# Performance Evaluation of IEEE 802.11 ah Networks With High-Throughput Bidirectional Traffic
## 4 Performance Evaluation and Discussion
### Video Streaming Scenario
#### 4.2.2 Video Streaming Scenario: Results  
The trade-off between the maximum attainable data rate per station and the total number of cameras in the network is illustrated in Figure 5. The EDCA/DCF experiments have been performed with the same number of stations and the same effective medium access time. Already for 10 stations, TIM and RAW can achieve a 10% higher data rate than EDCA/DCF. For 10-80 stations, RAW and TIM introduce an improvement of at least 10-30% in maximum attainable data rate, whereas, for denser networks, this improvement further grows. However, low-quality video streaming is only feasible for up to 20 stations as the maximum attainable data rates become too low in denser networks. An average of 160 kbps per station is feasible with 20 stations, whereas 10 stations achieve 255.2 kbps with IEEE 802.11 ah, which is significantly better than 115.2 kbps and 230.9 kbps achievable with EDCA/DCF, respectively. The MCS and TCP segment size are, as expected, crucial for sustaining higher data rates. A higher MCS enables much faster transmissions, reducing the drops due to the busy radio (already transmitting/receiving). Only two stations can have 200 kbps with MCS0. A bigger TCP segment size makes better use of the available channel bandwidth before waiting for TCP ACKs, which can be clearly seen in the rise in achievable data rate (Figure 6). Experiments showed that, in general, 2144 byte TCP segments perform best in any configuration (regardless of density and MCSs). The only measured exception was for MCS4 with 10 stations as shown in Figure 6a. A TCP segment size of 2144 bytes performs best because it is the largest TCP segment that does not need IP fragmentation (as evaluated in Section 4.4). Transmission of a single 1608 byte segment with MCS0 takes 21.84 ms, which is more than one fifth of our beacon interval. This means that, if RAW is used, its slots must be sufficiently long. TCP segment sizes larger than 1072 are not feasible with MCS0 for RAWs with more than four slots because the transmission time of a single segment would be longer than the RAW slot duration. With a maximum data rate of 41.1 kbps per station for 10 stations and 17.7 kbps for 20 stations, MCS0 is only feasible for less demanding applications. MCS4 fares better and attains a maximum of 89 kbps for  
Figure 5: IEEE 802.11 ah attains higher data rates per station with TIM segmentation and RAW slotting, and offers better scalability. (**a**) Maximum attainable data rates with TCP segment size 3216 bytes for MCS8 at 2 MHz over 200 m are higher with TIM segmentation and RAW slotting; (**b**) Maximum attainable data rate per stations with EDCA/DCF declines with increasing density relative to IEEE 802.11 ah with TIM and RAW. 20 stations. MCS8 has the least problems to attain the receiving rate: a TCP segment size of 1608 bytes or higher is enough to reach 128 kbps data rate for 20 stations without any issue. Contention in RAW slots reduces the maximum attainable data rate by 45-70%, as can be seen by comparing graphs (a-c) in Figure 6. Stations sense if the medium is busy at the start of the RAW slot and, if so, apply the exponential back-off algorithm which on average halves the available time and thus the possible data rate. RAW standing alone largely contributes to reducing contention, but adding TIM segmentation makes the difference in dense (or high-throughput) networks where RAW meets its limits by further contributing to the reduction of contention. Increasing the number of TIM groups can increase the data rate by 1-15% for MCS8 (Figure 7) and 6-20% for MCS0. However, increasing the number of TIM groups has a serious impact on the latency: four TIM groups with a beacon interval of 102.4 ms result in a minimum RTT of at least 409.6 ms assuming the frame was queued in an empty EDCA queue during the RAW slot of the station and the AP schedules the reply in the next DTIM cycle. Doubling the number of TIM groups also doubles the round trip time that can pose problems for TCP retransmissions.