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 -