# Underwater Wireless Communications for Cooperative Robotics with UWSim-NET
## 3 Results
### MAC Layer  
This second experiment demonstrated the ability of UWSim-NET to use different MAC protocols. A scene inspired by the TWINBOT project was used as an example of a real multi-robot scenario (see Figure 5a). This scenario consisted of two Girona500 [22; 23] intervention AUV (I-AUV), shown in yellow in Figure 5b, whose objective was to grasp an underwater pipe in a cooperative manner. These two vehicles shared a RF channel to communicate with each other. At the same time, a support vehicle (BlueROV) was connected through an umbilical to a buoy, which was connected with the operator using a WiFi link. The support vehicle tried to follow the trajectory of the I-AUVs in order to implement a communication bridge between the RF channel and the operator. In addition, four extra cooperative AUVs (BlueROV) shown in Figure 5a acted as explorers, making a survey over the seafloor in order to detect other objects of interest for the operator. All explorers shared an acoustic channel to communicate with a buoy in order to send visual data and receive commands from the operator. Seven ROVs were considered in this scenario: the two I-AUV and one BlueROV shared a RF channel and the four BlueROV shared an acoustic channel with the buoy on the surface. The block diagram of the software elements of this experiment is depicted in Figure 5c. The trajectories of each ROV during the experiment are shown in Figure 6.  
Figure 4: End to end delay measured with the model of an acoustic modem. For this experiment, all communication devices were configured to have a transmission rate of 1.9 kbps and a 1 ms intrinsic delay. Table 1 details the eight data flows defined for this experiment, three for RF communications and five sharing the same acoustic channel. On the one hand, each explorer sent large packets to the surface simulating the transmission of low resolution images. Simultaneously, the buoy sent small broadcast packets with commands destined to the explorers. On the other hand, the remaining three packet flows transmitted between the robots sharing the RF channel. Two of the RF packet flows corresponded to the communication between the two I-AUV, while the third one was the packet flow transmitted from the support vehicle to the I-AUVs. This experiment was repeated considering two MAC protocols: the ALOHA with carrier sensing (CS-ALOHA) and slotted FAMA (S-FAMA). First, the data rates required to send the operator's commands to the ROVS was established at 32 bps, identified as support and buoy in Table 1. The data flows transmitted by the I-AUV (leader and follower in Table 1) were expected to have a throughput as high as possible, since they were devoted to exchange the pose of leader and follower for the cooperative intervention. The same occurred with the transmissions from the explorers (E0-E3 in \begin{table}
\begin{tabular}{l c c c c c c c c} \hline \hline  & \multicolumn{3}{c}{**RF**} & \multicolumn{3}{c}{**Acoustic**} \\ \cline{2-9}  & **Leader** & **Follower** & **Support** & **Buoy** & **E0** & **E1** & **E2** & **E3** \\ \hline CS-ALOHA & 62.5 & 62.5 & 4 & 4 & 30 & 30 & 30 & 30 \\ S-FAMA & 27.5 & 27.5 & 4 & 4 & 27.5 & 27.5 & 27.5 & 27.5 \\ \hline \hline \end{tabular}
\end{table} 
Table 1: Throughput (Bytes per second) of each packet flow stream during the second experiment. Figure 5: Illustration of the scenario of the second experiment. The four BlueROVs in charge of the visual surveying share the acoustic channel with the buoy on the surface. The communication between the two I-AUVs and the survey vehicle is based on an RF network: (**a**) general view of the scene; (**b**) details of the RF communication network between the two I-AUVs and the survey AUV with the umbilical to the surface; and (**c**) block diagram of the elements considered in this second experiment. Table 1), which were expected to provide the operator with visual feedback of the seafloor, thus to adjust the data rate of each transmission at the maximum value allowed for each protocol.