Avoiding Head of Line Blocking in Directional Antenna

Vinay Kolar, Sameer Tilak,

Dr. Nael Abu-Ghazaleh

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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

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Presentation contents

Background Problem definition Mechanism to avoid HoL Results Conclusions

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Omni vs. Directional antenna

Omni

DirectionalA B

X Y

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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

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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

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Omni vs. directional mode

Packet to X

X in AoA Cache?

Z = Get AoA for XFrom AoA cache

Transmit packet directionally at Z degrees

YES

Transmit in omniNO

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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!

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Presentation contents

Background Problem definition Mechanism to avoid HoL Results Conclusions

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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

BCD

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Presentation contents

Background Problem definition Mechanism to avoid HoL Results Conclusions

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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?

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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.

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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?

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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

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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.

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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

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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

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Presentation contents

Background Mechanism to avoid HoL Problem definition Results Conclusions

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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

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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

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Results: Simple Topology Queue size is varied

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Results - Grid

Improvements: Throughput End to end delay

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Results – Grid

Throughput Upto 20% improvement

End to end delay Upto 25% improvement

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Presentation contents

Background Problem definition Mechanism to avoid HoL Results Conclusions

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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

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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

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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”.

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Questions, Comments or Suggestions welcome

Contact: vinkolar@cs.binghamton.edu