## 7 System Simulation Model
This graph presents a surface of the proposed model, where nodes turn \(On\) and \(Off\) according to the characteristics of the bird's sounds, in black, and the case when all the nodes are always active in the \(On\) mode, in red. We can see that

Figure 11: Mean Square Error for (**a**) Throughput, (**b**) Idle time slot, and (**c**) Collision. the throughput is very close among these schemes when the transmission probability is low (lower than 0.3). This is due to the fact that even if the nodes remain in the _On_ state all the time, they do not transmit, which generates a traffic very similar to the case when a node turns _On_ and _Off_. However, as \(\tau\) increases, nodes in the _On_ state attempt many more transmissions, consequently increasing the throughput. As such, when nodes do not go to the _Off_ mode, there are many more transmissions, but in the same way, there are many more collisions, as observed in Figure 12c, especially as \(\tau\) approaches 1. Building on this, we can clearly see that a higher throughput does not always entail good system performance since there are many packet collisions and high energy consumption with non-useful data transmission. Also note in Figure 12b that as \(\tau\) and \(N\) increases, the probability of finding empty slots reduces considerably, which is important to consider for a concealed channel since nodes would constantly generate sound exposing the presence of a surveillance system. The results presented in Figure 13a compare the lifespan of the system due to its energy consumption for the proposed _On/Off_ scheme and the case when nodes are always active. It can be seen that even though the difference in the throughput (shown in the previous figure) is higher at certain points for the always _On_ scheme, the energy consumption is lower for all the cases in our proposed scheme. Indeed, the conceal channel approach has the main objective of turning _Off_ nodes according to the natural behavior of birds, and it also achieves an important energy consumption reduction (higher system lifetime). Figure 13b presents the efficiency analysis, which examines the relationship between throughput and the system's lifespan. This relationship is analyzed by multiplying the throughput and lifespan results of both systems (\(Lifespan(N,\tau)\times Throughput(N,\tau)\)). It can be observed that efficiency increases as the probability of transmission increases. However, an increase in the number of nodes in the network has the opposite effect, causing efficiency to decrease. Now, we compare the performance of the sound-based system to a conventional RF-based network. We can see in Figure 14a that the proposed covert channel achieves a lower transmission rate compared to a classic RF channel using ZigBee at 20 and 200 Kbps. However, this lower transmission rate occurs for a system where communications are hidden using a different communication medium (sound) that is robust against conventional jamming attacks which are expected in many surveillance applications, while the ZigBee based systems would certainly collapse in case of such attacks while our proposal remains operational even under DoS-based attacks. Hence, these results clearly show the system capacity of the covert system, in such a way as to provide clear guidelines for the system administrator to know in advance the transmission rate. For instance, if the system is based on small packet transmissions, such as presence detection or variations in lighting or sound, the achieved data transmission of 978 bps would be more than enough to provide adequate monitoring services. However, if the system has to transmit images or video HD of 5 Mbps, then this proposed WSN would not be adequate due to the low transmission rate. The main limitations of this work are related to the transmission distance, which is significantly lower (hundreds of meters) compared to RF-based transmissions (several kilometers) as well as the transmission capacity, as discussed before, where only small packets can be transmitted.