# Performance Evaluation of IEEE 802.11 ah Networks With High-Throughput Bidirectional Traffic
## 5 Conclusions  
This article presents the influence of the joint usage and performance of TIM and RAW on scalability, throughput, latency and energy efficiency in networks with bidirectional traffic. In the presence of downlink traffic, the RAW mechanism limits uplink performance by rescheduling it to the next available channel access opportunity and giving precedence to downlink. The RAW performance in uplink-only networks is different than in the ones with downlink or bidirectional traffic. We conducted over 40,000 experiments in the ns-3 simulator to evaluate the behaviour of TIM and RAW in the presence of bidirectional traffic. We envisioned this IoT technology as a long-range technology that is able to support legacy Internet traffic and therefore evaluated the full network stack. IEEE 802.11 ah requires more changes at the various layers of the network stack than other 802.11 standards. The network and transport layer requires adjusted settings to incorporate the introduced delays due to intermittent channel access and the application layer has to limit the data transfer to prevent buffer overflows. However, when implemented properly, the promised power saving, range extension and higher station limit work very well without crippling the use cases it has been designed for. TCP adds extra header overhead, but it is still feasible to support legacy Internet traffic in IoT. IEEE 802.11 ah offers a good balance between throughput and distance in comparison with LPWAN and WPAN technologies. It enables reliable communication since there are no exclusive constraints on downlink traffic. TIM segmentation contributes to both high scalability and energy preservation and, combined with the RAW mechanism, they can enable reliable and high-throughput applications such as IP camera streaming and over-the-air firmware updates for up to 20 stations over 200 m with 160-256 kbps of throughput per station. TIM segmentation and RAW dominantly define the network performance. However, the standard does not specify how to configure them. The performance of both TIM and RAW are greatly dependent on traffic patterns and improper configuration could degrade the overall network performance. Proper TIM and RAW configurations introduce an approximately linear improvement in throughput with density in comparison with EDCA/DCF and represented by:  
\[I_{\%}=0.2529\cdot N_{STAs}+6.2074, \tag{13}\]  
where \(I_{\%}\) is throughput improvement in percentages introduced by TIM and RAW and \(N_{STAs}\) is total number of stations. By enlarging the TCP segment size to a maximal value that is not subject to IP fragmentation, the maximal attainable throughput for a given MCS can be reached. The number of RAW slots defines the amount of contention in each slot, with respect to the number of stations. Ergo, RAW has the largest influence on the maximum attainable data rate in IEEE 802.11 ah. However, in networks with high traffic loads or high density, combining TIM with RAW can further improve the data rates up to 15% if the DTIM interval is still small enough to not cause extensive TCP re-transmissions. Increasing the number of TIM groups but limiting the number of RAW slots improves the overall scalability as found in the reliable monitoring scenario. In addition, 6960 stations reporting 100-byte measurements every minute is supported with 12 TIM groups and two RAW slots without packet loss. Extensive RAW slotting can be a limiting factor for bidirectional traffic since it postpones transmissions for paged stations. This can lead to quicker buffer overflow. Grouping several measurements into one packet and sending them less frequently benefits the overall scalability. A larger packet size makes better use of the available channel time: it reduces the overhead of the headers, the amount of TCP acknowledgements and the collisions.