# A Novel Covert Channel for IEEE 802.11 Networks Utilizing MAC Address Randomization
## 5 Covert Channel Performance Evaluation  
### Scenario 1--Periodic Transmission with No Retransmission  
Figure 6 depicts the covert channel throughput analysis. Two parameters directly influence the throughput: the transmission interval (TI) and the number of regular STAs connected to the same network as the covert STA. As the TI is extended, the throughput decreases regardless of the number of regular STAs connected to the network. For instance, when the TI is set to 100 ms in the isolated environment configuration, the throughput experiences a tenfold reduction from its initial value (from 4.8 Kbps to 0.48 Kbps), representing a significant drop. Notably, longer TI leads to throughput values that converge to the extent that having 10 additional STAs is nearly equivalent to having 50 STAs generating traffic. This decline in throughput can be attributed to the latency introduced by longer TIs during transmission. The impact of adding regular STAs to the network is illustrated in Figure 6. Increasing the number of STAs in the network leads to a degradation in throughput for a TI. For instance, when the shortest TI of 10 ms was used, it was observed that the throughput significantly decreased as soon as 10 STAs joined the network (from 4.8 Kbps to 2.9 Kbps). Furthermore, with the addition of 20 STAs, the throughput further dropped to 2.3 Kbps, which is half the initial value. In denser environments, where 20 to 50 STAs were considered, the difference in throughput reductions tended to become less evident. This indicates that the effects of having 20 STAs are nearly equivalent to having 50 STAs. The impact of additional STAs on the throughput can be attributed to increasing collisions as more STAs compete for channel access. Next, we evaluate the transmission efficiency of the covert channel. In Figure 7, it is evident that the TI does not significantly impact the transmission efficiency. However, the number of regular STAs connected to the network affects the transmission efficiency. This observation indicates that the response-to-request ratio is not solely dependent on the frequency of frame transmission but on the presence of external traffic generated when the frame is sent. As depicted in Figure 7, in an isolated environment with no external traffic interference, no frame loss was initially detected in the covert channel. However, upon adding ten STAs to the network, a significant decrease in efficiency was immediately observed, resulting in approximately 63% efficiency. Furthermore, the inclusion of 20 STAs led to a frame loss of nearly 50%. In more congested environments with 30, 40, and 50 STAs, the covert channel experienced a further decline, with over 50% of frames being lost. The delay analysis is presented in Figure 8. In an isolated environment, a nearly constant delay of approximately 0.5 ms is recorded, regardless of the TI. However, the delay increases when additional STAs are added to the network. Notably, the shortest TI exhibits similar delays across different numbers of STAs, oscillating around 4 ms. From 20 ms and beyond, the delay slightly rises as the transmission interval increases. This observation  
Figure 6: Covert channel throughput as a function of transmission interval in scenario with retransmission disabled. Figure 7: Covert channel efficiency as a function of transmission interval in scenario with retransmission disabled. highlights the impact of multiple factors, including TI and the number of STAs, on the delay. As more STAs join the network, channel resources are exhausted, and the AP frame queue grows, affecting the response time. As shown in Figure 9, the covert channel exhibits jitter based on the delay. In the isolated scenario, the jitter was relatively insignificant compared to the scenario with an external load. Observations revealed that with a transmission interval of 10 ms, the jitter remained below 3.5 ms.