## 5 Simulation Results
### I-Dmac  
I-DMAC springs from a modification to DMAC's channel decoupling access mechanism and emphatically addresses the airtime fairness in the multi-rates network scenarios. Therefore, we also present the evaluating results of I-DMAC in terms of throughput, utilization, delay, and fairness in this section. We utilize four different data rate nodes (1, 2, 5.5, and 11 Mbps) that join the network as one group; therefore, one group increases the network density once. All of the nodes are assumed to be saturated and their packet sizes are 1000 bytes. In simulations, we use the Jain's Fairness Index (JFI) [28] as the metric of the channel fairness and compare I-DMAC with TTBS, since they both concentrate on fairness.  
As indicated in Figure 6a,b, I-DMAC1 and I-DMAC2 always keep stable in terms of throughput and utilization and they have almost the same channel utilization. From the perspective of throughput, I-DMAC2 can obtain more significant throughput improvement, which, due to that, I-DMAC2 can continuously transmit several packets in the data transmission period once it competes for the channel, thus reducing the collision rather than the other algorithms. On the contrary, the TTBS only changes the size of the packet and enables the high-rate nodes to transmit larger packets once. Definitely, it cannot relieve the collision, especially in the dense scenario, which leads to the fact that the throughput decline steadily as the number of nodes increases.  
Figure 5: Delay in saturated and non-saturated scenarios. (**a**) Completion time in saturated scenario; (**b**) Completion time in non-saturated scenario.  
Analogously, we also investigate the transmission time of these algorithms for all nodes transmitting one mega size file to AP. It can be observed from Figure 7 that the I-DMAC1 significantly outperforms the other algorithms due to the fair channel utilization and lower collision occurrence. The completion time of TTBS is the highest and it obviously fluctuates. The reason is that the high-rate nodes occupy excessively the airtime and some low-rate nodes can always be in the state of \"starvation\". In the dense scenario (100 nodes), I-DMAC2 has the similar situation with the TTBS. This is also because high-rate nodes occupy too much transmission opportunities in the data transmission period.  
Finally, we also investigate the airtime fairness for all contenders by letting them operate for 3600 seconds. Except for TTBS, the packet size for all algorithms is 1000 bytes. The JFI curves in Figure 8 show JFI for all tested protocols. It can be observed that the fairness index for all algorithms does not change significantly as the network density increases. Remarkably, the JFI of TTBS is close to 1, which reflects that it obtains the best airtime fairness. The JFI of I-DMAC2 and I-DMAC1 also exceed 0.9 and 0.8, respectively. However, DMAC and DCF perform worse performance due to their no consideration for multi-rate environment.  
Figure 6: Throughput utilization in multi-rate scenario. (a) System throughput; and, (b) Channel utilization.  
Figure 7: Transmission time under multi-rate scenario.  
In summary, I-DMAC1 and I-DMAC2 are not as good as TTBS in term of airtime fairness. However, they obtain better throughput, utilization, and delay. In addition, except for delay, I-DMAC2 always performs better performance than I-DMAC1. Therefore, I-DMAC1 is suitable for some delay-sensitive applications and I-DMAC2 can provide the better system throughput and airtime fairness for the other applications.