# Performance Evaluation of IEEE 802.11 ah Networks With High-Throughput Bidirectional Traffic
## 5 Conclusions
With 10 s sampling interval and 100-bytes payload, 1440 stations can be supported with no packet loss, whereas grouping two or three such measurements together and sending them two or three times less frequently results in 2080 or 2560 stations, respectively. IEEE 802.11 ah enables all 5500 stations to turn off the radio for 97% of the total simulation time when using 20 TIM groups, five RAW slots and sampling interval of 60 s. With 10 s, 1800 stations can also sleep 97% of the time using 12 TIM groups and 5 RAW slots. Already from four TIM groups, sleeping time is over 90% and almost constant. However, doubling the number of TIM groups increases latency almost twice and doubles the RTT. There is a trade-off between energy efficiency and latency. Considering the presented evaluation, the best approach to maximize the efficiency of a bidirectional communication would be:  
1. Choose a sampling interval that is as high as possible, but still acceptable for the application. Furthermore, if it is acceptable to delay transmissions by grouping multiple measurements together into a bigger packet, it is advised to do so. If TCP is used, group measurements as such to fill the largest non-fragmented TCP segment size. 2. Choose the number of TIM groups so that \(T_{DTIM}\) is at least four times smaller than the smallest sampling interval in the network. 3. In the chosen interval and TIM group, maximize the number of RAW slots so that they are long enough to be able to serve stations assigned to them, having in mind the maximal possible number of transmissions in a single slot. Our immediate reply mechanism greatly improves bidirectional communication. In the presence of downlink traffic, RAW limits uplink by rescheduling it to the next available channel access opportunity and giving precedence to downlink. Our mechanism not only halves the RTT of 30-70% of packets by scheduling the reply from the AP in the current RAW slot if possible, but also enables uplink packets to be transmitted in the next cycle by eliminating paged stations and allowing stations to attempt uplink transmission (stations are not paged in the next cycle because they already got DL packet). Higher improvements are attainable in non-saturated networks, whereas, in highly dense networks with an almost fully utilized channel, there is no time left in RAW slots to schedule replies, decreasing the potential improvement. Finally, this evaluation and obtained results enable smart usage of TIM segmentation and the RAW mechanism in reliable and high-throughput applications, as well as in reliable low-throughput applications with bidirectional communication. We aim to use this evaluation to design a self-adaptable network management algorithm to enable rea-time TIM and RAW regrouping, which would optimize throughput and/or latency and/or energy efficiency in closed loop scenarios. Judging by the conducted survey among Industrial Advisory Committee (IAC) members (Figure 15), there will be much more research on this technology and we will get to see its grand rise as it was the only technology from the survey not available on the market at the time, and yet rated the second most interesting. Figure 15: In 2016, IEEE 802.11 ah was voted the second most interesting technology by Industrial Advisory Committee (IAC) members in spite of the lack of hardware support at the time. **Acknowledgments:** This work was carried out in the context of following projects: (1) IDEAL-IoT (Intelligent DEnse And Longe range IoT networks) is an Strategic Basic Research (SBO) project funded by the Fund for Scientific Research-Flanders (FWO-V) under grant agreement #S004017N, (2) 'Processing visual sensor data in low-power wide area networks' is a project funded by the Fund for Scientific Research-Flanders (FWO-V) under grant agreement #G084177N and the (3) Ghent University GOA (Geconcerteerde Onderzoeksacties) \"Disposable and biodegradable wireless networks for extreme conditions\" project. **Author Contributions:** The work was realized with the collaboration of all of the authors.