Wireless Ad Hoc and Sensor Networks

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Distributed Power Control and Rate Adaptation 245NAV (G)(with pulses)EIFS EIFS EIFS EIFS EIFS EIFS EIFSTx powerMaxDPC levelRTSCTSDATAACKTimeFIGURE 6.4Periodic increase in power during frame transmission.6.5.2 Protocol DesignIn the proposed protocol, only the initial RTS-CTS frames for a new linkhave to be transmitted using the maximum power defined for the network.Subsequently, all frames, including RTS-CTS-DATA-ACK frames,will use the power calculated according to the proposed DPC scheme.The MAC header needs to be changed to allow power information to besent between the communicating nodes.In other words, the MAC frames have to embed the power informationthat is used for the current packet, as well as for any subsequentresponse. This process repeats for all transmissions occurring betweenany two nodes A and B resulting in an increase in overhead, whichcould cause a decrease in throughput. However, an increase inthroughput observed because of better channel utilization overcomesthe penalty introduced by the additional overhead. Moreover, thisoverhead can be further reduced by using discrete power levels. Consequently,the number of bits necessary for storing the power value inthe header will decrease, thus reducing the overhead. Additionally,these fields can be defined as an option in the frame header by usinga one-bit flag.Whenever the power level changes, this flag will be set indicating thereceiving node to calculate its transmitter power for the subsequent transmission.Otherwise, the flag will be cleared and the node will continue touse its previous value.Because a lower power is selected for transmission of the MAC frames,during severe fading, certain frames will be unsuccessfully decodedbecause of poor reception and they will be dropped. It is important tonote that predicting the channel during severe fading conditions is quitedifficult and impossible. To mitigate this problem, the proposed protocol
246 Wireless Ad Hoc and Sensor Networksincreases the power by a predefined safety factor before each retransmission,to reduce the packet drops.6.5.3 Channel UtilizationThough hidden-terminal problem occurrence increases with lower transmitterpowers, an improvement in channel utilization and throughputcan be seen by using the proposed MAC protocol. In fact, Figure 6.2depicts the enhancement in utilization that will occur when a lower transmitterpower is used for subsequent RTS-CTS transmissions wheneverthe RTS-CTS handshake between any two given nodes A and B has failed.In this scenario, node B will not respond to request from node A. Thiswill occur, for example, if node F is transmitting at the same time as nodeA is trying to send the RTS frame. Therefore, node B is unable to receivethe RTS frames because of a collision. After the predefined number ofretransmissions, node A will cease to send the packet. In such a case, nodeC will be able to start transmission earlier than in the case of usingmaximum power for RTS-CTS. As a result, the contention time for framesfrom certain nodes, such as C, decreases.Consider the scenario when maximum power is utilized for all RTS-CTS frames. Node C will decode the RTS frame because it was sent withthe maximum transmitter power defined for the network. Consequently,node C will update its NAV vector using the RTS frame. No transmissionoccurs; hence, the channel is idle. On the other hand, if the RTS frame issent at a power level calculated by the DPC, the node C will only detectthe RTS frame and will set its NAV vector to the EIFS time. Hence, shortlyafter EIFS, node C is free to initiate communication. Because of the availabilityof a channel to C, the throughput increases.This improvement applies to all nodes within the improvement areadepicted in Figure 6.2. Given the high density of nodes in the case ofwireless ad hoc networks, the probability of a node accessing the channelis quite high. Therefore, an increase in aggregated throughput is observedwith the proposed protocol.Because of higher channel utilization, the spatial reuse factor, whichis defined as the number of successful transmissions within a giventime interval for a given area, will increase for the proposed DPCscheme. For 802.11, the NAV vector will be set for an entire expectedduration of flow transmission; hence, there will be time intervals whenno transmissions take place. As the result, there will be fewer transmissionsfor a given time interval in comparison with the theoreticalcapacity of the radio channel. In our scheme, these idle periods aredetected, nodes are allowed to transmit sooner and, thus, the totalnumber of successful transmissions within a given time intervalincreases. Consequently, the spatial reuse factor increases for the proposedDPC when compared to 802.11.
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Wireless Ad Hocand SensorNetworksPr
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18. Chaos in Automatic Control, edi
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CRC PressTaylor & Francis Group6000
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Table of Contents1 Background on Ne
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3 Congestion Control in ATM Network
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5.4.2 Distributed Power Controller
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7.3 Adaptive and Distributed Fair S
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9.2.2 Overview.....................
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the number of connections in each l
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y Maheswaran Thiagarajan, and tirel
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2 Wireless Ad Hoc and Sensor Networ
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10 Wireless Ad Hoc and Sensor Netwo
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2BackgroundIn this chapter, we prov
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Background 51u(k)x n (k + 1)x n (k)
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Background 53with x( 0)being the in
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Background 55with tr(.) the matrix
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Background 57nGiven a function ft (
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Background 592.3.2 Lyapunov Stabili
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Background 61as well as Jagannathan
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Background 63The subsequent example
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Background 65Evaluating this along
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Background 67Now, select the feedba
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Background 69is SISL if and only if
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Background 71L 1 (x)L(x, t)L 0 (x)0
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Background 73for some R > 0 such th
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Background 75Evaluating the first d
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Background 77Section 2.4Problem 2.4
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4Admission Controller Design for Hi
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Admission Controller Design for Hig
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7Distributed Fair Scheduling in Wir
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Distributed Fair Scheduling in Wire
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8Optimized Energy and Delay-Based R
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Optimized Energy and Delay-Based Ro
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9Predictive Congestion Control for
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Predictive Congestion Control for W
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10Adaptive and Probabilistic Power
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Adaptive and Probabilistic Power Co
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