Wireless Ad Hoc and Sensor Networks
Wireless Ad Hoc and Sensor Networks Wireless Ad Hoc and Sensor Networks
Predictive Congestion Control for Wireless Sensor Networks 433Then, the DPC protocol is briefly described, and the used metrics arehighlighted.9.2.1 Congestion Prediction and MitigationTo predict the onset of congestion, the proposed scheme uses both queueutilization and the transmission power under the current channel state ateach node. When nodes in a network become congested, the traffic willaccumulate at the nodes because there will be an excess amount of incomingtraffic over the outgoing one. Hence, the queue utilization has beenselected as an indicator of the onset of congestion.On the other hand, in wireless networks during fading, the availablebandwidth is reduced and the outgoing rate will be lowered. Consequently,input and output buffers will accumulate the incoming trafficindicating the onset of congestion. The channel fading is estimated byusing the feedback information provided by the DPC protocol (Zawodniokand Jagannathan 2004) for the next packet transmission. The DPCalgorithm predicts the channel state for the subsequent time interval andcalculates the required power. If this power exceeds the maximum threshold,then the channel is considered to be unsuitable for transmission, andthe proposed congestion control scheme can initiate the backoff processby reducing incoming traffic. Hence, information from DPC can be utilizedto predict the onset of congestion due to fading channels.Once the onset of congestion has been assessed, different strategies canbe applied to prevent it from occurring. We propose a scheme, with a goalto prevent and minimize the effect of congestion while ensuring weightedfairness. When applied at every node in the wireless sensor network, itwill render a fair and distributed congestion control scheme. Theemployed algorithms minimize queue overflows at a particular node byregulating the incoming flow. The admissible incoming traffic is calculatedbased on three factors:Predicted outgoing flow — The outgoing flow is periodically measured,and an adaptive scheme is used to accurately predict theoutgoing flow in the next period; moreover, the next hop nodecan reduce the outgoing flow assessment by applying a controlover its incoming flows.Wireless link state — The predicted outgoing flow rate is furtherreduced when the DPC protocol predicts a severe channel fading,which will disrupt communication on the link.Queue utilization — The algorithm restricts the incoming flow basedon the current queue utilizations and predicted outgoing flow,thus reducing buffer overflows.
434 Wireless Ad Hoc and Sensor NetworksReceiverTransmitterF(SIR, modulation,backoff)DatarateWireless channelG(Shadowing, path loss,noise, interference,uncertainty)DatarateBER, SIRData rateTarget SIRBER, SNRBack off periodOptimalStrategyPower, data ratebackoffPowerRTS/CTS signalsFIGURE 9.2DPC with rate adaptation.Weights are utilized in the proposed scheme to provide fair allocationof resources to all traffic flows based on the initial values. The weightedfairness guarantees that data from each source will be delivered to thedestination in a timely and orderly manner. The weights associated withthe packets in transit are used to ensure fair service.9.2.2 OverviewA novel scheme, shown in Figure 9.2, is then derived based on the channelstate, transmitter intended rate, and backlog. The scheme can be summarizedin the following steps:1. The buffer occupancies at the transmitter and receiver nodesalong with the transmitter power required to overcome the channelstate at the subsequent time interval will be used to detect anonset of congestion. The rate selection algorithm is then executedat the receiver to determine the appropriate rate (or availablebandwidth) for the predicted channel state.2. The available bandwidth (or rate) is allocated for the flows accordingto the flow weights. This ensures weighted fairness in termsof bandwidth allocation among the neighboring nodes. Theweights can be selected initially and held subsequently orupdated over time.3. The DPC and rate information is communicated by the receiverto the transmitter for every link. At the transmitter node, a backoffinterval is selected by using the proposed scheme based on theassigned outgoing rate.
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434 <strong>Wireless</strong> <strong>Ad</strong> <strong>Hoc</strong> <strong>and</strong> <strong>Sensor</strong> <strong>Networks</strong>ReceiverTransmitterF(SIR, modulation,backoff)Datarate<strong>Wireless</strong> channelG(Shadowing, path loss,noise, interference,uncertainty)DatarateBER, SIRData rateTarget SIRBER, SNRBack off periodOptimalStrategyPower, data ratebackoffPowerRTS/CTS signalsFIGURE 9.2DPC with rate adaptation.Weights are utilized in the proposed scheme to provide fair allocationof resources to all traffic flows based on the initial values. The weightedfairness guarantees that data from each source will be delivered to thedestination in a timely <strong>and</strong> orderly manner. The weights associated withthe packets in transit are used to ensure fair service.9.2.2 OverviewA novel scheme, shown in Figure 9.2, is then derived based on the channelstate, transmitter intended rate, <strong>and</strong> backlog. The scheme can be summarizedin the following steps:1. The buffer occupancies at the transmitter <strong>and</strong> receiver nodesalong with the transmitter power required to overcome the channelstate at the subsequent time interval will be used to detect anonset of congestion. The rate selection algorithm is then executedat the receiver to determine the appropriate rate (or availableb<strong>and</strong>width) for the predicted channel state.2. The available b<strong>and</strong>width (or rate) is allocated for the flows accordingto the flow weights. This ensures weighted fairness in termsof b<strong>and</strong>width allocation among the neighboring nodes. Theweights can be selected initially <strong>and</strong> held subsequently orupdated over time.3. The DPC <strong>and</strong> rate information is communicated by the receiverto the transmitter for every link. At the transmitter node, a backoffinterval is selected by using the proposed scheme based on theassigned outgoing rate.
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