## 5 Performance Evaluation
### Results  
All the figures shown in this section include solid lines referring to the analytical results and markers related to the simulation results. It is also worth noting that the results of energy efficiency of the evaluated mechanisms as a function of the traffic load, MSDU length, and PHY data transmission rate were presented in [20]. (1) Impact of the Traffic Load  
The network throughput and energy efficiency of the evaluated mechanisms with \(\alpha=1\) and \(\alpha=3\) rounds of data transmissions as a function of the total offered traffic load are shown in Figure 2a and Figure 2b, respectively. As it can be seen in Figure 2a, the throughput of the mechanisms linearly increases as the traffic load increases. Each mechanism achieves a maximum stable value of the throughput when the network is in saturation. The saturation throughput of the mechanisms using \(\alpha=3\) is higher than that achieved by the mechanisms when \(\alpha=1\) is employed. The reason is that the AP and the STAs can send up to three data frames in each channel access opportunity, hence enabling the reduction of the overall channel access overhead. In addition, it can be seen that the throughput of DCF and TXOP PSM is the same across all traffic loads whereas PSM attains lower throughput than DCF and TXOP PSM under high traffic loads. This is because PSM, in contrast to TXOP PSM, modifies the operation of the STAs to receive data frames from the AP in order to improve energy efficiency. It is also worth noting that DCF, PSM, and TXOP PSM with \(\alpha=3\) produce a throughput improvement of up to 32% when compared to those with \(\alpha=1\). In Figure 2b, results show the energy efficiency for each mechanism. More specifically, when DCF is employed, the network energy efficiency has a similar shape to that of the throughput, and under saturation conditions it increases up to 29% for DCF using \(\alpha=3\) as compared to DCF with \(\alpha=1\). This is due to the fact that the energy consumption of the AP and the STAs does not increase significantly with higher traffic loads, while the increase in the amount of transmitted data frames is significant. As opposed to DCF, the energy efficiency of PSM is the highest under low traffic loads since the STAs can sleep for a long period of time. The energy efficiency of PSM is significantly reduced under high traffic loads, close to the energy efficiency value of DCF. This occurs because the majority of the STAs remain awake most of the time to transmit and receive data. Note that the energy efficiency of PSM with \(\alpha=3\) is reduced more slowly than that of PSM with \(\alpha=1\) as the traffic load increases. Figure 2: Network throughput and energy efficiency of the evaluated mechanisms with \(\alpha=1\) and \(\alpha=3\) rounds of data transmissions as a function of the traffic load. (**a**) Throughput. (**b**) Energy efficiency. The reason is that the STAs do not wake up until there are at least three data frames to transmit or their holding times expire. For this reason, the energy efficiency of PSM using \(\alpha=3\), being always above that of PSM using \(\alpha=1\), increases a little and then decreases under low to medium traffic loads. In such range of traffic loads and when \(\alpha=3\) is employed, TXOP PSM attains higher energy efficiency than DCF but lower energy efficiency than PSM, as shown in Figure 2b. However, under high traffic loads, TXOP PSM improves the energy efficiency of both DCF and PSM by up to 110%. This occurs because the AP and the STAs normally send two or three data frames together in a single channel access attempt, hence enabling listening STAs to sleep and save energy. It is also worth noting that TXOP PSM with \(\alpha=1\) cannot improve the energy efficiency of DCF under high traffic loads. The reason is that, considering the selected system parameters, no STA can sleep during the transmission of a single data frame. This means that the duration of data transmissions are shorter than the transition times of the STAs between awake and sleep states.