1 MAC for Directional Antenna Redes Ad Hoc Sem Fio Prof. Marco Aurélio Spohn DSC UFCG 2010.1.
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Transcript of 1 MAC for Directional Antenna Redes Ad Hoc Sem Fio Prof. Marco Aurélio Spohn DSC UFCG 2010.1.
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MAC for Directional Antenna
Redes Ad Hoc Sem FioProf. Marco Aurélio Spohn
DSC UFCG2010.1
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MAC for Directional Antenna
Benefits of Directional Antenna
More spatial reuse With omni-directional antenna, packets intended to one
neighbor reaches all neighbors as well
Node A
Node B
Node C
Node A
Node B
Node C
Omni-directional transmission Directional transmission
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MAC for Directional Antenna
Benefits of Directional Antenna
Increase “range”, keeping transmit power constant
Reduce transmit power, keeping range comparable with omni mode
Reduces interference, potentially increasing spatial reuse
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More Spatial Reuse
A B
C D
Omni-directional antenna
While A is transmitting to B, C cannottransmit to D
A B
C D
Directional antenna
Both A and C can transmit simultaneously
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Antenna Model
2 Operation Modes: Omni and Directional
A node may operate in any one mode at any given time
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Antenna Model
In Omni Mode:
Nodes receive signals with gain Go
While idle a node stays in omni mode
In Directional Mode:
Capable of beamforming in specified direction
Directional Gain Gd (Gd > Go)
Symmetry: Transmit gain = Receive gain
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Multi-Beam Adaptive Array (MBAA) directional Antennas
An MBAA antenna can successfully receive and transmit one or more overlapping packets at the same time by pointing its beams toward individual packet directions, while annulling all other unwanted directions.
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Multi-Beam Adaptive Array (MBAA) directional Antennas
Node d transmitting to nodes b and c at the same time.
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Two “directional” models
Steerable: can point to any direction (there is a minimum beam-width)
Switched-beam: many antenna components cover all directions (i.e., 4 antenna elements covering each 90 degrees)
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Directional Packet Transmission
A B
B’s omni receive range
D-O transmission
A B
B’s directional receive beam
D-D transmission
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MAC Designs for Directional Antenna
Most proposals use RTS/CTS dialog
They differ in how RTS/CTS are transmitted
Omni-directional transmit: ORTS, OCTS
Directional transmit: DRTS, DCTS
Current proposals: ORTS/OCTS [Nasipuri’00]
DRTS/OCTS [Ko’00]
DRTS/DCTS [Choudhury’02]
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ORTS/OCTS
Sender sends omni-directional RTS Receiver sends omni-directional CTS
Receiver also records direction of sender by determining the antenna on which the RTS signal was received with highest power level
Similarly, the sender, on receiving CTS, records the direction of the receiver
All nodes overhearing RTS/CTS defer transmissions
Sender then sends DATA directionally to the receiver
Receiver sends directional ACK
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ORTS/OCTS – cont.
Protocol takes advantage of reduction in interference due to directional transmission/reception of DATA
All neighbours of sender/receiver defer transmission on receiving omni-directional RTS/CTS
spatial reuse benefit not realized
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D-MAC
Uses directional antenna for sending RTS, DATA and ACK in a particular direction, whereas CTS sent omni-directionally
Directional RTS (DRTS) andOmni-directional CTS (OCTS)
RTS and CTS block antenna elements, making it possible to transmit/receive even though a node hear a RTS/CTS
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DATA
DRTS(B)
OCTS(B,C) OCTS(B,C)
ACK
A B C ED
DRTS(D)
DATA
ACK
OCTS(D,E)
DMAC: DRTS/OCTS
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Directional NAV
Physical carrier sensing still omni-directional
Virtual carrier sensing directional – directional NAV
When RTS/CTS received from a particular direction, record the direction of arrival and duration of proposed transfer
Channel assumed to be busy in the direction from which RTS/CTS received
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Nodes overhearing RTS or CTS set up directional NAV (DNAV) for that Direction of Arrival (DoA)
X
D
Y
CCTS
Directional NAV (DNAV)
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Nodes overhearing RTS or CTS set up directional NAV (DNAV) for that Direction of Arrival (DoA)
X Y
Directional NAV (DNAV)
D
C DNAV
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Directional NAV (DNAV)
A
B
Cθ
DNAVD
New transmission initiated only if direction of transmission does not overlap with DNAV, i.e., if (θ > 0)
RTS
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Deafness: A MAC problem with directional antennas
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ToneDMAC
Use out-of-band tone acting as a feedback mechanism to all neighbors
After the completion of every dialog, a communicating node transmits an out-of-band tone, OMNIDIRECTIONALLY!
This tone helps other nodes to distinguish deafness from congestion
If the tone is received from the intended receiver, a node deduces deafness and sets backoff timer appropriately
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Acknowledgments
Presentation adapted from the following sources: Prasant Mohapatra, UC Davis, ://www.cs.ucdavis.edu/~prasant/ECS257/NOTES/Adhoc-Sensor.ppt Katia Obraczka and J.J. Garcia-Luna-Aceves, UCSC http://www.cse.ucsc.edu/classes/cmpe257/Spring05