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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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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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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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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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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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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”.