Wireless Ad Hoc and Sensor Networks

452 Wireless Ad Hoc and Sensor NetworksIn terms of throughput, the 802.11 protocol performs unsatisfactorilybecause it cannot handle the increased number of collisions that occur ina heavily congested network as it was unable to predict the onset ofcongestion to prevent it. The network becomes deadlocked because frequentcollisions prevent the 802.11 protocol from successfully deliveringpackets to the destination node, as observed in Figure 9.5. As a result, theaverage delay for 802.11 is significantly high compared to other schemes.On the other hand, the DPC detects the idle channel after collisions andresumes transmission sooner than the 802.11 protocol (Zawodniok andJagannathan 2004), thus improving throughput in a congested network.In the case of the proposed scheme, the throughput is further increasedbecause it prevents the onset of congestion. By applying feedback in theform of backpressure signal, the proposed scheme prevents packet losses;therefore, it minimizes energy and bandwidth wastage, and maximizesend-to-end throughput. Consequently, the proposed algorithm outperformsother protocols.Figure 9.6 and Figure 9.7 present throughput and drop rate for flow 1,for instance. It can be noticed that, during the time interval of 21 to 31sec, the link from relay node to destination experiences severe fading andno transmission is possible. The proposed protocol prevents overflowingbuffer at the relay node by sending backpressure indication to the sourcesand preventing them from overflowing the relay node. Thus, no increasein drop rate is observed for the rate-based protocol.Figure 9.8 and Figure 9.9 illustrate the “weight * delay” metric, whichdescribes a weighted fairness in terms of end-to-end delay. The desired350300Throughput for flow 1802.11Rate-basedThroughput (Kbps)2502001501005000 5 10 15 20Time25 30 35 40FIGURE 9.6Throughput for flow 1.