## 4 Results
### Physical Testbed Results  
To evaluate the Modbus protocol on the DUT, a purely physical OT network testbed performed 4000 cycles of Modbus requests for individually reading and writing single registers, function codes \(0\times 04\) and \(0\times 06\), to characterize the device's response and reliability during normal operations. These tests were then conducted with varying network traffic over the actual test network. First, tests were conducted with additional UDP and TCP packets sent to the device. However, the link reliability was only affected when a valid Modbus header was present without a valid Modbus device address. While this is not necessarily a normal network traffic characteristic, these tests help characterize the device's response to possible false data injection attacks. The collected results can be seen in Figure 4.  
This figure reveals a noteworthy trend in the device's performance, indicating that even a relatively low frequency of errant packets can significantly degrade link reliability. Specifically, during the tests, it was observed that out of 4000 packets transmitted, 21 packets failed to receive a valid response. Despite this initial degradation in reliability, the impact did not worsen substantially as the frequency of attack packets increased. In fact, even with a higher volume of errant packets, 3920 of the packets still received valid responses, suggesting a degree of resilience within the system. This finding underscores the importance of understanding how even minimal disruptions in packet transmission can affect system performance, particularly in critical network applications where reliability is paramount. There is also a large drop shown between 40 packets per second and 50 packets per second. This is likely due to the limited number of samples, as 30 pps had 3970 successful responses, 40 pps had 3976 pps, and 50 pps exhibited 3950 pps. Given the small number of packet differences between these, the authors anticipate this curve would flatten out with additional samples.  
Figure 4: Link reliability under network load.  
The measured response times, as illustrated in Figure 5, show a high level of consistency across all tested traffic levels, further reinforcing the system's ability to maintain stable performance under varied conditions. The figure presents both individual runs of traffic levels, plotted as lines in the figure, and the solid histogram, which represents the average response time across all tests. The average response distribution, derived from all measured samples, was employed in subsequent phases of network simulation and emulation. This distribution was particularly useful for these purposes, as it closely mirrors the latency characteristics observed in each individual test scenario, ensuring that the network simulations accurately reflected real-world performance under diverse conditions. Thus, the consistency of the latency distribution across different traffic scenarios provides a reliable basis for modeling and subsequent analysis from the perspective of network behavior and performance.  
There is observable variation in the device's response times under specific packet load conditions. The red line in Figure 5 represents the average response time when a single request is sent per packet. It demonstrates that the response time increases on average when additional request packets are sent per second. This variation is limited in the other tests performed.