Wireless Ad Hoc and Sensor Networks

102 Wireless Ad Hoc and Sensor Networksless than the MCR of 3444 cells/sec. The cell losses decrease with anincrease in buffer size for the case of one- and two-layer NNs. Due to aslight decrease in CLR with increasing buffer size, the two-layer NN withtraining method indicates that a buffer size of 150 cells provides a resultsimilar to having buffer size of 350 cells. First of all, low CLR observedfor all methods are due to the feedback, and large buffer size or feedbackimplies that longer delays occur with low CLR.Further, the one-layer and multilayer NN methods render a low CLRcompared to the threshold and adaptive ARMAX methods because theNN approximation property helps to accurately predict the traffic, basedon past measurements. In fact, the two-layer NN method with a prioritraining provides a CLR near zero. The NN-based congestion controllerswere able to achieve the low CLR because the NN accurately predictedtraffic inflow and onset of congestion. Subsequently, the NN controller wasable to generate a desirable source rate to prevent congestion and thesources were able to quickly accommodate their behavior in response tothe delayed feedback. On the other hand, threshold approach generates amuch higher CLR, compared to other schemes, even when the thresholdvalue is kept at 40%, taking longer to transmit all the cells. Increasing thethreshold value will increase CLR but the delay will be smaller. Similar tothe case of ON/OFF traffic, transmission delays were noticed for all thecontroller schemes and the NN-based scheme was able to transfer theinformation with a delay of less than 2%, whereas others took much longer.Figure 3.13 and Figure 3.14 (same as Figure 3.13, but with thresholdingscheme removed) show the result of the simulated congestion created bythrottling the outgoing transmission link at the network switch when abuffer size of 250 cells was used. Throttling was performed by reducingthe service capacity at the switch to 1920 cells/sec during the time intervalof 66.71 to 100 sec as follows:S r = 4680 cells/sec 0≤t ≤33. 33sec,= 3360 cells/sec= 1920 cells/sec33. 37 ≤t≤66. 66sec,66. 71 ≤t≤100sec,= 3360 cells/sec 100. 04 ≤t≤133. 33sec,(3.33)The CLR for all the methods remains close to 0 when the service capacityis higher than the MCR during the time intervals of 0 to 33.33 sec and133.37 to 167 sec. As expected, the CLR attains a maximum value whenthe service capacity is reduced to less than the MCR of the combined traffic.