# Ship's Digital Twin--A Review of Modelling Challenges and Applications
## 3 Connectivity Issues and Communication between the Ship and SDT  
An SDT can be defined as a synthesis of a sensor network installed on a ship, providing insight into a real ship by observing various systems onboard, and simulation software where operational insight into users is given as a result of processing acquired data whether onboard or onshore, in a cloud, operating centre, etc. This means that the communication between an SDT and a real object, the ship, is very much alive and represents another issue in the process that needs to be considered in terms of concept development, which is also identified in [12] as a challenge that limits SDT's potential. Regarding the level of connectivity between the DT and a real object, six DT topologies were proposed in [42], namely: disconnected DT, connected DT, embedded DT, aggregated DT, multi-device DT, and combined DT. A disconnected DT has no connections with a real physical twin, due to connection nonfeasibility, unlike a connected DT, which has a stable connection to the physical twin all the time. An embedded DT is embedded in a physical twin, while an aggregated DT is composed of multiple DTs. A multi-device DT is connected to multiple physical twins, while a combined DT combines several aforementioned topologies. As communication between the ship and its twin should be in real time, in the best case, it can be assumed that DT concepts should stream towards a connected DT topology. However, this issue is expected to be a subject to compromise between different influencing factors, including the economical side as well. Commonly used communication protocols and standards in DT formulation, in accordance with the Open Systems Interconnection (OSI) reference model, are reviewed in [43; 44]. The OSI reference model is a conceptual model, enabling network communication between diverse communication systems or devices. It is based on seven stacked-up layers, each responsible for a specific purpose in the communication process and responding to the requests of an adjacent layer. Seven layers are, starting from the first: physical, data link, network, transport, session, presentation, and application layer, as presented in Figure 2.  
The physical layer defines properties of a physical medium, i.e., communication channel for data exchange. The data link layer defines the format of the data exchanged over the network. The network layer breaks up data into smaller units, i.e., data packets, reassembles them on the receiving device, and routes data to reach their destination. The transport layer is responsible for error control to ensure that the data are completely received or transmitted. The session layer is responsible for opening and closing a communication session for data transmission to prevent waste of processing resources. The presentation layer ensures a correct and usable data format. The last layer, application layer, being the only layer that is in direct interaction with the user, is responsible for presenting data in a user-perceivable format. Commonly used physical layer standards are Ethernet, Wireless-HART as explained in [45], RG-6, CANBus, RS-485, and RS-232. The data link layer utilises standards such as CDMA (code-division multiple access), CSMA/CD (carrier-sense multiple access with collision detection), CSMA/NBA (carrier-sense multiple access with nondestructive bitwise arbitration), Modbus, OpenFlow, PTP (precision time protocol), and wireless-HART as explained in [45]. The network layer includes IPv4 and IPv6 protocols as explained in [46]. The transport layer protocols include the TCP/IP (transmission control protocol/Internet protocol), UDP (user datagram protocol), ControlNet, and DeviceNet, while the session layer includes the CIP protocol family, Modbus, TCP as used in [5], and EtherCAT. The presentation layer includes the CIP (Common industrial protocol) family and EtherCAT.