RTSS/CTSS: Mitigation of Exposed Terminals in Static 802.11-Based Mesh Networks

1. RTSS/CTSS: Mitigation of Exposed Terminals in Static 802.11-Based Mesh Networks Kimaya Mittal and Elizabeth Belding MOMENT Lab, Dept. of Computer Science University of California, Santa Barbara

2. Introduction • Capacity of mesh networks severely constrained • Limited bandwidth • Shared medium • Contention along multihop paths • Efficient spatial usage of medium critical Kimaya Mittal

3. CS range X Z Exposed Terminals W Y • Common artifact of CSMA • Wasted transmission opportunities • Reduced spatial reuse • Objective: To increase spatial reuse and capacity utilization by mitigating the exposed terminal problem Kimaya Mittal

4. CS range Hidden Terminals Q S P R • Complementary effect • Adjustment of CS range / transmit power decreases one, increases other • Prevalence of hidden/exposed terminals depends on • Topology • CS range • Capture capability • Data rate Kimaya Mittal

5. Prevalence of Hidden and Exposed Terminals • QualNet simulation of grid topology • Identification of strong links • Pair-wise simultaneous transmissions • To identify exposed links, carrier sense must be disabled! Exposed terminals Hidden terminals CS range X Z Q S W Y P R Kimaya Mittal

6. Prevalence in Grid Topology • 25 nodes (5 x 5) • 8,688 link pairs tested at 11 Mbps • 37,476 link pairs tested at 2 Mbps Kimaya Mittal

7. Proposed Solution • Two phases • Phase 1: Empirical detection of exposed terminals • Phase 2: Coordination of simultaneous transmissions over exposed links Kimaya Mittal

8. Detection of Exposed Terminals • Distance-based assumptions avoided • Empirical approach • Adapted from broadcast interference estimation technique [Padhye et al. IMC 05] • O(n2) rather than O(n4) • Key modifications • Tests repeated with CS disabled • Tests repeated at different data rates • ACK collisions explicitly accounted Kimaya Mittal

9. Coordination of Simultaneous Transmissions • New control messages • Request-To-Send-Simultaneously (RTSS) • Clear-To-Send-Simultaneously (CTSS) • CTSS synchronizes transmissions of mutually exposed links CS range X Z W Y CTSS Kimaya Mittal

10. Coordination of Simultaneous Transmissions • New control messages • Request-To-Send-Simultaneously (RTSS) • Clear-To-Send-Simultaneously (CTSS) • CTSS synchronizes transmissions of mutually exposed links CS range X Z W Y Kimaya Mittal

11. CTSS Implementation • CTSS frame before every data packet generates high overhead • PLCP preamble and header sent at lowest rate • CTSS implemented as header on data packet PLCP Hdr CTSS MAC Hdr Payload Kimaya Mittal

12. CS range X Z CTSS Processing W Y CTSS • CTSS header processed independently as soon as received • Interface switched to transmit mode Kimaya Mittal

13. The RTSS Message • Motivation: CTSS mechanism to be used only when necessary • RTSS broadcast by node when required • Triggered by queue size • Lists links • Broadcast periodically till needed • CTSS sent only if RTSS received recently Kimaya Mittal

14. Lost/Unused CTSS • Causes: • Collisions • Transmission errors • Unavailability of data • Sensed interference • Low CTSS overhead critical Kimaya Mittal

15. CTSS Destination Selection • Multiple CTSS candidates may exist • Policy for CTSS destination selection is key • Impacts fraction of CTSS successfully used • Proposed policy based on received signal strength of RTSS • Motivation: To minimize CTSS loss R RTSS Selected CTSS destination (highest received strength) RTSS P Q RTSS S Kimaya Mittal

16. CTSS Data Rate • CTSS and data can be transmitted at different data rates • Tradeoff between range and overhead • Physical layer must know CTSS data rate • Can be a fixed value • Can be indicated in PLCP header Kimaya Mittal

17. Evaluation • Simulation-based • QualNet version 3.9, CBR traffic • Evaluation outline • Baseline • Impact of CTSS destination selection policy • Impact of CTSS data rate Kimaya Mittal

18. Evaluation Baseline • Aggregate throughput improvement • 60% in two links topology • 50% in parallel lines topology A B C D E A B C D F G H I J Two links topology Parallel lines topology Kimaya Mittal

19. Grid Topology • Inter-node spacing = 150m • 4 gateways at corners of grid • Pre-configured static routes from each node to a gateway • Routes minimize hops and distribute nodes evenly among gateways A B C D E F G H I J K L M N O P Q R S T U V W X Y Gateway Kimaya Mittal

20. Impact of CTSS Destination Selection Policy • Two sample traffic scenarios • Performance compared with random selection policy A B C D E F G H I J K L M N O CBR source in Scenario I P Q R S T CBR source in Scenario II U V W X Y Kimaya Mittal

21. Impact of CTSS Destination Selection Policy • Throughput improvement in Scenario I • RSS = 40% • Random = 27% • Throughput improvement in Scenario II • RSS = 16% • Random = 18% Kimaya Mittal

22. Impact of CTSS Data Rate • Grid topology • Number of flows varied from 2 to 8 • Sources selected randomly • Evenly distributed among gateways • CTSS data rate set to 1, 2, and 11 Mbps in different tests Kimaya Mittal

23. Impact of CTSS Data Rate Kimaya Mittal

24. Conclusion • RTSS/CTSS approach effective • Benefits • Very low overhead • Maintains distributed contention-based nature of MAC protocol • No time synchronization • Complementary to solutions that tune CS range • Drawbacks • Increased complexity of PHY layer • Implementation with current hardware infeasible Kimaya Mittal

25. Conclusion (cont.) • Throughput improvement depends on topology and data rate • Between 15% and 60% in simulated scenarios • Approach most beneficial in dense networks with strong links • Future Work: • Dynamic detection of exposed links • Enhancements to CTSS destination selection policy Kimaya Mittal

26. Thank you! Questions / Comments?