Opportunistic Routing in Multi-hop Wireless Networks

1. Opportunistic Routing inMulti-hop Wireless Networks Shunyuan Ye 12/16/05

2. Paper • "EXOR: Opportunistic Multi-Hop Routing for Wireless Networks“, Sanjit Biswas and Robert Morris, In Proc. of ACM/SIGCOMM 2005

3. Outline • Introduction • Basic idea • Protocol design details • Measurements • Conclusion

4. Traditional routing • Identify a route, forward over links • Abstract radio to look like a wired link

5. Radios aren’t wires • Packet is broadcast • Reception is probabilistic

6. Opportunistic routing • Motivation • Exploiting probabilistic broadcast • Goal • High throughput and network capacity • ExOR • Extremely opportunistic routing

7. Outline • Introduction • Basic idea • Protocol design details • Measurements • Conclusion

8. Why ExOR good? • Assumes independent losses • Traditional routing: 1/0.25 + 1 = 5 tx • ExOR: 1/(1-(1-0.25)^4) + 1 = 2.5 tx

9. Basic idea • Probability falls off gradually with distance • Traditional route through N2, N4 • EXOR exploits lucky long receptions • Node closest to the dst has highest priority

10. Outline • Introduction • Basic idea • Protocol design details • Measurements • Conclusion

11. Protocol Design • Source’s behavior • Intermediate nodes’ behavior • Destination behavior

12. Source’s behavior • Collects enough packets of the same destination to form a batch • ExOR operates on batches of packets for efficiency • Selects a set of nodes to be candidate forwarders, and includes the prioritized list in the overhead of every packet

13. Priority ordering • Goal: nodes “closest” to the destination send first • Higher delivery probability, closer to the destination

14. Priority ordering (2) • ETX=1/(delivery probability) • Sort by ETX metric to dst • Nodes periodically flood ETX “link state” measurement

15. Protocol Design • Source’s behavior • Intermediate nodes’ behavior • Destination behavior

16. Forwarders’ behavior (1) • How can a node know whether it is one of the forwarders or not? • Check the forwarder list in the overhead of the received packet • If the node finds itself in the list, buffer the packet and keep state of this batch • If no, discard the packet

17. Forwarders’ behavior (2) • How can a node know whether the packet it receives has also been received by a node with higher priority or not? • ExOR designs a “batch map” to record, for every packet in the batch, the highest-priority node known to have received that packet.

18. Batch map example • src generates a batch map before tx • src inserts the batch map into the overhead of every packet in this batch

19. Batch map example • When N1 first receives a packet from src, it keeps the batch map in local • N1 updates its batch map when receiving new packets of this batch

20. Batch map example • N2 receives packets from src, so it doesn’t know that N1 has received packet 1 and 2

21. Batch map example • After scr finished transmission, N2 begins to broadcast packets it received • N1 also received these packets and then update its local batch map

22. Batch map example • src updates its local map and gets into know that all the packets in the batch has been received by dst or intermediate nodes, no acknowledge is needed.

23. Forwarders’ behavior (3) • How can a node know when is its turn to transmit? • ExOR add a “fragment size” and “fragment number” in the overhead of every packet. • Fragment size: number of packets the node has to send • Fragment no. : the index of the sending packet in the fragment

24. Fragment example • N2’s fragment size is 2 and fragment no. is 1 for packet 3 and 2 for packet 2

25. Forwarding estimate • When N1 receives packet 3 from N2, it knows that there is still one packet N2 has to send, then N1 can estimate the time to forwarding its fragment

26. Protocol Design • Source’s behavior • Intermediate nodes’ behavior • Destination behavior

27. Destination’s behavior • Actually destination is the last intermediate node and has the highest priority. • After the finish of src’s transmission. Destination sends out ten packets only including the batch map, to inform other nodes about the packets it has received

28. Outline • Introduction • Basic idea • Protocol design details • Measurements • Conclusion

29. 65 Roofnet node pairs

30. Exor: 2x overall improvement • Median throughputs: 240Kbit/s for ExOR 121Kbit/s for Traditional

31. 25 highest throughput pairs

32. 25 lowest throughput pairs

33. Conclusion • ExOR achieves 2x throughput improvement • Future work will focus on: • Choosing best 802.11 bit rate • Cooperation between simultaneous flows