## 5 Experimental Results
### Tests in a Mobile Scenario
#### 5.4.2 Experimental Results  
In Figure 22, the PMF of the hop index, at which messages are received, is shown. From the results in the figure, it can be observed that the \"average\" network topology emerges from the test. In particular, it can be observed that for all the SUs, 95% of the fragments are received within the first two communication hops. In other words, in the majority of the cases, the SUs have been in direct visibility (or at least two hops away) from the PU. Therefore, we expect a performance comparable to that observed in Scenario \(1\).  
From the point of view of the number of missing packets after the first transmission of the PU, from Figure 23, it can be observed that the performance is comparable with that obtained by \(\mathrm{SU}_{2}\) and \(\mathrm{SU}_{3}\) in Scenario \(1\). A remarkable exception is represented by the node, \(\mathrm{SU}_{1}\), which exhibits excellent performance in Scenario 3.  
Figure 21: Circuit followed by the vehicles during the tests. Image taken from Open Street Map.  
Figure 22: PMF of the hop index at which _DATA_ messages are received in Scenario 3.  
Similarly, also from Figure 24, it can be observed that the behavior, in terms of requested messages and satisfied messages, is comparable to that observed in Scenario \(1\). However, there is a significant difference with respect to Scenario \(1\). In the latter, the larger fraction of requests is satisfied by the PU, while in Scenario 3, the SUs play a more important role.  
By observing Figure 25, it is possible to get some interesting insights, concerning the main differences between Scenario \(1\) and Scenario \(3\). First of all, it can be observed that the archive is received only after a first round of _REQUEST_ messages and subsequent retransmissions. Moreover, a network bimodal behavior can be observed:  
* the archive is received in a very short time (after 1 or 2 _REQUEST_ message rounds)--this happens when the network conditions are good and stationary;
* the archive is received after a large number of rounds (5 or 6)--this happens when the network homogeneity disappears because of phenomena that break the network topology, such as the presence of other vehicles fragmenting the platoon or an excessive inter-vehicle distance.  
Figure 23: PMF of the number of missing _DATA_ fragments before the transmission of the first _REQUEST_ message in Scenario 3.  
Figure 24: Number of requested _DATA_ messages and number of replica _DATA_ messages sent upon the reception of a request, in Scenario 3.  
Finally, in Figure 26, the CDF of the received _DATA_ messages, before a _REQUEST_, is shown. The obtained results justify the conclusions reached in the previous paragraph (relative to the results in Figure 25). In fact, it can be observed that a small number of rounds is often sufficient to collect the majority of fragments. However, a significantly longer time is required for completing the reception of the entire archive.  
To summarize, the obtained results show that the performance on a realistic scenario, even if simplified by keeping an almost constant vehicle speed and a limited inter-vehicle distance, has been shown to be similar to that obtained in Scenario \(1\) and even better than that obtained in Scenario \(2\).