CMAP: Harnessing Exposed Terminals in Wireless Networks

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

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

3. u v y z x Today’s Solution: CSMA • Exposed terminal problem. Energy > carrier sense threshold

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

5. u v y z x CMAP: Conflict Map • Map of conflicting transmissions based on empirical evidence. • Built in distributed, online manner. • Exposed terminals – 2x gain on CSMA. u v & x  yNO!

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

23. Two Senders In Range CMAP traces ideal curve.

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

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

26. More results in the paper • 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.

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

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. 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.