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Medium Access Control with Directional Antennas 201 average power density. The gain is measured in dBi, where i is used to indicate that this is the gain in decibels over an ideal isotropic antenna. The main lobe of the antenna represents the direction of peak gain during a transmission or a reception. The antenna beamwidth typically corresponds to the angle subtended by the two directions on either side of the peak gain that are 3 dB lower in gain as compared to the peak gain. Note that this is a reduction by half in terms of the signal power as compared to the power in the direction of peak gain (not in decibels). This angle is also sometimes referred to as the 3 dB beamwidth. 7.2.3 Directional Antenna Models The presence of side-lobes causes interference to other simultaneous transmissions in spite of using a directional antenna. Sophisticated antenna arrays can steer these side lobes so as to create nulls towards other simultaneous users of the channel. However, simpler (and hence cheaper) antennas suffer from the presence of these sidelobes. In ad hoc network literature, most of the work thus far adopts one of two models for characterizing the radiation pattern of a directional antenna: (a) The Flat Topped Radiation Pattern and (b) The Cone and Sphere Radiation Pattern. With the flat topped radiation pattern model, it is assumed that the gain of the antenna is a constant within a defined beamwidth of radiation. It is also assumed that the side lobes are absent. If the beamwidth is the gain is computed to be With the cone and sphere radiation pattern, the side lobes are accounted for by a spherical footprint that is attached to the apex of a cone. The axis of this cone passes through the direction of peak gain of the antenna. If the gain in the direction of the main lobe and the beamwidth of the main lobe are known, it is a simple exercise to compute the gain of the spherical side-lobe in the cone and sphere radiation pattern [16]. We depict the cone and sphere radiation pattern in Figure 7.2. 7.3 Medium Access Control with Directional Antennas In this section, we describe the current state of the art literature on medium access control with directional antennas. Medium access control refers to the arbitration of channel bandwidth among a plurality of multiple-access users. We can classify medium access protocols into two types (a) on-demand or unscheduled access and (b) scheduled access. On-demand or unscheduled access mechanisms are based on contention access. Nodes in the ad hoc network contend for the channel. Carrier Sensing (both virtual and physical) are used to reduce the extent of packet losses due to collisions. Traditionally, the MAC protocol defined in the IEEE 802.11 standard has been popularly adopted as the
202 Use of Smart Antennas in Ad Hoc Networks Figure 7.2. The Cone and Sphere Radiation Pattern contention-based MAC protocol in ad hoc network research. Scheduled access, on the other hand, attempts to schedule transmissions in advance to reduce the possibility of collisions. Protocols that use scheduled access might proactively allocate bandwidth based on a number of criteria that may include the topology, the generated traffic and priority of various nodes. We begin with a very brief discussion of the IEEE 802.11 MAC protocol and then go on to point out the problems that one would face if this protocol is used “as is” with directional antennas. We then discuss various approaches that have been proposed for addressing these problems. Finally, we discuss the few approaches that have been proposed for scheduled access. 7.3.1 The IEEE 802.11 MAC Protocol in Brief The Distributed Co-ordination Function (DCF) specified in the IEEE 802.11 MAC standard has been popularly advocated for ad hoc networks. The DCF function is based on co-ordinating medium occupancy using carrier sense multiple access with collision avoidance (CSMA/CA). The approach alleviates the hidden terminal problem that arises in wireless networks by the use of a simple CSMA scheme. In the IEEE 802.11 MAC protocol, a transmitter sends a Request to Send (RTS) message to a recipient neighbor when it wishes to send a data packet to that neighbor. The RTS message implicitly informs the neighboring nodes within the omni-directional range of the transmitting node that a data transfer is being initiaited. If possible, the receiver would then respond with a Clear to Send or CTS message. The CTS message implicitly informs the nodes in the
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AD HOC NETWORKS Technologies and Pr
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AD HOC NETWORKS Technologies and Pr
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Contents List of Figures List of Ta
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Contents vii 4.7. Conclusions and F
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Contents ix 9.2. Potential Attacks
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List of Figures 1.1 Internet in the
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List of Figures xiii 6.3 COMPOW com
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List of Figures xv 9.1 9.2 An IDS a
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List of Tables 2.1 2.2 2.3 2.4 2.5
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Contributing Authors Samir R. Das i
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Preface Wireless mobile networks an
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Acknowledgments xxiii We wish to ex
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Chapter 1 AD HOC NETWORKS Emerging
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Introduction and Definitions 3 coul
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Introduction and Definitions 5 Secu
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Ad Hoc Network Applications 7 tunis
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Ad Hoc Network Applications 9 The d
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Ad Hoc Network Applications 11 Figu
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Design Challenges 13 example of cro
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Evaluating Ad Hoc Network Protocols
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Overview of the Chapters in this Bo
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Overview of the Chapters in this Bo
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Conclusions 21 1.6 Conclusions This
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Chapter 2 COLLISION AVOIDANCE PROTO
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Performance of collision avoidance
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Framework and Mechanisms for Fair A
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Conclusion 59 In the first part of
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Conclusion 61 [13] [14] [15] [16] [
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Chapter 3 ROUTING IN MOBILE AD HOC
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Flooding 65 vector and link state p
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Flooding 67 centralized approximati
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Proactive Routing 69 status of each
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Proactive Routing 71 Topology Broad
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On-demand Routing 73 reply packets
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On-demand Routing 75 during the lin
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Proactive Versus On-demand Debate 7
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Proactive Versus On-demand Debate 7
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Location-based Routing 81 to find a
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Location-based Routing 83 Figure 3.
