# Performance Evaluation of IEEE 802.11 ah Networks With High-Throughput Bidirectional Traffic
## 3 IEEE 802.11 ah Overview  
Thanks to the novel hierarchical Association Identification (AID) structure shown in Figure 1, IEEE 802.11 ah supports up to 8191 associated stations, a much larger value than in previous 802.11 releases. However, this number is a direct consequence of the AID field size (13 bits enabling 213 or 8192 unique AIDs, with AID 0 being reserved) and it is yet to be experimentally verified under which circumstances it is possible to connect 8191 devices to one AP in reality. Unlike previous IEEE 802.11 MAC mechanisms, IEEE 802.11 ah stations can be configured to use only specific time slices between beacons where they are allowed to contend for the medium and communicate with the AP. This enables the stations to sleep most of the time and only wake up at the beginning of these intervals. The collision probability is greatly reduced by distributing the slices evenly between associated stations. IEEE 802.11 ah defines two modes of operation: (1) TIM mode and (2) non-TIM mode. Stations operating in one of these two modes are referred to as TIM- and non-TIM stations, respectively. TIM stations have periodic access to the medium and are typically used for high bandwidth requirements and receive both down- and uplink access, therefore being the mode used in this research. TIM stations wake up periodically to receive the beacon broadcasted by the AP. On the other hand, in non-TIM mode (power saving mode) stations do not need to periodically wake up to receive beacons. Instead, they transmit at least one PS-Poll or trigger a frame transmission to the associated AP every listen interval. Non-TIM stations are destined to exchange low amounts of data and as such can request buffered DL traffic from the AP or transmit UL traffic any time they wake up. In order to reduce collisions and interference, the standard introduces the Restricted Access Window (RAW) mechanism (Section 3.2), which enables reserving a specific time window for specific stations only. As such, RAW can be used for restricting access to any specified group of stations. It is useful for improving performance in dense IoT networks where a large number of stations are contending simultaneously [33]. This section describes the TIM and RAW mechanisms in more detail in Sections 3.1 and 3.2, respectively. TIM and RAW can be used independently or in parallel. Moreover, IEEE 802.11 ah has introduced a number of novel MAC mechanisms in comparison to previous Wi-Fi amendments. For more details on these MAC mechanisms, which are summed up in Table 2, we refer the readers to [10], where the authors present a brief overview of all of them, or to the standard itself [2]. \begin{table}
\begin{tabular}{l l l l l} \hline \hline
**Notable Features** & & **802.11-2007** & **802.11n** & **802.11 ac** & **802.11 ah** \\ \hline Backwards compatibility & & X & X & X & \\ Distributed Channel Access (DCF) & & X & & & \\ Point Coordinated Function (PCF) & & X & & & \\ \hline Hybrid Coordination Function (HCF) & HCCA & X & X & & X \\ EDCA & X & X & X & X & X \\ \hline Transmission Opportunity (TXOP) & Forward & X & X & X & X \\  & RD Protocol & & X & X & X \\  & BDT & & & & X \\ \hline Response Indication Deferral (RID) & & & & X \\ Frame Aggregation & & & X & X & X \\ Block Acknowledgement & & X & X & X & X \\ Multi User (MU) Aggregation & & & & X & X \\ Null Data Packet (NDP) & & & X & X & X \\ Group-ID & & & & X & X \\ BSS color & & & & & X \\ Dynamic Bandwidth Management & & & & X & X \\ Subchannel Selective Transmission & & & & X & X \\ Traffic Indication Map (TIM) & X & X & X & X & X \\ Delivery Traffic Indication Map (TIM) & & & X & X & X \\ Target Wakeup Time (TWT) & & & & & X \\ Grouping of Stations & & & & & X \\ Hierarchical AID & & & & & X \\ Dynamic AID reassignment & & & & & X \\ Restricted Access Window (RAW) & & & & X \\ Group sectorization & & & & & X \\ Relay operations & & & & X \\ Power saving at AP & & & & X \\ Low power mode of operations & & & & X \\ \hline \hline \end{tabular}
\end{table}