## 5 Performance Evaluation
### Reference Mechanisms and Simulation Description  
This paper considers two main reference mechanisms, namely IEEE 802.11 DCF and PSM, to compare their performance with that of TXOP PSM. Such mechanisms, as well as TXOP PSM, are assumed to enable the RTS/CTS handshake and can operate either with or without burst transmissions, i.e., \(\alpha>1\) or \(\alpha=1\). DCF and PSM with \(\alpha=1\) are considered according to the IEEE 802.11 legacy specification in [6]. DCF with \(\alpha>1\) (similar to 802.11e EDCA TXOP) operates following the definition in [25], which is also used in TXOP PSM. PSM with \(\alpha>1\) (similar to 802.11e APSD) is based on the operation of DCF with \(\alpha>1\) in [25], and is described in more detail below. Finally, TXOP PSM with \(\alpha=1\) accounts for the operation of the mechanisms in [7; 8; 9; 11], whereas TXOP PSM with \(\alpha>1\) operates as the mechanism in [12].  
A Python simulation program that implements all the operation rules of the aforementioned mechanisms using the network operation and assumptions described in the previous sections was developed. The main motivations for using a custom simulator rather than other available network simulators (e.g., OPNET and ns-3) are the faster execution of the simulations and the fact that advanced sleep mode operations are not yet implemented in other existing simulators. The simulator used in this paper supports all required functionalities to validate the proposed model and to assess how selected system parameters influence the performance of the evaluated mechanisms.  
More specifically, the simulated network operation considers an AP and \(n\) STAs generating data frames of the same size by following a Poisson process. All the STAs generate data frames addressed to the AP at an equal rate \(\lambda_{1}\), whereas the rate at which the AP generates data frames is \(\lambda_{2}=n\lambda_{1}\). The destination of each data frame sent by the AP is chosen at random among all the STAs with equal probability \(1/n\). This setting provides balanced bidirectional data flows between the AP and each STA on average. In addition, the AP and the STAs enable a holding time during which they can aggregate up to \(\alpha\) data frames with the same destination address before attempting to transmit. This holding time allows the creation of more opportunities for multiple data transmissions in each channel access attempt.  
The simulated operation of PSM using the holding time and \(\alpha>1\) is described as follows. The AP broadcasts a fixed-size beacon frame at each beacon interval, after waiting for the channel to be idle during a Point Coordination Function Interframe Space (PIFS) period. All the STAs wake up to receive the beacon frame and determine if there are buffered data frames for them at the AP. The beacon frame includes buffer status information only for STAs that have \(\alpha\) buffered data frames at the AP or a buffered data frame with an expired holding time. When an STA has buffered data at the AP, it transmits a PS-Poll frame to the AP and the AP replies with an ACK frame. The AP then delivers up to \(\alpha\) buffered data frames to an STA, in accordance with the order of received PS-Poll frames, as soon as it gets access to the channel. In addition, an STA can wake up to transmit data only after the expiration of the holding time or whenever there are \(\alpha\) data frames ready to be transmitted.  
The simulated time for each simulation experiment was set to 15 seconds and each simulation experiment was repeated 10 times. The average values of the collected simulation data were employed to compute the simulation results for each parameter set. In addition, confidence intervals of 95% were calculated. Since the width of these confidence intervals is at most 2% of the mean values, they are omitted in the figures for the sake of visualization.