Connectivity Framework
7: Connectivity Standards
The distinction between transport and framework layers is important. To be considered a connectivity framework at a minimum, a connectivity transport would have to be paired with a data type system. For instance, a connectivity transport such as MQTT could be paired with data type system technology such as protocol buffers 1, and could be used to create a custom connectivity framework. However, there is currently no standard that describes such a pairing.
The connectivity framework and transport standards shown in Figure 7-1 are discussed below. Detailed assessments are provided, starting from Annex A.
7.1 IIOT CONNECTIVITY FRAMEWORK STANDARDS 7.1.1 DATA DISTRIBUTION SERVICE( DDS)
Data Distribution Service( DDS) is an open connectivity-framework standard specifically targeted at IIoT applications. The Object Management Group( OMG) maintains the DDS family of specifications in the DDS Portal, including Remote Procedure Call over DDS 2.
DDS is generally used in the control, application, information, operations domains, and sometimes in the business domain( see Figure 1-1). DDS’ s main purpose is to connect components( devices or gateways or applications) to other components to enable real-time systems and system-of-systems. Components interact with a shared data space, and never directly with each other. Therefore, it is referred to as a data-centric middleware standard. It has roots in high-performance defense, industrial, and embedded applications.
DDS implements direct component-data-component communication via a relational data model. DDS is also referred to as a databus because it is the data-in-motion analog to a database that manages for“ data-at-rest”. Both a database and a databus implement the“ data-centric” abstraction; applications interact with the infrastructure, not directly with each other. The difference is that a database saves past data that can later be searched by relating properties of the stored data. A databus manages future data by filtering on properties of the incoming data. Data centricity makes a database essential for large storage systems. Data centricity makes a databus a fundamental technology for large IIoT software integration and autonomous operation.
The DDS wire protocol is The Real-Time Publish-Subscribe Protocol( RTPS) DDS Interoperability Wire Protocol Specification( DDSI-RTPS) 3 connectivity transport standard, as shown in Figure 7- 1. DDSI-RTPS is independent of the underlying transport. Many applications use the UDP transport. DDS can also run over TCP, shared memory, backplane connections, hypervisor transports and many others. DDS does not require any special transport properties such as reliability support.
1
See [ GOO-PB ]
2
See [ OMG-DDS ] and [ OMG-DDSRPC ]
3
See [ OMG-DDSI-RTPS ] IIC: PUB: G5: V1.0: PB: 20170228- 47-