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Priority Scheduling in Wireless Ad Hoc Networks

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  1. Priority Scheduling inWireless Ad Hoc Networks Xue Yang and NitinVaidya University of Illinois at Urbana-Champaign

  2. Problem Definition • High priority & low priority packets • High priority packets should be transmitted before low priority packets • Packets may reside at different nodes in ad hoc network

  3. Related Work • Reservation Based Schemes • Reserve channel for high priority in advance • Contention Based Schemes [Aad01] • High priority source stations occupy channel with blackburst while contending. • Simple variations of IEEE 802.11 DCF • Modify backoff stage of 802.11 • Modify inter-frame space stage of 802.11

  4. Contention Using Blackburst • Station transmitting longer burst wins • Longer bursts for high priority packets A B C

  5. Contention Using Blackburst • Hidden terminals may not sense the blackburst A B C

  6. Variations on IEEE 802.11 Figure source: IEEE 802.11 standard spec.

  7. Modifying Backoff Stage of 802.11 • Backoff intervals for low priority packets biased to be larger than high priority packets • Priority reversal problem

  8. Priority Reversal Station 1 (H) Station 2 (H) Station 3 (L) Station 4 (L) High priority backoff in [0-15] Low priority backoff in [16-31]

  9. Modifying Inter-Frame Space • Low priority source stations use larger inter-frame space (IFS) than high priority stations • PMAC [Aad01] • CWh: maximum contention window for high priority • LIFS (HIFS): IFS of low (high) priority stations. • LIFS = HIFS + CWh • Low priority packets wait too long when no high priority packets

  10. Summary of Issues

  11. Priority Scheduling in Multi-hop Networks Impact of hidden terminals on priority scheduling High priority flow Low priority flow 0 1 2 3 Station 0 is hidden from 2  need some mechanism to make station 2 aware of high priority packets at station 0

  12. Priority Scheduling in Multi-hop Networks Low priority flow 3 0 1 2 High priority flow 4 Collisions among high priority packets increases priority reversals

  13. Trade-Off • Some existing schemes trade priority scheduling with throughput for low priority flows • Example: LIFS = DIFS + CWh • Can we improve on this?

  14. A Plausible Solution

  15. Busy Tone Priority Scheduling (BTPS) • Allows two priority classes • Two busy tone used to inform hidden terminals • BT1 and BT2

  16. Busy Tone Priority Scheduling (BTPS) • High priority source stations send BT1 pulse every M backoff slots

  17. Busy Tone Priority Scheduling • Stations sensing BT1 • Low priority stations defer transmission for a short duration • Transmit BT2 pulse  propagate BT1 to hidden terminals Busy Medium

  18. Busy Tone Priority Scheduling • The IFS of low priority source stations DIFS + M slots.  access priority to high priority

  19. Busy Tone Priority Scheduling • On receiving BT2, Low priority stations defer transmission for a short duration

  20. Busy Tone Priority Scheduling (BTPS) • Busy tones inform hidden terminals of pending high priority packets • Low priority stations defer transmission after BT1 and BT2 for a short duration (= DIFS + M slots)  allows high priority packets to be transmitted first

  21. Simulations

  22. Fixed Simulation Topology Number of high priority flows is increased from 1 to 6. Bit rate 2 Mbps.

  23. Conflict Graph Flows 5 and 8 have the highest contention degree, while flows 1 , 3, 10 and 12 have the lowest contention degree

  24. Delivery Ratio of High Priority Packets PMAC BTPS Delivery Ratio of High Priority Packets IEEE 802.11 Number of high priority flows

  25. Aggregate Throughput (Kbps) IEEE 802.11 Aggregate Throughput (Kbps) BTPS PMAC Number of high priority flows

  26. Delivery Ratio of individual flows with six high priority flows (4,5,6,7,8,9) Note flows 5 and 8 which have the highest contention degree.

  27. Random Topology • Number of nodes between 10 to 80, randomly located in a 1000m x 1000m rectangle • The flows are randomly picked between two nodes within transmission range. • Number of flows between 7 to 73. • Number of high priority flows between 4 to 37.

  28. Delivery ratio of high priority packetsRandom Topology BTPS Delivery Ratio of High Priority Packets PMAC Number of nodes

  29. Aggregate Throughput (Kbps)Random Topology BTPS PMAC Number of nodes

  30. Conclusion • Advantages • BTPS provides channel access priority for high priority packets • In absence of high priority packets, BTPS does not degrade throughput for low priority unnecessarily • Disadvantages • Protocol more complicated than desired • Uses busy tones