# A Novel Covert Channel for IEEE 802.11 Networks Utilizing MAC Address Randomization
## 5 Covert Channel Performance Evaluation
### Scenario 2--Periodic Transmission with Retransmission  
In the second scenario, the presence of a covert STA introduces retransmission, thereby increasing the probability of successful frame delivery. As depicted in Figure 11, the impact of retransmission is initially not perceptible in the isolated environment since no frames are lost, as in the first scenario. However, as more STAs join the network, the retransmission effect becomes more apparent compared to the first scenario. To compare the two extremes of TI used in the experiments, 10 ms as the shortest and 100 ms as the longest, we observed that the covert channel's behavior remains similar to the previous scenario, where the throughput decreases as the TI increases. However, when retransmission is enabled, and additional STAs are added, significant throughput improvements are observed. For the shortest transmission interval, with ten more STAs, the throughput increased by 1169 Kbps. Adding 20 more STAs increased to approximately 1098 Kbps. In denser environments, where 30, 40, and 50 additional STAs were added, throughput gains of 763 bps, 1069 bps, and 900 bps were achieved, respectively. Notably, the benefits of retransmission extend to even the longest TI, as in the case of 100 ms for each case from 10 to 50 additional STAs, the gain of more than 100 bps was consistently observed. Figure 10: The impact of the covert station on the network saturation throughput. The introduction of retransmission significantly enhances the efficiency of the covert channel. It is worth noting that, as observed, the transmission efficiency is closely correlated with the number of regular STAs rather than the values of the TI. Comparing it to the first scenario, when having ten additional STAs, efficiency improved from approximately 60% to values exceeding 80% (see Figure 12). When 20 more STAs were added, the efficiency experienced a notable increase of 22%. Furthermore, as 30, 40, and 50 more STAs joined the network, the transmission efficiency improved by 19%, 22%, and 18%, respectively. In contrast to the first scenario, none of the configurations in the second scenario exhibited frame loss exceeding 50%. While retransmission enhances throughput and efficiency, it is necessary to note that it introduces an additional delay, as evidenced by the results presented in Figure 13. The retransmissions lead to an increase of up to 2.5 ms in each configuration compared to the delays observed in the first scenario. This effect can be attributed to the additional traffic generated by the covert STA. As more STAs join the network, many frames require retransmission, increasing collisions and amplifying channel delays. Despite introducing this additional delay, the benefits of retransmission, including improved throughput and transmission efficiency, outweigh the observed delay. Likewise, as illustrated in Figure 14, it was noticed that retransmissions contribute to increased channel jitter. Enabling retransmissions resulted in an average jitter increment of 1.5 in scenarios involving external traffic. Figure 11: Covert channel throughput as a function of transmission interval in scenario with retransmission enabled. Figure 12: Covert channel efficiency as a function of transmission interval in scenario with retransmission enabled. Adapting the Selective Repeat ARQ Protocol--The Concept of the Discontinuous Sliding Window Protocol  
The obtained results demonstrate that the performance of the covert channel is significantly influenced by the number of stations associated with the BSS and their respective loads. As the number of stations increases, several factors are impacted, including decreased throughput, reduced efficiency, increased delay, and elevated jitter. This effect is particularly noticeable in dense environments, where the presence of additional stations (30 to 50) worsens the impact of external traffic on the covert channel.