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 ]
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