# Seamless 5G Multi-Hop Connectivity Architecture and Trials for Maritime Applications
## 3 Maritime Network Architecture and Solutions for Seamless Connectivity
### Single- and Multi-Hop Ship-Shore Connectivity  
As has been discussed previously, the 5G multi-hop method offers an interesting option to extend the coverage on the sea. With the available information about the real-time vessel movements and historical data, a multi-hop network could be planned for vessels so that it would gather the most important maritime corridors by utilizing operative vessels as moving base stations or multi-hop nodes. Such a solution could save infrastructure investments and enable more flexible communication for maritime scenarios. From a risk and resilience analysis point of view, such an option could also be interesting due to the freedom from infrastructure built onshore. For example, in emergency situations, multi-hop node vessels could provide fast and agile support for rescue teams and operations by sharing their connectivity capacity. By creating a \"dynamic\" 5G mesh network in the corridors of intensive vessel traffic, it is possible to provide backup channels for ensuring 5G connectivity. In cases where one connection channel is lost, a vessel could connect to another vessel without losing connectivity. We assume that, with such approach, 5G could be more reliable compared with the existing solutions. Figure 10 depicts different options to achieve IP connectivity for the onboard-onshore splits discussed in the previous section. One or more always best connected (ABC) router(s) are the central elements for enabling flexible and resilient connectivity between onboard and onshore components. A local area network (LAN) is assumed on the ship, which also includes the special case of the LAN only consisting of an ABC router. The LAN transports \begin{table}
\begin{tabular}{c c c c c c} \hline \hline
**Split** & **BBU** & **Core (UP)** & **Core (CP)** & **MEC** & **IP Failure** & **Cost** \\ \hline Wireless backhaul (Figure 7) & Onboard & Onshore & Onshore & No & Immediate network outage & Lowest \\ CP onshore (Figure 8) & Onboard & Onboard & Onshore & Yes & Delayed network outage & Medium \\ Core aboard (Figure 9) & Onboard & Onboard & Onboard & Yes & Local services survive & Highest \\ \hline \hline \end{tabular}
\end{table} 
Table 1: Summary of the main differences among the three different network splits. Figure 9: Core onboard. at least the N6 interface to the onboard network. Optionally, depending on the split option discussed in the previous section, N2, N3 and/or N4 interfaces could also be provided. The wide area network (WAN) interface of the ABC router could select from among different options, namely satcom or 5G (incl. a fallback to earlier cellular network generations). If capable, the ABC router could also aggregate several connectivity options and also have more than one modem of each type. It is also not precluded that one or more of the modems depicted separately in Figure 10, e.g., the 5G one, would be integrated in the ABC router. For the use of satellite communication in conjunction with 5G, different architecture options exist, as described in the introduction. Within the implementation options described by ETSI [31], the following two were identified as suitable for the maritime scenario evaluated within the trials described in Section 4:  
* ETSI Scenario A4: indirect access via an NTN transport network. * ETSI Scenario A5: indirect untrusted access using non-3 GPP networking functionality (N3 IWF) with a secure entity (SER) between the micro cell and the NTN terminal to be integrated into the 5G core network. A block diagram for both options, A4 and A5, employing state-of-the-art very-small-aperture terminals (VSAT) is depicted in Figure 11.  
Figure 11: Options for realizing satellite connectivity for ferry trials. Figure 10: IP connectivity with always best connected (ABC) router. Due to the ease of establishing seamless integration with the 5G core network (without the need to implement the N3 IWF functionality), the first option (scenario A4) is currently being implemented as a baseline, as will be described in Section 4.3.