Avoiding Head of Line Blocking in Directional Antenna

1. Avoiding Head of Line Blocking in Directional Antenna Vinay Kolar, Sameer Tilak, Dr. Nael Abu-Ghazaleh

2. Synopsis • Directional Antennas - A new technology emerging Ad hoc networking. • MAC layer faces unique challenges that were absent in omni-directional antenna system. • Scope: • Identify Head of Line (HoL) blocking and propose new queuing policy. • Incorrect Virtual carrier sensing

3. Presentation contents • Background • Problem definition • Mechanism to avoid HoL • Results • Conclusions

4. A X B Y Omni vs. Directional antenna • Omni • Directional

5. Directional MAC (DMAC) • Assumptions: • Capable of operating in omni and directional mode • AoA for a signal can be captured from antenna • RTS-CTS handshake similar to 802.11 • Omni RTS • Directional RTS

6. Angle of Arrival (AoA) cache • Table of <node, angle> tuples. • Add/Update: • If X hears from Y at angle z, then X adds/updates <Y,z> in its AoA cache. • Delete • If X fails to reach Y in direction z for DIRECTIONAL_TRANSMIT_LIMIT. • Timer expires

7. Omni vs. directional mode Packet to X X in AoA Cache? NO Transmit in omni YES Z = Get AoA for X From AoA cache Transmit packet directionally at Z degrees

8. RTS Busy! A Directional Virtual Carrier Sensing (DVCS) • Directional NAV (DNAV) table • If RTS-CTS is overheard in direction ‘z’ • Mark sector as busy • For a constant θ, [(z- θ),(z+ θ),duration] • Before transmitting: • Check if channel is busy

9. Presentation contents • Background • Problem definition • Mechanism to avoid HoL • Results • Conclusions

10. C D Spatial reuse in Directional Antennas • Spatial reuse: • But, is spatial reuse being used in DMAC? • Head of Line (HoL) Blocking • Spatial reuse is being limited because of HoL blocking B

11. Presentation contents • Background • Problem definition • Mechanism to avoid HoL • Results • Conclusions

12. Avoiding HoL blocking • Existing Queueing mechanism • Strict priority FIFO queuing • Ineffective for DMAC • What is needed to avoid HoL? • Mechanism to find out the time interval for which the channel might be busy in a particular direction • Sensing the channel in direction of each packet?

13. Avoiding HoL blocking • If such mechanism is present: • Use greedy approach • Schedule the packet with least wait time. • Use DNAV!! • For given directions, check DNAV and record wait times for each packet. • Choose packet with minimum wait time.

14. Avoiding HoL blocking • Is DNAV accurate? • What if the node was deaf and DNAV was not updated? • Live with it !! • Chances of marking wrong angle in DNAV?

15. Avoiding HoL blocking • Marking right information in DNAV • When X gets a packet from Z when it is locked: • Update only the wait time • Do not update the angle • Update angle and wait time when X is in omni mode

16. Avoiding HoL blocking • Terminologies • Interlinking queue • Routing layer inserts the packet into this queue • MAC picks up the packet from this queue • MAC Queue • New queue for the proposed protocol from which the DMAC will pick the packets for transmitting • A MAC Queue can accommodate a maximum of MAC-QUEUE-SIZE packets.

17. Proposed queuing policy • If MAC Queue is not full • Buffer packets from Interlinking queue to MAC Queue • Check MAC Queue for the packet of least wait time (respecting priority) • Transmit that packet

18. Omni-directional packets • Have the maximum wait time • If an omni packet is head of Interlinking queue • Transmit all packets from MAC Queue • Schedule omni packet • Disadvantage: • Packets which are behind the omni packet will not be scanned till the omni packet is sent. • Starving of omni packet

19. Presentation contents • Background • Mechanism to avoid HoL • Problem definition • Results • Conclusions

20. Results: Simple Topology Throttling connection • 1-2 obstructs 4-3 flow • If 1-2 is very high, then chances of a packet 4-3 being transmitted is low • 4-5 packet gets blocked

21. Results: Simple Topology • Demonstrate throughput improvement of 4-5 • When connection rate 4-3 is varied • Good improvement when 4-3 connection interval is low

22. Results: Simple Topology • Queue size is varied

23. Results - Grid • Improvements: • Throughput • End to end delay

24. Results – Grid • Throughput • Upto 20% improvement • End to end delay • Upto 25% improvement

25. Presentation contents • Background • Problem definition • Mechanism to avoid HoL • Results • Conclusions

26. Conclusions • Spatial reuse can be made more effective • Identified and proposed a solution to solve the HoL • Proposed a scheme to solve incorrect AoA updates • Good results with incorrect DNAV • Greater improvement if deafness is solved

27. Future Work • Study HoL with DMACs which reduce deafness • Reduce the omni-directional packet block • Without letting omni-packets to starve • Study the effects when the number of sectors are varied

28. References [1] Choudhury, R. R., and Vaidya, N. H. “Deafness: A Problem in Ad Hoc Networks when using Directional Antennas” [2] Choudhury, R. R., and Vaidya, N. H. “Impact of Directional Antennas on Ad Hoc Networks Routing”. [3] Korakis, T., Jakllari, G., and Tassiulas, L. “A MAC protocol for full exploitation of directional antennas in ad-hoc wireless networks” [4] Takai, M., Martin, J., Bagrodia, R., and Ren, A. “Directional virtual carrier sensing for directional antennas in mobile ad hoc networks”. [5] Choudhury, R. R., Yang, X., Vaidya, N. H., and Ramanathan, R. “Using directional antennas for medium access control in ad hoc networks”. [6] Xu, S., and Saadawi, T. “Revealing the problems with 802.11 medium access control protocol in multi-hop wireless ad hoc networks”.

29. Questions, Comments or Suggestions welcome • Contact: vinkolar@cs.binghamton.edu Thank you !