Wireless Ad Hoc and Sensor Networks

450 Wireless Ad Hoc and Sensor Networksthus causing congestion at the intermediate nodes. In the simulations,the traffic consists of five flows, which had been simulated for two cases:(1) with the same weights, equal to 0.2, and (2) with weights equal to 0.4,0.1, 0.2, 0.2, and 0.1, respectively. All the sources generate equal traffic,which exceeds channel capacity, so that congestion can be created. Theinitial rates for each flow have been assigned proportional to the weight,such that they saturate the radio channel. It has to be noted that in sucha heavily congested network, the RTS-CTS-DATA-ACK handshake andthe standard windows contention mechanism of the 802.11 protocol areunable to prevent collisions. Moreover, the high number of collisionsoccurring leads to underutilization of the wireless channel in case of the802.11 protocol. The DPC protocol improves the channel utilization in thepresence of collisions, as described by Zawodniok and Jagannathan(2004). However, the imbalance between incoming and outgoing flowsdue to congestion is not addressed by the DPC, thus still resulting inbuffer overflows and a significant number of dropped packets.Figure 9.4 and Figure 9.5 depict the throughput/weight (normalizedweights) ratio for each flow. Ideally, the throughput over the initial weightratio plot should be a straight line parallel to the x-axis for fair schedulingschemes. The proposed protocol results in a fair allocation of bandwidthwhen compared to the DPC and to the 802.11 MAC protocols, even forthe case of variable weights assigned to the flows, as observed in Figure 9.5.The proposed protocol achieves this by taking weight into account duringa packet scheduling and flow control. Consequently, flows with differentThroughput/weight (Kbps)500 Proposed schemeDPC schemeIEEE 802.1140030020010001 2 3 4 5Flow IDFIGURE 9.4Performance for the tree topology.