using directional antennas for medium access control in ad hoc networks

Using Directional Antennas for Medium Access Control in Ad Hoc Networks

MOBICOM 2002R. Roy Choudhury et al.

2002.10.16Prepared by Hyeeun Choi

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Contents

Introduction Related Works Preliminaries Basic Directional MAC (DMAC) Protocol Multi-Hop RTS MAC (MMAC) Performance Evaluation Future Work Conclusion

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Introduction

The Problem of utilizing directional Antennas to improve the performance of ad hoc networks is non-trivial

Pros Higher gain (Reduced interference) Spatial Reuse

Cons Potential possibility to interfere with

communications taking place far away

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Omni-directional Antennas

S

D

A

B

Silenced Node

C

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Directional Antennas

S

D

A

B

C

Not possible

using Omni

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Related Works

MAC Proposals differ based on How RTS/CTS transmitted (omni, directional) Transmission range of directional antennas Channel access schemes Omni or directional NAVs

Gain of directional antennas is equal to the gain of omni-directional antennas

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Preliminaries (1/2)

Antenna Model Two Operation modes

: Omni & Directional

Omni Mode: Omni Gain = Go Idle node stays in Omni mode.

Directional Mode: Capable of beamforming in specified direction Directional Gain = Gd (Gd > Go)

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Preliminaries (2/2)

IEEE 802.11

IEEE 802.11 DCF – RTS/CTS access scheme

Physical Carrier Sense

Physical Carrier Sensing

Virtual Carrier Sensing

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Problem Formulation

Using directional antennas Spatial reuse

Possible to carry out multiple simultaneous transmissions in the same neighborhood

Higher gain Greater transmission range than omni-

directional Two distant nodes can communicate with a

single hop Routes with fewer hops

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Basic DMAC Protocol (1/2)

Channel Reservation A node listens omni-directionally when idle Sender transmits Directional-RTS (DRTS) using

specified transceiver profile Physical carrier sense Virtual carrier sense with Directional NAV

RTS received in Omni mode (only DO links used) Receiver sends Directional-CTS (DCTS)

DATA,ACK transmitted and received directionally

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Basic DMAC Protocol (2/2)

Directional NAV (DNAV) Table Tables that keeps track of the directions

towards which node must not initiate a transmission

E

H

B

2*ß ε θ

ε = 2ß + Θ

If Θ> 0 ,New transmission can be initiated

DNAV

CCTS

RTS

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Problems with Basic DMAC (1/4)

Hidden Terminal Problems due to asymmetry in gain A does not get RTS/CTS from C/B

C

A B

DataRTS

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Problems with Basic DMAC (2/4)

Hidden Terminal Problems due to unheard RTS/CTS

CB

D

A

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Problems with Basic DMAC (3/4)

Shape of Silence Regions

Region of interference for directional transmissionRegion of interference for

omnidirectional transmission

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Problems with Basic DMAC (4/4)

Deafness

RTS

RTS A B

X

Z

DATA

X does not know node A is busy. X keeps transmitting RTSs to

node A

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MMAC Protocol (1/3)

Attempts to exploit the extended transmission range Make Use of DD Links

Direction-Direction (DD) Neighbor

C

A

A and C can communication each other directly

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MMAC Protocol (2/3)

Protocol Description : Multi-Hop RTS Based on Basic DMAC protocol

D

R

G

S

T

B

A

C

F

DO neighbors

DD neighborsRTS

DATA

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MMAC Protocol (3/3)

Channel Reservation Send Forwarding RTS with Profile of node F

R

G

S

T

BC

Fowarding RTS

DATA

A FD

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Performance Evaluation (1/6)

Simulation Environment Qualnet simulator 2.6.1 Beamwidth :45 degrees Main-lobe Gain : 10 dBi 802.11 transmission range : 250meters DD transmission range : 900m approx Two way propagation model Mobility : none

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Performance Evaluation (2/6)

A

B C

D E F

A B CD E F

High Spatial Reuse

Aggregate Throughput (Kbps)

IEEE 802.11 : 1189.73Basic DMAC : 2704.18

High Directional Interference

Hidden terminal Problem

Aggregate Throughput (Kbps)

IEEE 802.11 : 1194.81Basic DMAC : 1419.51

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Performance Evaluation (3/6)

150m

Aligned Routes

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Performance Evaluation (4/6)

Less aligned Routes

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Performance Evaluation (5/6)

Randomly Chosen Routes

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Performance Evaluation (6/6)

Random Topology

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Future Work

Design of directional MAC protocols that incorporate transmit power control

New protocols that rely less on the upper layers for beamforming information

Impact of directional antennas on the performance of routing protocols

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Conclusion

Directional MAC protocols show improvement in aggregate throughput and delay But not always

Performance dependent on topology Random topology aids directional

communication

MMAC outperforms DMAC & 802.11 802.11 better in some scenarios

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Related Works

Nasipuri et al Assume that the gain is equal to the gain of

omni-directional antennas Ko et al

A node may transmit in directions that do not interfere with ongoing transmission

Bandyopahyay et al Present another MAC which uses additional

messages to inform neighborhood nodes about ongoing communications

Takai et al Directional virtual Carrier Sensing (DVCS)

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Extra Slides

Gain of Antenna Used to quantify the directionality of an

antenna Relative power in one direction compared to

an omni-directional antenna