# A Trustworthy Communication Hub for Cyber-Physical Systems
## 2 Related Works  
The targeted grand challenge of this research dealing with communications of cyber-physical systems arises from the accessibility, trustworthiness, heterogeneity, mobility, and ownership of physical entities. The interaction with physical resources (devices) over the communication networks shall concern the ownership, communications, and trust aspects. The adopted solution approach is based on the application of communication overlay and security technologies in a novel combined way for experimental realization of the communication spaces concept, represented in the proceeding step of this research [2]. The communication spaces concept applies the idea of making a kind of virtual home for people and their resources. The respective idea has been applied earlier, for example, in the concept of virtual home environment for supporting roaming ecosystems in the mobile context for the mobile device of a user [3]. The same type of concepts has also been developed for sensors devices, for example, virtual sensor services, called the \(\upmu\)SMS (micro subscription management system) middleware, for smart environments over sensor networks from tiny in-network services based on agent technology is provided in the work of [4]. The respective required functionalities are included e.g., in the smart instant messaging (SIM) system with presence management, user centric configuration and adaptive grouping [5]. However, the requirements arising from ownership, trust, and communications when dynamic resources owned by different stakeholders are exchanging information are not known to be considered earlier in such energy flexibility and mobility cases. A review of some communication overlay technologies is represented in Table 1 (see also the works of [2; 6]), and discussed in the following. An essential required feature in CPS systems is delivery of information from one source to one (1-1) or multiple destinations (1-N) according to the needs of the destinations. Therefore, the capability to realize publish/subscribe paradigm was seen to be more relevant than the pure client/server model [7]. For example, extensible messaging and presence protocol (XMPP) technology [8; 9] has such a publish/subscribe mechanism, which is based on the concept of a specific entity called \"pubsub\" node [10; 11; 12]. Such a \"pubsub\" node acts in a way as the publish/subscribe service established to work in the XMPP server. A publisher can publish information into the \"pubsub\" node. A subscriber can subscribe the referred information, and receive event notification when the information is published. However, the referred XMPP based publish/subscribe means have proved to be quite complicated. The publish/subscribe capabilities of the message queue based telemetry transport (MQTT) protocol have proved to be much more simple and lightweight for physical resources [13; 14]. There are also other respective message queues; broker-based (e.g., data distribution service (DDS), advanced message queuing protocol (AMQP), and simple text oriented messaging protocol (STOMP)) or broker-less (e.g., zero message queue protocol (ZeroMQ)).