# Performance Evaluation of IEEE 802.11 ah Networks With High-Throughput Bidirectional Traffic
## 3 IEEE 802.11 ah Overview
### Restricted Access Window (RAW)  
In order to reduce collisions and interference between stations, the standard introduces the RAW mechanism that could be seen as a combination of deterministic and stochastic media access control mechanisms. RAW enables reserving specific slotted time windows only for specific stations assigned to those windows. During those windows, assigned stations use Enhanced Distributed Channel Access/Distributed Coordination Function (EDCA/DCF) to access the medium within their corresponding slot (Figure 3). RAW can be used for restricting channel access to any specified group of stations. It is useful for achieving performance improvements in dense IoT networks where a large number of stations are contending simultaneously [33]. This section presents the RAW operation. An AP may allocate one or more RAWs for a group(s) of stations within a (short) beacon interval and broadcast this information using the RAW Parameter Set (RPS) information element carried in the preceding beacon. The RPS element specifies which stations are assigned to RAW(s), the configuration of RAW(s), as well as start time of RAW(s) relative to either the beacon ending time (for the first RAW) or the previous RAW. Each RAW can be divided into time slots referred to as RAW slots, and stations assigned to RAW are evenly split across the RAW slots using round robin assignment. If a station belongs to a RAW group, it is allowed to contend for medium access at the start of its assigned RAW slot and will not contend for medium access within any other RAW slot during that RAW. The number of slots \(N_{RAW}\), Slot Format and the Slot Duration Count \(C\) are also specified in the RPS element. The duration of the RAW slot \(T_{slot}\) is defined by [2]:  
\[T_{slot}=500\,\mathrm{\SIUnitSymbolMicro s}+C\cdot 120\,\mathrm{\SIUnitSymbolMicro s}. \tag{1}\]  
The Slot Duration Count \(C\) sub-field is either \(y=11\) or \(y=8\) bits long depending on whether the Slot Format sub-field is set to 1 or 0, respectively. The minimal RAW slot duration is thus 500 \(\mathrm{\SIUnitSymbolMicro s}\) for \(C=0\), whereas the maximal slot duration \(T_{slot}^{max}\) is 246.14 ms for \(y=11\) (\(C=2^{11}-1\)) and 31.1 ms for \(y=8\) (\(C=2^{8}-1\)). The Number of Slots \(N_{RAW}\) field is \(14-y\) bits long. Consequently, the maximal number of slots \(N_{RAW}^{max}\) is \(2^{14-11}=8\) for \(y=11\) and \(2^{14-8}=64\) for \(y=8\). Therefore, the maximal RAW group duration is \(N_{RAW}^{max}\cdot T_{slot}^{max}\) and amounts to 1.9904 s for \(y=8\) and 1.96912 s for \(y=11\). It is important to note that (1) the RAW group duration must be shorter than the (short) beacon interval so RAW must be configured with respect to the (short) beacon interval and (2) if Cross-Slot Boundary (CSB) is forbidden (equals zero), the RAW slot duration must be at least as long as the packet transmission time, thus RAW must be configured with respect to the traffic patterns and distances in the network (i.e., packet sizes and MCSs). Otherwise, disrespecting (1) would cause the next beacon to cut-off the latter part of RAW (that continues after TBTT or TSBTT on the timeline), which would disable all the stations assigned to the cut-off slots to ever access the medium. Moreover, disrespecting (2) would  
Figure 3: RAW feature: the beacon carries an RPS element that defines the duration of RAW, the number of RAW groups within the beacon interval, their duration, the number of equal-sized slots within each group and the assigned stations. disable every transmission or reception. If CSB is allowed (i.e., equals 1), a station is allowed to cross its assigned RAW slot boundary to complete the ongoing frame exchange sequence. Otherwise, a station will not transmit if the transmission would exceed the boundary of its allocated RAW slot.