## 5 Simulation Results  
PER and throughput of each link are presented in Figures 9 and 10, respectively. Figure 9 compares the PER per link obtained with the three physical layers described in Section 4.1 with the PER measured during the sea trial (green diamond).  
Uwphysical (Figure 9a) is extremely optimistic and provides a very low PER. In particular, with the considered settings the PER of the links is equal to zero up to a transmission range of 1.1 km, and increases to 1 when the distance between nodes is more than 1.6 km. This implies that the link connecting the two farthest nodes (node 1 and node 3, that are 1.2 km from each other) has a non-zero PER, but still the real values are underestimated. Conversely, the PER obtained both with the two-state (Figure 9b) and with the three-state (Figure 9c) models is very similar to the one observed in the sea trial, with the three-state model having a PER that almost perfectly (within the CI) matches the experimental one (depicted with green diamonds), definitely outperforming the other two models.  
Figure 9: PER results yielded by the simulations (bars) with respect to Haifa Harbor measurements (green diamond) for UWPhysical (**a**), UWHMMPhysical (**b**), and UWHMMPhysicalExt (**c**).  
Similarly, the throughput observed with Uwphysical (Figure 10a) is almost the same for all of the links, and is equal to 3.7 bps: only in the link between node 1 and 3 the throughput is approximately 1.85 bps, as that link was removed at the simulation time 9000 s, when the network topology was changed from topology 1 to topology 2. With a higher PER per link, the throughput observed with the two- and three-state HMM is significantly different link by link, presenting results that are definitely closer to those that can be observed during a sea experiment.  
Finally, we report some plots showing the variability of throughput in time (i.e., computed every 300 s) for the links from node 4 to node 2 and from node 1 to node 3, and again we see how optimistic the results obtained using the simplest PHY module are (Figure 11). We can observe the jump at 9000 s for the link \(1\to 3\) due to the switch from topology 2 (where the link was not in place) to topology 1. Besides, the values for the throughput are constant for the UWPhysical module, but for the links \(1\to 3\) and \(3\to 1\), which are the only ones having a PER greater than zero. Conversely, the throughput obtained with the two HMMs models (Figures 12 and 13) is definitely lower, due to the higher PER, and has a higher variance, well characterizing the channel variability. While we could directly compare the PER obtained in simulation with that experienced during the experiment, we could not perform the same operation for the throughput, as the simulation used an application layer generating traffic with a different rate than the one used during the sea trial. This tool can be used to test protocol stack configurations that are different from the one used in the experiment, exploiting the measures obtained during the sea trial to model the packet error rate time evolution and observing other performance indicators as a result, such as the throughput per link.  
Figure 10: THR results yielded by the simulations with respect to Haifa Harbor measurements for UWPhysical (**a**), UWHMMPphysical (**b**), and UWHMMPphysicalExt (**c**).