# A Trustworthy Communication Hub for Cyber-Physical Systems
## 3 Concepts of the CPS Communication Hub
### Physical CPS Entities with CPS Communication Spaces  
The operation of physical CPS entities (i.e., device systems, resources) is challenging today, because they usually need to interoperate with all the zones of the system when interacting with the information services of cyber world; Figure 2. For example, a mobile CPS device needs to get access over the radio access systems (mobility zone) into the Internet (demilitarized zone) to be able to interact with service platforms (IoT platforms zone) and/or specific SPs (vertical enterprises zone), so that the all authentications with the related stakeholders are successful. This is particularly challenging  
Figure 4: Trust checking with CPS communication spaces. IoT, Internet of Things; SP, service provider.  
because the physical CPS resources are very heterogeneous, ranging from big machines and vehicles to constrained wearable low power mobile devices, and sometimes a gateway structure is needed, like clarified in Figure 1.  
The operation of physical CPS entities with CPS communication spaces has two options; Figure 5. A physical CPS entity can be attached to the CPS communication hub directly (option 1) or indirectly via the enterprise SP cloud (option 2). The direct attachment requires that the physical CPS entity can be programmed to contain the _connector of the CPS communication hub_ with information of the owner of the device, address of the hosting CPS hub and required owner credentials for authentication with CPS hub. Let us assume that we classify the devices based on who chooses the set of tasks embedded in the device--the manufacturer (Class I), the service provider (Class II), or the user (Class III) [26]. When applying this classification, the direct attachment option looks most relevant for Class III devices or devices operating via the Class III gateway.  
The indirect operation model looks better for the Class I devices, into which the manufacturer usually programs the address of the hosting SP (usually the manufacturer) parameters. The same is true for the Class II devices, where the service provider parameters have been programmed into the devices. In these cases, the indirect operating model requires that the enterprise SP cloud apply the connector of the CPS hub on behalf of the device and the owner.  
Some of the CPS devices are constrained in terms of memory and power capabilities. Usually, they require special optimized operation capabilities to enable feasible collaboration with the related enterprise SP cloud. In these cases, the indirect operating model works better, because the manufacturer usually knows better how the referred optimization shall be done. However, when a separate programmable gateway capabilities/device can be applied, then the direct operating model could also be possible. In such a case, the connector of the CPS hub operating on behalf of the owner is needed in the gateway. In addition, some of the CPS devices could be programmable, for example, via application of virtualization/programmable operation capabilities. In that case, direct operating model could be also applied for Class I or Class II devices, if the manufacturer/enterprise SP allows such programmable operation.  
Figure 5: Capabilities of physical CPS resources and options for operation with CPS communication spaces.