Ad Hoc Networks : Technologies and Protocols - University of ...

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Medium Access Layer 235 neighbors, throughput determination becomes a global problem rather than a local problem. So, in general in an ad-hoc network using DCF the throughput received by a node depends on the whole topology. Note that the DCF mechanism attempts to provide access per-node and not per-link. Figure 8.3. IEEE 802.11 DCF We now describe the DCF mechanism is detail. Each node that has a packet to send picks a random slot for transmission in where is the contention window used for backoffs. Initially is set to In the chosen slot, the node sends a MAC layer control packet called RTS (request-to-send), to the receiver. If the receiver correctly receives the RTS and is not deferring transmission, it responds with a CTS (clear-to-send). This is followed by transmission of the data packet by the sender, and a subsequent acknowledgment from the receiver. The transmissions of these four packets are separated by short durations called SIFS (Short Inter-Frame Space). The SIFS allows time for switching the transceiver between sending and receiving modes. The sequence of transmission of these four packets is shown in Figure 8.3. The MAC header of all these packets (see the packet structures in Figure 8.4) contains a “duration” field indicating the remaining time till the end of the reception of the ACK packet. Based on this advertisement, the neighboring nodes update a data structure called NAV (Network Allocation Vector). This structure maintains the remaining time for which the node has to defer all transmissions. If the packet transmission fails, the sender doubles its contention window and backs off before attempting a retransmission. The number of retransmissions is limited to 4 for small packets (including RTS packets) and 7 for larger (typically DATA) packets. If these counts are exceeded, the data packet is dropped and is reset to If the data packet is successfully delivered, both the sender and the receiver reset to
236 QoS Issues in Ad-hoc Networks Figure 8.4. Packet formats for basic 802.11 8.4.2 802.11 Point Coordination Function (PCF) PCF operates in the Infrastructure mode of 802.11. The standard requires that an AP implementing the PCF mode (contention-free period) must alternate it with the DCF mode (contention period). In the PCF mode, the point-coordinator (AP) sends packets to other nodes and polls a list of nodes giving them an opportunity to transmit. Unlike the DCF mode, in the PCF mode nodes can transmit only if they are polled by the AP. The beginning of the contentionfree period (the period in which PCF operates), is marked by a beacon from the AP which also advertises the length of the contention-free period. During this period, the transmission schedule is completely determined by the AP. The contention-free period could be foreshortened by the AP by transmitting a special packet called the CF-End packet. The polls and the acknowledgments are piggybacked on the data packets as shown in Figure 8.5. Note that before sending the beacon, the AP waits for a period called the PIFS (PCF Inter Frame Space) which is larger than SIFS. This ensures that all communication related to the contention period has ceased. The PIFS interval is also used to wait for a response to a poll by the AP. After this interval elapses, the AP concludes that the node being polled either does not have packets to send or did not receive the poll. It then moves ahead by polling the next node after a PIFS period. 8.4.3 The QoS Extension: 802.11e The IEEE 802.11e extension provides mechanisms for supporting different priorities in WLAN networks. Being a distributed protocol, it is hard to ensure strict priorities. Hence, the priorities are probabilistic in nature. Such priorities can be viewed as a form of QoS metric.
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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.
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Summary 151 References [1] [2] [3]
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Chapter 6 ENERGY CONSERVATION Robin
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Energy Consumption in Ad Hoc Networ
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Energy Consumption in Ad Hoc Networ
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Communication-Time Energy Conservat
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Idle-time Energy Conservation 175 a
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Idle-time Energy Conservation 177 T
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Idle-time Energy Conservation 179 k
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Idle-time Energy Conservation 181 F
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Idle-time Energy Conservation 183 F
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Page 216 and 217: Conclusion 191 [6] [7] [8] [9] [10]
Page 218 and 219: Conclusion 193 [33] [34] [35] [36]
Page 222 and 223: Chapter 7 USE OF SMART ANTENNAS IN
Page 224 and 225: Smart Antenna Basics and Models 199
Page 226 and 227: 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 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
Page 276 and 277: Potential Attacks 251 attacks, such
Page 278 and 279: Attack Prevention Techniques 253 9.
Page 280 and 281: Intrusion Detection Techniques 255
Page 290 and 291: Conclusion 265 an important and sti
Page 294 and 295: Index Ad hoc network multicast rout
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