Using Directional Antennas for
Medium Access Control
in Ad Hoc Networks
MOBICOM 2002
R. Roy Choudhury et al.
2002.10.16
Prepared by Hyeeun Choi
Contents
Introduction
Related Works
Preliminaries
Basic Directional MAC (DMAC) Protocol
Multi-Hop RTS MAC (MMAC)
Performance Evaluation
Future Work
Conclusion
2/26
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
3/26
Omni-directional Antennas
Silenced
Node
D
B
S
A
C
4/26
Directional Antennas
Not
possible
using Omni
D
B
S
C
A
5/26
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
6/26
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)
7/26
Preliminaries (2/2)
IEEE 802.11
Physical Carrier
Sensing
Sense
Virtual
Carrier
Sensing
IEEE 802.11 DCF – RTS/CTS access scheme
8/26
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
9/26
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
10/26
Basic DMAC Protocol (2/2)
Directional NAV (DNAV) Table
Tables that keeps track of the directions towards
which node must not initiate a transmission
H
ε = 2ß
RTS
E
2*ß
ε θ
DNAV
C
CTS
B
+Θ
If Θ> 0 ,
New transmission can
be initiated
11/26
Problems with Basic DMAC (1/4)
Hidden Terminal Problems due to asymmetry in
gain
A does not get RTS/CTS from C/B
Data
RTS
A
B
C
12/26
Problems with Basic DMAC (2/4)
Hidden Terminal Problems due to unheard RTS/CTS
D
A
B
C
13/26
Problems with Basic DMAC (3/4)
Shape of Silence Regions
Region of interference for
omnidirectional transmission
Region of interference for
directional transmission
14/26
Problems with Basic DMAC (4/4)
Deafness
Z
RTS
A
B
DATA
RTS
X
X does not know node A is busy.
X keeps transmitting RTSs to
node A
15/26
MMAC Protocol (1/3)
Attempts to exploit the extended transmission
range
Make Use of DD Links
Direction-Direction (DD) Neighbor
A
C
A and C can communication each other directly
16/26
MMAC Protocol (2/3)
Protocol Description : Multi-Hop RTS
Based on Basic DMAC protocol
DO neighbors
RTS
DD neighbors
C
B
G
A
D
DATA
T
F
R
S
17/26
MMAC Protocol (3/3)
Channel Reservation
Send Forwarding RTS with Profile of node F
Fowarding RTS
C
B
A
D
DATA
G
T
F
R
S
18/26
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
19/26
Performance Evaluation (2/6)
High Spatial Reuse
D
A
E
F
IEEE 802.11 : 1189.73
Basic DMAC : 2704.18
C
B
Aggregate Throughput (Kbps)
High Directional Interference
Hidden terminal Problem
A
D B
E
C
F
Aggregate Throughput (Kbps)
IEEE 802.11 : 1194.81
Basic DMAC : 1419.51 20/26
Performance Evaluation (3/6)
Aligned Routes
150m
21/26
Performance Evaluation (4/6)
Less aligned Routes
22/26
Performance Evaluation (5/6)
Randomly Chosen Routes
23/26
Performance Evaluation (6/6)
Random Topology
24/26
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
25/26
Conclusion
Directional MAC protocols show improvement in
aggregate throughput and delay
Performance dependent on topology
But not always
Random topology aids directional communication
MMAC outperforms DMAC & 802.11
802.11 better in some scenarios
26/26
Related Works
Nasipuri et al
Ko et al
A node may transmit in directions that do not
interfere with ongoing transmission
Bandyopahyay et al
Assume that the gain is equal to the gain of omnidirectional antennas
Present another MAC which uses additional
messages to inform neighborhood nodes about
ongoing communications
Takai et al
Directional virtual Carrier Sensing (DVCS)
27/26
Extra Slides
Gain of Antenna
Used to quantify the directionality of an antenna
Relative power in one direction compared to an
omni-directional antenna
28/26
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