1. Opportunistic Routing in Multi-hop Wireless Networks Sanjit Biswas and Robert Morris MIT CSAIL http://pdos.csail.mit.edu/roofnet/

2. ExOR: a new approach to routing in multi-hop wireless networks Dense 802.11-based mesh Goal is high-throughput and capacity 1 kilometer

3. packet Initial approach: Traditional routing Identify a route, forward over links Abstract radio to look like a wired link src AB dst C

4. Radios aren’t wires Every packet is broadcast Reception is probabilistic 1234561 23635 1 42345612456 src AB dst C

5. packet ExOR: exploiting probabilistic broadcast src AB dst C packet Decide who forwards after reception Goal: only closest receiver should forward Challenge: agree efficiently and avoid duplicate transmissions

6. Outline Introduction Why ExOR might increase throughput ExOR protocol Measurements Related Work

7. Why ExOR might increase throughput (1) 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 srcdstN1N2N3N4 75% 50% N5 25%

8. Why ExOR might increase throughput (2) Traditional routing: 1 / 0.25 + 1 = 5 tx ExOR: 1 / (1 – (1 – 0.25) 4 ) + 1 = 2.5 transmissions Assumes independent losses N1 srcdst N2 N3 N4 25% 100%

9. Outline Introduction Why ExOR might increase throughput ExOR protocol Measurements Related Work

10. ExOR batching Challenge: finding the closest node to have rx’d Send batches of packets for efficiency Node closest to the dst sends first –Other nodes listen, send remaining packets in turn Repeat schedule until dst has whole batch src N3 dst N4 tx: 23 tx: 57 -23  24 tx:  8 tx: 100 rx: 23 rx: 57 rx: 88 rx: 0 tx: 0 tx:  9 rx: 53 rx: 85 rx: 99 rx: 40 rx: 22 N1 N2

11. Reliable summaries Repeat summaries in every data packet Cumulative: what all previous nodes rx’d This is a gossip mechanism for summaries src N1 N2 N3 dst N4 tx: {1, 6, 7... 91, 96, 99} tx: {2, 4, 10... 97, 98} summary: {1,2,6,... 97, 98, 99} summary: {1, 6, 7... 91, 96, 99}

12. Priority ordering Goal: nodes “closest” to the destination send first Sort by ETX metric to dst –Nodes periodically flood ETX “link state” measurements –Path ETX is weighted shortest path (Dijkstra’s algorithm) Source sorts, includes list in ExOR header Details in the paper src N1 N2 N3 dst N4

13. Using ExOR with TCP Node Proxy ExOR Gateway Web Proxy Client PC Web Server TCP ExOR Batches (not TCP) Batching requires more packets than typical TCP window

14. Outline Introduction Why ExOR might increase throughput ExOR protocol Measurements Related Work

15. ExOR Evaluation Does ExOR increase throughput? When/why does it work well?

16. 65 Roofnet node pairs 1 kilometer

17. Evaluation Details 65 Node pairs 1.0MByte file transfer 1 Mbit/s 802.11 bit rate 1 KByte packets Traditional RoutingExOR 802.11 unicast with link- level retransmissions Hop-by-hop batching UDP, sending as MAC allows 802.11 broadcasts 100 packet batch size

18. ExOR: 2x overall improvement Median throughputs: 240 Kbits/sec for ExOR, 121 Kbits/sec for Traditional Throughput (Kbits/sec) 1.0 0.8 0.6 0.4 0.2 0 0200400600800 Cumulative Fraction of Node Pairs ExOR Traditional

19. 25 Highest throughput pairs Node Pair Throughput (Kbits/sec) 0 200 400 600 800 1000 ExOR Traditional Routing 1 Traditional Hop 1.14x 2 Traditional Hops 1.7x 3 Traditional Hops 2.3x

20. 25 Lowest throughput pairs Node Pair 4 Traditional Hops 3.3x Longer Routes Throughput (Kbits/sec) 0 200 400 600 800 1000 ExOR Traditional Routing

21. ExOR uses links in parallel Traditional Routing 3 forwarders 4 links ExOR 7 forwarders 18 links

22. ExOR moves packets farther ExOR average: 422 meters/transmission Traditional Routing average: 205 meters/tx Fraction of Transmissions 0 0.1 0.2 0.6 ExOR Traditional Routing 01002003004005006007008009001000 Distance (meters) 25% of ExOR transmissions 58% of Traditional Routing transmissions

23. Future Work Choosing the best 802.11 bit-rate Cooperation between simultaneous flows Coding/combining

24. Related work Relay channels [Van der Meulen][Laneman+Wornell] Flooding in meshes / sensor nets [Peng][Levis] Multi-path routing [Ganesan][Haas] Selection Diversity [Miu][Roy Chowdhury][Knightly][Zorzi]

25. Summary ExOR achieves 2x throughput improvement ExOR implemented on Roofnet Exploits radio properties, instead of hiding them

26. Thanks! For more information and source code: http://pdos.csail.mit.edu/roofnet/