## 4 Standards Enabling CPES
### Opc UA (Iec 62541)  
OLE for Process Control (OPC) [50] is the successor of OPC Unified Architecture (OPC UA) [51]. This framework was created for data exchange in automation technology and has since become widely used in industry. OPC UA is maintained by the OPC Foundation [52]. The standards IEC 62541-3 describe the object-based address space model of OPC UA. This framework follows a service-oriented, internet-enabled approach that is available across platforms. It provides a communication model and a uniform data model for processing data, alarms, and historical data, which can be used on a system-wide basis. Objects, properties, relationships, and methods can be used to create an information model of the physical application. A basic set of nodes is already defined in the standard system [53; 54]. This basic scaffold can be extended to specific information models of any application domain, such as CPES. Standardized, cross-manufacturer information models are referred to as Companion Specifications (CS). The uniform semantics of this CS enables interoperability between type-like devices of different brands. A client-server model via TCP/HTTPS, as well as a connectionless publisher-subscriber model using existing standards, such as the User Datagram Protocol (UDP), Advanced Message Queuing Protocol (AMQP), and Message Queue Telemetry Transport Protocol (MQTT), are supported for data exchange [55]. The scalability in larger infrastructures is given by the possibility to use multiple OPC UA servers and clients. In addition to the information models and transport mechanisms, OPC UA offers an integrated security architecture to protect data and access. Different security policies and selectable mechanisms for authentication and authorization make the protocol flexible. [56]. In order to further reduce complexity, OPC UA has several server profiles with different, interdependent functional ranges [57]. These profiles gradually build on each other:  
* _Nano Embedded Device Server_: the smallest possible profile includes the core server, binary message encoding, and TCP support. * _Micro Embedded Device Server_: additionally includes at least two simultaneous sessions and a simple subscription protocol. * _Embedded UA Server_: in this profile, additional messages can be signed and encrypted. The number of possible subscriptions is also increased. * _Standard UA Server_: this full-featured profile allows server configuration customization, allows the use of X509 certificates, and further increases subscription capacity. For the mapping of the CPES system topology, the authors of [15] recommend an object-oriented representation of the numerous, heterogeneous components. OPC UA meets this requirement because the objects in its information model allow states, methods, values, and hierarchy-free relationships via links. Moreover, variable structural dynamics are mentioned as a challenge, which is given in OPC UA over the extendable address space. OPC UA clients with the appropriate authorization can add any other objects to the address space. After all, the extensible markup language (XML) meets the demand for a model language that can be read by humans, as well as machines. The most important must-have properties of CPES are summarized in [4]. Table 2 examines the OPC UA standard for coverage of these criteria.