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Concluding Remarks 85 relative meri
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Concluding Remarks 87 [14] [15] [16
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Concluding Remarks 89 [42] [43] [44
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Chapter 4 MULTICASTING IN AD HOC NE
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Introduction 93 The mobility of the
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Classifications of Protocols 95 red
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Multicasting Protocols 97 this sect
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Multicasting Protocols 99 as close
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Multicasting Protocols 101 In ODMRP
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Multicasting Protocols 103 hoc netw
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Multicasting Protocols 105 A member
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Multicasting Protocols 107 Figure 4
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Broadcasting 109 “talk” to a ra
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Broadcasting 111 broadcasted packet
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Protocol Comparisons 113 are cluste
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Overarching Issues 115 is user data
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Overarching Issues 117 and RTS/CTS
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Conclusions and Future Directions 1
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Conclusions and Future Directions 1
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Chapter 5 TRANSPORT LAYER PROTOCOLS
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TCP and Ad-hoc Networks 125 Modifie
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TCP and Ad-hoc Networks 127 mechani
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TCP and Ad-hoc Networks 129 timeout
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TCP and Ad-hoc Networks 131 to freq
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TCP and Ad-hoc Networks 133 Figure
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Transport Layer for Ad-hoc Networks
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Modified TCP 137 MAC layers without
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Modified TCP 139 Figure 5.6. TCP-EL
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TCP-aware Cross-layered Solutions 1
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Ad-hoc Transport Protocol 147 in it
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Ad-hoc Transport Protocol Figure 5.
Page 176 and 177: Summary 151 References [1] [2] [3]
Page 178 and 179: Chapter 6 ENERGY CONSERVATION Robin
Page 180 and 181: Energy Consumption in Ad Hoc Networ
Page 182 and 183: Energy Consumption in Ad Hoc Networ
Page 184 and 185: Communication-Time Energy Conservat
Page 200 and 201: Idle-time Energy Conservation 175 a
Page 202 and 203: Idle-time Energy Conservation 177 T
Page 204 and 205: Idle-time Energy Conservation 179 k
Page 206 and 207: Idle-time Energy Conservation 181 F
Page 210 and 211: Idle-time Energy Conservation 185 t
Page 212 and 213: Idle-time Energy Conservation 187 c
Page 216 and 217: Conclusion 191 [6] [7] [8] [9] [10]
Page 218 and 219: Conclusion 193 [33] [34] [35] [36]
Page 220 and 221: Conclusion 195 [59] [60] [61] [62]
Page 222 and 223: Chapter 7 USE OF SMART ANTENNAS IN
Page 224 and 225: Smart Antenna Basics and Models 199
Page 228 and 229: Medium Access Control with Directio
Page 242 and 243: Routing with Directional Antennas 2
Page 248 and 249: Broadcast with Directional Antennas
Page 250 and 251: Broadcast with Directional Antennas
Page 252 and 253: Summary 227 [4] [5] [6] [7] [8] [9]
Page 254 and 255: Chapter 8 QOS ISSUES IN AD-HOC NETW
Page 256 and 257: Introduction 231 The concept of ad-
Page 258 and 259: Physical Layer 233 The 802.11a stan
Page 260 and 261: Medium Access Layer 235 neighbors,
Page 262 and 263: Medium Access Layer 237 Figure 8.5.
Page 264 and 265: QoS Routing 239 where is the number
Page 266 and 267: QoS at other Networking Layers 241
Page 268 and 269: Inter-Layer Design Approaches 243 8
Page 270 and 271: Conclusion 245 are specifically des
Page 272 and 273: Conclusion 247 [13] S. Mangold, S.
Page 274 and 275: Chapter 9 SECURITY IN MOBILE AD-HOC
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Potential Attacks 251 attacks, such
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Attack Prevention Techniques 253 9.
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Intrusion Detection Techniques 255
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Conclusion 265 an important and sti
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Conclusion 267 [24] [25] [26] [27]
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Index Ad hoc network multicast rout
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