# An Aquatic Mobile Sensing USV Swarm with a Link Quality-Based Delay Tolerant Network
## 6 Performance Evaluation
### Link Quality Estimation Comparison  
There is no real link quality metric of reference which other link quality estimators can be compared to. Therefore, to evaluate the metrics utilized in the link estimation, we compare PAmuLQE with a link quality estimation only based on the SSI.  
To measure the quality values, given by Equation (2), we performed test B1, where just USV 2 was sending packets. The evaluation is shown in Figure 17. As expected, PAmuLQE measures a higher quality value on the link with no data packets (node 4 to node 3), due to this estimator taking into account the time that a packet takes from the moment that it is sent to the moment that it is received. Because link 2-3 is being used for transmission besides the announcement packets, this link will introduce a higher delay on each packet, therefore lowering the link quality. This makes path P2 the best and the selected path for delivering packets from node 1 to node 3.  
For the evaluation of the link quality estimator, the delivery ratio is presented in Figure 18 for both tests B2 and B3. In this case, the topology scenario is the same for both scenarios, and USVs 1 and 2 generate packets. The PAmuLQE forwarding strategy is the one selected to assess the impact of the link quality estimator. As expected, when the packets from USV 1 to USV 3 go through path P1, which happens when the LQE is based on RSSI estimation because both paths have the same quality values (because the distance is the same), it takes longer to deliver all packets. However, when PAmuLQE is used, the best path in this case is path P2, which has a higher delivery ratio because this path uses a relay node that does not generate any packet, and therefore more bandwidth to the Gateway is available. Both real and simulated environments present similar behaviors and the same average results.  
To evaluate the adaptability of PAmuLQE, we performed tests B2 and B3 and analyzed the quality values measured by Equation (2), which is represented in Figure 19. In the beginning of test B2 and B3, USV 2 forwards packets directly to USV 3, and USV 1 calculates the best path to forward its packets. At this point, the quality values follow the behavior shown in Figure 17, making P2 the chosen path. In the first half of these tests (0-40 s), USV 2 delivers all 100 packets to USV 3, and USV 1 delivers all 100 packets to USV 4. Here, the quality value of link 4-3 is higher, as this connection is not delivering a large amount of packets. When node 2 stops delivering packets, the link 4-3 starts exhibiting a quality value lower than the one of link 2-3, since it stops being used to deliver data packets.  
In test B3, we increase the amount of packets that USV 2 has to deliver to USV 3, causing links 2-3 and 4-3 to be delivering packets simultaneously. Due to this situation, and as shown in Figure 20,  
Figure 17: Quality values given by Equation (2), for an unused link (node 4 to node 3) and a link with packets (node 2 to node 3) in Scenario B, test B1.  
the quality values are closer to each other. Link 4-3 has a mean quality value of 9.1505 and link 2-3 of 8.6520 (difference of 5%).  
Figure 19: Quality values given by Equation (2) for link 4–3 and link 2–3 in Scenario B.  
Figure 20: Quality values given by Equation (2) for link 4–3 and link 2–3 in Scenario B, test B4.