Connectivity Framework
7: Connectivity Standards
For more details, and to determine the suitability of DDS for a specific set of system requirements, please refer to the assessment template( see chapter 6) in Annex A.
7.1.2 WEB SERVICES USING HYPERTEXT TRANSFER PROTOCOL( HTTP)
Web services( Wikipedia) 1 using Hypertext Transfer Protocol( HTTP) refers to the applicationspecific connectivity frameworks, primarily devised for human user interaction interfaces. They rely on a RESTful 2 style of architecture( Fielding, Wikipedia 3) using the HTTP connectivity transport standard to exchange textual data, as shown in Figure 7-1. It requires the TCP transport. The IETF 4 maintains the HTTP open standard specification; the W3C 5 maintains the web( HTML5) specifications.
Web services using HTTP are generally used in the application domain( see Figure 1-1). Data is represented in textual form( either as JSON or XML), and embedded in a hypermedia( HTML) context. A Uniform Resource Identifier( URI) represents a data object on a server. A client( app) sends a request to a web server, specifying a data object URI, an operation and a payload. The server replies with failure or success and a response payload. The communication is text-based and designed for human speeds. It is not efficient for device-to-device communications and not suitable for real-time communications.
For more details, and to determine the suitability of web services using HTTP for a specific set of system requirements, please refer to the assessment template( see chapter 6) in Annex D.
7.1.3 OPEN PLATFORM COMMUNICATIONS UNIFIED ARCHITECTURE( OPC-UA)
OPC Unified Architecture( OPC-UA 6) is a connectivity framework standard used in the manufacturing industry. OPC-UA is designed to support multiple transports. Currently transport mappings are defined for TCP with a OPC-UA Binary encoding connectivity transport standard or the HTTP connectivity transport, as shown in Figure 7-1. Work on a Web Socket transport mapping has started. All current transport mappings require the TCP transport. The OPC Foundation maintains the OPC-UA family of specifications.
At its core, OPC-UA targets device interoperability. Before OPC-UA( or its predecessor OPC), applications simply accessed devices directly through proprietary APIs provided by their vendors. Unfortunately, this meant that applications became dependent on the particular device they controlled. Worse, higher level applications such as Human-Machine Interfaces( HMI) had no easy way to find, connect to, or control the various devices in factories. OPC-UA is generally used
1
See [ W3C-WSA ], for overview [ WKPD-WS ]
2
RESTful means to adhere to a REST communications architecture style.
3
See [ Fielding-2000 ], for overview [ WKPD-REST ]
4
See [ IETF ]
5
See [ W3C ]
6
See [ OPC-UA ]
IIC: PUB: G5: V1.0: PB: 20170228- 49-