1 mac protocols that use directional antennnas. 2 directional antenna directional communication ...
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MAC Protocols that use MAC Protocols that use Directional AntennnasDirectional Antennnas
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Directional AntennaDirectional Antenna Directional
communication Less Energy in the wrong
direction• Better Spatial reuse and
less multipath More Energy in the right
direction• Longer ranges more robust
links Reduce interference to
other neighbor nodes increase throughput
Antenna Model Typically, 2 operation mode Omni mode / Directional
Mode Directional Antenna Type
Switched Antenna : Select One
Steerable/Steered Antenna Adaptive Array Antenna
A B
X
Y
A B
X
Y
Omni-Directional Antenna
Directional Antenna
Red nodes cannot communicate
presently
Not Possible
using Omni
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MAC Protocol using Directional MAC Protocol using Directional AntennasAntennas
Each node has only 1 radio transceiver
A transceiver Can tx or rx only one packet at a given
time Equipped with M directional antennas
Antennas Each antenna has non-overlapping conical
radiation pattern Every antenna individually or all the
antennas can be switched to the active or passive modes
• The transceiver used only the antennas in active mode
• If all the antennas of the node are active, similar to omni-directional antenna
It is assumed that the radio range is the same for all directional antennas of the nodes
MNs do not know direction of the sender and receiver nodes
Make use of RTC/CTS exchange Direction of the sender is identified by the
antenna received with max power sender/receiver node tx/rx data packet
through the selected directional antennna
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Directional Busy Tone-based MACDirectional Busy Tone-based MAC Adapts the DBTMA for use
with directional antennas Assumption: Orientation of
sectors of each antenna element remains fixed (does not support MNs)
Sender: tx RTS in all direction Receiver
Determines the antenna on which RTS is received with max gain
Turn on BTr in the direction toward the sender
Send back a directional CTS Sender:
Turn directional BTt to the receiver Tx data packet through the
antenna on which the CTS packet was received with max gain
Omni-directional BT vs Directional BT
Directional BT is not collision-free !!CX may cause collision
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D-MAC: Directional MACD-MAC: Directional MAC
Young-Bae Ko, V. Shankarkumar, N. Vaidya (2000)
Assumption: Each node knows about (via GPS) Location of its neighbors Its own location
MAC protocol similar to 802.11, but on a per-antenna basis If a node has overheard an
RTS or CTS on a particular antenna, then the antenna is blocked for the transmission duration (NAV)
But, remaining antennas of the node can be used for Tx
D-MAC-1 Directional RTS (DRTS) /
Omni-Directional CTS (OCTS)
DRTS from E to A may collide with OCTS or ACK from B to A
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D-MAC (Cont’d)D-MAC (Cont’d) DMAC-2
DRTS or ORTS / OCTS• Send ORTS if non of
antennas are blocked• Send DRTS, otherwise
Reduce collision between control packets
After receiving ORTS from node D, node C would not respond node D: backoff and ReTx
Avoid this situation, introduce Directional wait-to-send (DWTS) packet Carries the expected
duration of AB
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Multichannel MAC Multichannel MAC ProtocolsProtocolsfor Data Transmissionfor Data Transmission
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MMAC: Multichannel MACMMAC: Multichannel MAC Multiple channels for data Tx
No dedicated control channel Need single transceiver Each node maintains a data structure
called Preferable Channel List (PCL)• High preference channel (HIGH): has
been selected and is being used by the node in the current beacon interval
• Medium preference channel (MID): is free and is not being currently used by neighbor
• Lowest preference channel (LOW): already being used by neighbor
ATIM (ad hoc traffic indication msg) Is used to negotiate for channels during
the current beacon interval Exists at the start of every beacon
interval ATIM msgs exchange on the default
channel Carries the PCL of the transmitting
node May be lost due to collision back-off
Higher throughput than IEEE 802.11 when network load is high
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MCSMA: Multichannel CSMA MACMCSMA: Multichannel CSMA MAC Available BW is divided into N channels
A channel BW = BW/N Channels are created by FDMA or CSMA, but not on TDMA
(because it requires global time synchronization) Idle node continuously monitors and marks IDLE
channels if TRSS < ST TRSS: total received signal strength, ST: sensing threshold
CS If free channel list is empty, waits for any channel to become
IDLE, • i.e. wait for LIFS + random back-off period
Otherwise, select an IDLE channel (check first the most recently successfully transmitted channel)
Before actual transmission If the selected channel is idle (TRSS < ST) for at least LIFS
period, Tx immediately Otherwise, LIFS + random back-off delay
When N is large or traffic is high, each node tends to reserve a channel greatly reduce collision
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Power Control MACPower Control MAC
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Energy / Power ConservationEnergy / Power Conservation
Power Saving Go to a doze state by
Powering off its wireless network interface
Ex) DEC Roamabout Radio
• TX: 5.76 W• RX; 2.88 W• Idle; 0.35 W
Power Control Vary Transmit Power
suitably to reduce power consumption.
A CBB transmits
to A
B’s transmission is overheard by C which causes unnecessary power consumption
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Power Saving SchemesPower Saving Schemes
PAMAS: Power Aware Multi-Access protocol with Signaling for Ad Hoc Networks C. Raghavendra, S. Singh (1998) Based on the MACA with the addition of a separate
signaling channel Powering off nodes that are not actively transmitting or
receiving. Issues
• For how long is a node powered off ?• What happens if a neighbor wishes to transmit a packet to
a node that has powered itself off ? Out-of-Band Signaling Channel
• Busy Tone; • To exchange Probe Messages to resolve powering off
interval.
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Power Control SchemesPower Control Schemes Power Control in the IEEE 802.11: BASIC
RTS/CTS are transmitted using the highest power level (Pmax)
Data/ACK are transmitted using the minimum power level (Pdesired) necessary to communicate
Different Transmission Power can lead to increase collision
PCM (Power Control MAC) Fix the shortcomings of the IEEE 802.11’s Power Control
A B C D
When A is transmitting a packet to B, this transmission may not be sensed by C
and D. So, when C and D transmit to each other using a higher power, their transmission
will collide with the on-going transmission from A to B
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BASIC Scheme in IEEE 802.11BASIC Scheme in IEEE 802.11 Pdesired
= Pmax/Pr x Rxthresh x c Pr: received power level Rxthresh: min necessary received
signal strength Assumption
attenuation is same in both direction noise level at the nodes is below a
predefined threshold value
Drawback X and Y defer their Tx during EIFS
period by overhearing RTS and CTS After EIFS period, X and Y may
attempt to Tx collision• RTS from X may cause collision with
ACK• RTS from Y may cause collision with
DATA Throughput degradation and
higher energy consumption (because of ReTx) than even the IEEE 802.11 without power control
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PCM: Power Control MACPCM: Power Control MAC
Eun-Sun Jung, N. Vaidya (2002) Based on BASIC scheme To avoid collision
Source node tx DATA packet at Pmax periodically (every EIFS period)
Duration of each such Tx > time required for physical CS Achieves throughput very close to that of
IEEE 802.11 while using much less energy