# Performance Evaluation of IEEE 802.11 ah Networks With High-Throughput Bidirectional Traffic
## 4 Performance Evaluation and Discussion
### Video Streaming Scenario
#### 4.2.1 Video Streaming Scenario: Simulation Setup  
In order to evaluate the limits on throughput and scalability in our video streaming network, we ensured there is always motion, so the cameras are streaming continuously. We varied six configuration parameters, as shown in Table 3, so as to analyze which parameters affect the attainable throughput most. We executed 19,008 ns-3 simulations to assess all combinations of the different parameters. Each simulation was 1000 s long.  
_MCS_ and bandwidth dictate the data rates and the attainable range. MCS0 with a 2 MHz channel has a theoretical limit of 650 kbps, MCS4 bumps it up to 3.9 Mbps and MCS8 is the fastest at 7.8 Mbps but is also the most constrained in terms of possible range (up to 100 m in outdoor environments).  
_Contention per RAW slot \(c_{s}\)_ represents the number of stations with which station \(x\) shares its RAW slot. Contention introduces collisions and back-offs in shared RAW slots. A value of 0 means a station \(x\) has the channel to itself and does not need to contend. A value of 1 means a station shares its slot with 1 other station, and so on. This parameter allows us to generate a constant traffic demand regardless of the amount of TIM groups and RAW slots. As the stations are evenly distributed over TIM groups and RAW slots in RAWs, the total number of stations in the network is:  
\[N_{STAs}=(c_{s}+1)\cdot N_{TIM}\cdot N_{RAW}. \tag{5}\]  
The AP schedules to next slot parameter enables forcing the AP to schedule each reply in the next available RAW slot (after the next DTIM beacon) instead of the current active one. This action introduces a delay that depicts realistic behaviour in case the TCP server is not running on the AP itself.  
TCP segment size changes the payload size on the wire. Using a larger segment size increases the number of sent bytes before a TCP ACK from the server is expected, but also increases the number of collisions.  
The parameters \(N_{TIM}\) and \(N_{RAW}\) stand for the number of TIM groups and the number of RAW slots, respectively. Varying \(N_{RAW}\) will make the channel access time for stations smaller or larger because the duration of RAWs in each beacon interval is maximized to maximize channel access opportunities. In this constellation, with one TIM group (beacon interval is \(T_{DTIM}=T_{TIM}=102.4\) ms) and five RAW slots, each station would have 20,420 us of channel access each DTIM cycle, 51,140 us for two slots and almost the full beacon interval with 102,380 us for a single slot.  
To measure the maximum attainable data rate along with maximal scalability, we measured both the sending rate at the station and receiving rate at the AP. When the receiving rate is lower than the sending rate, the EDCA queue and the TCP TX buffer fill up and packets start getting dropped,  
\begin{table}
\begin{tabular}{c c c c c c c c c c c c} \hline \hline \multicolumn{1}{c}{**Parameter**} & \multicolumn{10}{c}{**Values**} \\ \hline MCS (2 MHz) & MCS0 & MCS4 & MCS8 & & & & & & & & \\ Contention per RAW slot & 0 & 1 & 2 & 3 & & & & & & \\ AP schedules to next slot & true & false & & & & & & & & \\ IP cameras sending rate (kbps) & 2 & 4 & 8 & 16 & 32 & 48 & 64 & 96 & 128 & 256 \\ TCP segment size & 536 & 1072 & 1608 & 2144 & 2680 & 3216 & & & & \\ \(N_{TIM}\) & 1 & 2 & 4 & 8 & & & & & & \\ \(N_{RAW}\) & 1 & 2 & 5 & & & & & & & \\ \hline \hline \end{tabular}
\end{table}
Table 3: Varied configuration parameters in video streaming scenario.  
which is an indication of a congested network. The results of our simulations are presented and discussed in the following subsection.