Using Directional Antennas for Medium Access Control in Ad Hoc Networks

1
Using Directional Antennas for Medium Access
Control in Ad Hoc Networks

• MOBICOM 2002
• R. Roy Choudhury et al.
• 2002.10.16
• Presented by Hyeeun Choi

2
Contents

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

3
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

4
Omni-directional Antennas
Silenced Node
B
D
S
A
C
5
Directional Antennas
Not possible using Omni
B
D
S
C
A
6
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

7
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 gt Go)

8
Preliminaries (2/2)

• IEEE 802.11

Virtual Carrier Sensing
9
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

10
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

11
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
e 2ß T If Tgt 0 , New transmission can be
initiated
RTS
2ß
?
E
e
B
DNAV
CTS
C
12
Problems with Basic DMAC (1/4)

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

C
B
A
13
Problems with Basic DMAC (2/4)

• Hidden Terminal Problems due to unheard RTS/CTS

D
B
C
A
14
Problems with Basic DMAC (3/4)

• Shape of Silence Regions

Region of interference for directional
transmission
Region of interference for omnidirectional
transmission
15
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
16
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
17
MMAC Protocol (2/3)

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

RTS
C
B
G
T
D
F
A
S
DATA
R
18
MMAC Protocol (3/3)

• Channel Reservation
• Send Forwarding RTS with Profile of node F

Fowarding RTS
C
B
G
T
F
D
A
S
DATA
R
19
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

20
Performance Evaluation (2/6)
High Spatial Reuse Aggregate Throughput
(Kbps) IEEE 802.11 1189.73Basic DMAC 2704.18



D
E
F
C
A
B
High Directional Interference Hidden terminal
Problem Aggregate Throughput (Kbps) IEEE 802.11
1194.81Basic DMAC 1419.51
C
D
A
B
E
F
21
Performance Evaluation (3/6)

• Aligned Routes

150m
22
Performance Evaluation (4/6)

• Less aligned Routes

23
Performance Evaluation (5/6)

• Randomly Chosen Routes

24
Performance Evaluation (6/6)

• Random Topology

25
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

26
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