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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