1. CMAP: Harnessing Exposed Terminals in Wireless Networks Mythili Vutukuru Joint work with Kyle Jamieson and Hari Balakrishnan

2. 2 The Problem u v x y  Which transmissions concurrently?  Increase throughput by maximizing concurrency. X z

3. 3 u vy z  Exposed terminal problem. Today’s Solution: CSMA x Energy > carrier sense threshold

4. 4 Key Insight  Existing solutions: rules to predict which concurrent transmissions increase throughput.  Instead, watch and discover which concurrent transmissions increase throughput.

5. 5 CMAP: Conflict Map  Map of conflicting transmissions based on empirical evidence.  Built in distributed, online manner.  Exposed terminals – 2x gain on CSMA. u vy z x u  v & x  y NO!

6. 6 Roadmap  CMAP Design Conflict maps ACK & Backoff Policy  Implementation  Evaluation

7. 7 What is a “conflict”? u v y x  Conflict – throughput lower when concurrent.  Loss rate threshold to decide conflicts. No Conflict. Conflict. 50% loss

8. 8 Discovering Conflicts u vy x  Loss rate of u  v when x is concurrent >50% then infer conflict at v.  Conflict entries timed out periodically. When u transmits to me, x causes interference. p q

9. 9 Populating the Conflict Map u v y x When u transmits to me, x causes interference. Do not transmit to v when x  anyone. Do not transmit to anyone when u  v. Conflict map z

10. 10 Channel Access Decisions  Nodes always overhear channel.  Consult conflict map before transmission.  Carrier sense always disabled!

11. 11 Roadmap  CMAP Design Conflict maps ACK & Backoff Policy  Implementation  Evaluation

12. 12 Windowed ACKs  Sliding window of packets at sender. u y z x X

13. 13 Backoff Policy  Cannot defer when hidden terminals.  Exponential backoff.  When loss rate in ACKs > threshold. u v y x Do not transmit to v when x  anyone. u must hear x.

14. 14 Roadmap  CMAP Design Conflict maps ACK & Backoff Policy  Implementation  Evaluation

15. 15 Implementation Challenges 1.At receiver: Identify colliding senders. 2.At sender: Identify ongoing transmissions. PHY MAC Trailer

16. 16 Implementation Options  Software radios – Partial Packet Recovery. [Jamieson and Balakrishnan, SIGCOMM 2007]  Commodity hardware – separate header and trailer packets. Header pktTrailer pkt

17. 17 Prototype Implementation MadWifi Driver Atheros 802.11 card Conflict MapsACKs & Backoff CSMA, ACKs & Backoff disabled. Click Kernel Module PHY MAC

18. 18 Roadmap  CMAP Design Conflict maps ACK & Backoff Policy  Implementation  Evaluation

19. 19 Evaluation  50-node 802.11a indoor testbed.  Does CMAP improve throughput by increasing concurrency?

20. 20 Two Senders In Range  Senders in range.  1400-byte UDP @ 6 Mbps.  50 unique sets of four nodes.  CMAP, CSMA, no CS no acks.  Exposed terminals.  Interfering transmissions.

21. 21 Two Senders In Range CSMA better. No CS better.

22. 22 Two Senders In Range Ideal is max of CSMA & No CS

23. 23 Two Senders In Range CMAP traces ideal curve.

24. 24 Multiple Concurrent Senders  AP-client networks.  Tree-based mesh networks.

25. 25 Multiple Concurrent Senders AP-client – 20-47% better over CSMA. Mesh – 52% better over CSMA.

26. 26  Hidden terminals – backoff ensures CMAP similar to CSMA.  CMAP without windowed ACKs gets only half the gains → windowed ACKs useful.  CMAP’s gains hold across multiple bit-rates. More results in the paper

27. 27 Related Work  RTS but no CTS → exposed. [Karn, Shukla et al.]  Offline training to identify exposed terminals. [Mittal and Belding] u y z x RTS CTS X

28. 28 Limitations  Losses until conflict map entries populated.  Unequal pkt sizes → longer to detect conflicts.  Cannot detect conflicts when interfering node’s headers cannot be decoded.

29. 29 Contributions  MAC to improve throughput by increasing concurrency.  Key idea: watch and discover conflicts.  Experiments show increased throughput. 2x improvement over CSMA with exposed terminals. ~50% improvement in AP and mesh networks.