## 5 Conclusions  
The 5G SENSOR@SEA project realized a low-cost tracking and monitoring system for \"general purpose\" ISO containers that integrates 5G cellular technologies for massive Internet of Things with a satellite-optimized backhauling link. With the cooperation of four industrial partners and a telecommunication research center, the so-called \"5 GT System\" was designed, developed, tested and validated up to field trials. Several modules of the system were designed, built and finally installed on the ship and in the teleport: the container tracking devices placed on the containers, the NB-IoT cellular network with optimized satellite backhauling, the Ku-band GEO satellite terminals and the maritime service platform based on the OneM2M standard. The field trial conducted during the intercontinental liner voyage of a container ship showed the validity of the project. The 5 GT System is still a prototype that, although designed to operate under the challenging conditions of containerized ocean transportation, had never been tested under real operating conditions, before the field trial. The performances of the hybrid cellular/satellite network in the field trial are a world first, and it is also possible to compare them directly with the performances of the commercial cellular network, because \"twin\" tracking devices have been deployed. The field trial also provided some insights on how to make the system installation and devices more robust, in subsequent developments. During the whole travel, all the components of the 5 GT System worked correctly, including the Maritime Service Framework applications and the management of the lock/unlock switching of the onboard Smart Cell. The performance indicators (lost messages and latency) show, in general, that CTDs transmit better in the \"open sea\" scenario than in the \"near sea\" and terrestrial scenarios. The packet loss values of the 5 GT System with satellite backhauling in operation (\"open sea\") are often lower than the values recorded on commercial cellular networks, while the delay values are equivalent. The worst-performing scenario is the \"near sea\" travel, in line with expectations, since, in this scenario, the sailing ship is reached by the cellular signal from shore cells that are up to 21 miles away. In this scenario, the CTDs record lower RSSI values than the other scenarios and, consequently, higher packet loss and higher latencies due to retransmissions by NB-IoT. The field test also showed a relevant number of lost messages: from 3% up to 22% in \"open sea\" and even more in the scenarios relying on commercial NB-IoT networks only. Figure 28: Plot NB-IoT message loss vs. max latency—open-sea scenario. The loss of messages resides in the NB-IoT Link mostly. It could be mitigated by acting in three directions: (i) the optimization of transmission protocols in the core network; (ii) the adoption of a more robust transmission protocol by the CTD, e.g., CoAP (suitable for high-loss connections) instead of raw UDP; (iii) improvements in CTD software introducing retransmission algorithms at the application layer in the case of lost messages. Regarding cellular transmission bands on the ship, the trial showed that, per current international regulations [36], the B28 band, while favorable in terms of signal propagation on the ship's decks, cannot be used outside territorial waters, between 12 nm and 41 nm. In order to ensure the operation of the 5 GT System in this zone as well, it will be necessary to use a permitted band (e.g., B3 band) or propose changes in the regulations to the relevant bodies. More generally, in this area, important developments are expected to be achieved with the integration of NB-IoT NTN direct communication with GEO and LEO satellite communication technologies and 6G cellular networks for the massive development of the Internet of Things. Conceptualization, M.F. and S.N. ; methodology, all; software, M.F., M.G., A.I., F.F., A.M., A.A. and M.T. ; validation, all; formal analysis, M.F.