## 5 Case Studies on the OMF-Enabled Testbed
### Case Study 4: Packet Loss and Jitter Analysis in VoIP Applications  
This case study evaluates the performance of Voice over IP (VoIP) applications under varying network conditions, focusing on packet loss and jitter, two critical metrics that significantly affect real-time communication quality. The experiment demonstrates the OMF-enabled test environment's flexibility in simulating diverse network scenarios and validating their impact on VoIP performance. This test environment utilized again a two-hop 802.11 g wireless ad hoc testbed, as shown in Figure 3d. Research nodes S and R served as the sender and receiver, respectively, with intermediate wireless hop introduced to emulate real-world network conditions. The experiment used the D-ITG tool to generate VoIP traffic, specifically simulating G.711 codec streams with a packet size of 160 bytes transmitted at 50 packets per second. To introduce controlled network impairments, the OMLized netem tool was configured on the intermediate node to simulate varying levels of packet loss and jitter. Packet loss rates were incrementally adjusted from 0% to 5%, while jitter values ranged from 10 ms to 50 ms in steps of 10 ms. Each configuration was tested for a duration of 5 min to ensure statistically reliable measurements. Two key performance metrics were evaluated for this case study:  
* Mean Opinion Score (MOS): This metric provides a subjective measure of perceived audio quality, calculated using the E-model. The MOS value is computed as the following: \[MOS=1+0.035R+7\times 10^{-6}R(R-60)(100-R)\] (8)  
Figure 6: Achievable throughput measurements for TCP and UDP on a two-hop 802.11 g wireless ad hoc network. where \(R\) is the quality rating derived from network metrics, including packet loss, jitter, and delay. Higher MOS values indicate better perceived quality, with a maximum score of 5. * Packet Delivery Ratio (PDR): The PDR is a measure of the percentage of packets successfully delivered to the receiver compared to those sent. This metric evaluates the reliability of packet delivery under varying network conditions. It is expressed as the following: \[PDR=\left(\frac{\text{Packets\ Delivered}}{\text{Packets\ Sent}}\right)\times 100\] (9)  
The OML was used to collect and preprocess experimental data, including packet timestamps, loss rates, and delays. MOS and PDR values were derived using these measurements. Figure 7a presents the MOS values as a function of packet loss rates. The results show a sharp decline in perceived audio quality when packet loss exceeds 2%. For instance, at a 5% loss rate, MOS values dropped below 2, indicating poor communication quality unsuitable for real-time applications. Figure 7b illustrates the impact of jitter on PDR. As jitter values increased beyond 30 ms, the percentage of successfully delivered packets decreased significantly, demonstrating the sensitivity of VoIP traffic to variable delays. At 50 ms of jitter, the PDR dropped below 85%, suggesting compromised reliability. These results validate the capability of the OMF-enabled test environment to replicate real-world network impairments and evaluate their effects on VoIP performance under realistic conditions. This case study highlights the adaptability of the OMF-enabled test environment in creating realistic scenarios for evaluating performance metrics critical to real-time applications. By dynamically configuring network parameters like packet loss and jitter, the framework enables researchers to assess application resilience and performance under diverse conditions. These findings underscore the practicality of the test environment for testing not only VoIP applications but also other latency-sensitive systems such as video conferencing and online gaming, further demonstrating its versatility and utility in network research.