# Dynamic Carrier-Sense Threshold Selection for Improving Spatial Reuse in Dense Wireless LANs
## 4 Performance Evaluation
### Simulation Results
#### 4.2.2 Varying Number of APs  
In the second experiment, we have varied the number of APs from 9 to 169. The number of nodes were fixed at 100. When AP density is high, the average distance between a mobile node and its AP becomes shorter. Thus, more concurrent transmissions are expected to take place. The result is shown in Figure 6.  
First we look at the total throughput. When the number of APs is small, the total throughput is similar for all protocols. However, as the number of APs become large, the difference in throughput also becomes larger. For the legacy 802.11 DCF, the throughput is increased with AP density, but not so much compared to the other protocols. This is because the protocol uses fixed CST. Regardless of whether the mobile node is close to the AP or not, the region that the node blocks while transmitting is the same. Thus, spatial reuse does not improve much even when the average distance between the nodes become small. Still, some throughput increase is observed for the legacy protocol, when the AP density is high. This is because when multiple nodes transmit simultaneously because of random backoff, there is higher chance that the packets will be successfully received by the receivers. DSC achieves the highest total throughput, but causes starvation at the edge nodes as can be observed from the bottom 25% throughput and the fairness. The proposed protocol achieves higher total throughput and comparable bottom 25% throughput, similar to the previous experiment.  
For the bottom 25% throughput, the throughput starts to decrease as the number of APs increase, and then after some point starts to increase. A similar trend can be observed in the fairness graph. The reason behind this increase of fairness is due to the increased delivery ratio. Even in the face of packet collisions, the probability that the packets will be successfully received by their receivers increases when the distance between the mobile node and the AP becomes smaller. When the number of APs is very small, possibility of concurrent transmissions is limited because an AP can only transmit to one mobile node at a time. The space is not saturated, and the edge nodes have higher opportunity to send packets. As the number of APs increase, number of concurrent transmissions increases and the fairness is decreased as discussed in the previous experiment. Once the space is saturated, the fairness (and the bottom 25% throughput) starts to increase again due to improved delivery ratio. One peculiar point in the graph is that the performance of legacy 802.11 DCF is very bad when the number of AP is small. This is because some APs at the center of the area are continuously blocked by the fixed CST, when the APs on the edge of the area transmit packets. For example, when the number of APs is 9 (placed in a 3\(\times\)3 grid), the AP at the center rarely gets a chance to transmit, because the APs at the outside take turns to block the AP and prevent it from counting down its backoff counter. Thus, all mobile nodes associated with the AP are starved and the channel share becomes significantly unfair. For other protocols, the center AP may get more opportunity when the other APs transmit, because when the AP on the outside transmits, the required CST could be set high if the mobile node is placed closely to the AP. In that case, the center AP may not be blocked by the transmission, and thus can obtain opportunity to transmit its packet.  
Figure 6: Performance of protocols varying number of APs. Number of APs is varied from 9 to 169, whereas the number of nodes is fixed to 100. (**a**) total throughput, (**b**) bottom 25% throughput, (**c**) [PERSON]’s fairness index, (**d**) packet delivery ratio.