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 Omni vs. Directional antenna Omni Directional AB XY

5 Types of Directional Antennas Switched beam  Cheaper and less complex  Each sector can point beam in one fixed direction Steerable antenna  More intelligent Precision focus Null steering  Greater complexity Sector of the antenna Antenna with 8 sectors

6 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  Omni RTS  Directional RTS

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

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

9 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 A RTS Busy!

10 Presentation contents Background Problem definition Mechanism to avoid HoL Results Conclusions

11 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 C D

12 Presentation contents Background Problem definition Mechanism to avoid HoL Results Conclusions

13 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?

14 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.

15 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?

16 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

17 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.

18 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

19 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

20 Presentation contents Background Mechanism to avoid HoL Problem definition Results Conclusions

21 Results: Simple Topology 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 Throttling connection

22 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

23 Results: Simple Topology Queue size is varied

24 Results – Queue 30 Que 30 Orig: diffquesizes]$ grep "Through" ch2-6.omni.stat.orig.20sec.que pkt.bal | grep Server 2,,[1024], Application, CBR Server,Throughput (bits/s) = ,,[1024], Application, CBR Server,Throughput (bits/s) = ,,[1025], Application, CBR Server,Throughput (bits/s) = Persec diffquesizes]$ grep "Through" ch2-6.omni.stat.persec.20sec.que pkt.bal | grep Server 2,,[1024], Application, CBR Server,Throughput (bits/s) = ,,[1024], Application, CBR Server,Throughput (bits/s) = ,,[1025], Application, CBR Server,Throughput (bits/s) = Total from 4: = = Around 2% improvement!

25 Results - Grid Improvements:  Throughput  End to end delay

26 Results – Grid Throughput  Upto 20% improvement End to end delay  Upto 25% improvement

27 Results - Grid Normalized throughput for grid  Why do we get lesser gain when MAC Queue is increased from 20 to 30?

28 Results - Grid Improvement in End to end delay

29 Presentation contents Background Problem definition Mechanism to avoid HoL Results Conclusions

30 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

31 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

32 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 medium access control protocol in multi-hop wireless ad hoc networks”.

33 Questions, Comments or Suggestions welcome Contact: