides

2. Overview • Traditional Routing • ExOR: key intuitions and ideas • ExOR: Realization • Evaluation

3. Traditional Routing • Links are abstracted as wires. B C E A D

4. Link transmission is a Broadcast • Probability of reception decreases with distance • However, there is always a chance that data travels longer 10% B C 60% 90% E A D

5. ExOR Exploits Broadcast • Best traditional route over 50% hops: 3(1/0.5) = 6 tx • Throughput 1/# transmissions • ExOR exploits lucky long receptions: 4 transmissions • Assumes probability falls off gradually with distance src N1 N2 N3 N4 N5 dst 75% 50% 25%

6. ExOR Exploits Broadcast N1 • Traditional routing: 1/0.25 + 1 = 5 tx • ExOR: 1/(1 – (1 – 0.25)4) + 1 = 2.5 transmissions • Assumes independent losses 25% 100% N2 25% 100% src dst 100% 25% N3 100% 25% N4

7. ExOR Realization: Batching • Packets are queued and sent in Batches • A list of forwarders prioritized by their ETX values is included • In the below topology ---> Source: A, Destination: E • Priority order : E C D B A • In other words, if E C D B A receive packets, they should forward in that order • Other nodes listen • They forward packets only if a higher priority node has failed to do so

8. An example • A has transmitted a batch of 10 packets 1-10 • E receives packets 1, 2 • C receives 1 3 4 10 • D receives 1 2 5 9 10 • B receives 1 2 3 4 5 6 7 8 9 10 • E received 1,2 1 2 3 4 5 6 7 8 9 10 • Now C forwards 3, 4,10 1 234 5 6 7 8 9 10 • D forwards 5,9 1 2345 6 7 8 910 • B forwards 6, 7, 8 1 23456 7 8 910

9. Batching • A batch map indicates highest priority node that received each packet in the batch • The map is updated and sent over along with data • Gossip mechanism: updated batch map propagates from high priority nodes to low priority nodes and ultimately to source • When the source receives the updated batch map, it restarts transmission if all packets haven’t got through

10. Evaluation • Comparison between traditional 802.11 is done with ExOR • Throughput between 65 randomly selected node pairs evaluated • 1 mega-byte file exchanged • Batch size is 100 • Data rate 1 megabit/second

11. ExOR: 2x overall improvement 1.0 • Median throughputs: 240 Kbits/sec for ExOR, 121 Kbits/sec for Traditional 0.8 0.6 Cumulative Fraction of Node Pairs 0.4 0.2 ExOR Traditional 0 0 200 400 600 800 Throughput (Kbits/sec)

12. 25 Highest throughput pairs 3 Traditional Hops 2.3x 2 Traditional Hops 1.7x 1 Traditional Hop 1.14x 1000 ExOR TraditionalRouting 800 600 Throughput (Kbits/sec) 400 200 0 Node Pair

13. 25 Lowest throughput pairs 1000 ExOR 4 Traditional Hops 3.3x TraditionalRouting 800 600 Throughput (Kbits/sec) 400 200 0 Node Pair Longer Routes

14. 58% of Traditional Routing transmissions 25% of ExOR transmissions ExOR moves packets farther • ExOR average: 422 meters/transmission • Traditional Routing average: 205 meters/tx 0.6 ExOR Traditional Routing Fraction of Transmissions 0.2 0.1 0 0 100 200 300 400 500 600 700 800 900 1000 Distance (meters)

15. Summary • ExOR opportunistically exploits wireless broadcast • long distance transmission • Avoids retransmission by allowing a low priority node to forward

16. Issues • Periodic link state flooding • Queuing for batching causes delay for interactive applications • Uses constant data rate for evaluation

17. Thank You • Questions ?