The Industrial Internet Consortium has released an important part of its Reference Architecture guidance: its Connectivity Framework document. This is actually pretty important; this document dives into the detail on connectivity for IIoT systems, establishes criteria for evaluating connectivity technologies/standards and puts forward some likely technologies for core connectivity standards, including DDS, OPC-UA and WebServices. In other words, there is some really valuable guidance here.
What is Connectivity for IIoT Systems?
According to the Industrial Internet of Things Connectivity document, “connectivity provides the ability to exchange information amongst participants within a functional domain, across functional domains within a system and across systems. The information exchanged may include sensor updates, events, alarms, status changes, commands, and configuration updates.” More concretely, connectivity is the critical, cross-cutting function that supports interoperability within and across IIoT systems. Moving beyond the current mish-mash of proprietary and vertical-industry-specific standards to an open IIoT standards-based framework is the goal of this work.
Looking at Connectivity from a technical viewpoint, figure 1 shows where the Connectivity function lies on a Network, Connectivity, and Information stack, and it divides Connectivity into 2 different layers: Transport and Framework. The Transport layer provides technical interoperability, with “Bits and Bytes shared between endpoints, using an unambiguously defined communication protocol.” The Framework layer provides syntactic interoperability, with “Structured data types shared between endpoints. Introduces a common structure to share data; i.e., a common data structure is shared. On this level, a common protocol is used to exchange data; the structure of the data exchanged is unambiguously defined.” Addressing connectivity needs up through the syntactic interoperability provided by the connectivity framework layer and assessing connectivity framework standards is one of the important contributions of this document.
Figure 1. Connectivity, using the Networking functions below – Internet Protocol, provides the layers for communicating messages and data between system participants.
The Concept of a Core Connectivity Standard.
To ensure interoperability within and across IIoT systems, the Connectivity Framework document recommends the use of a core connectivity standard. Figure 2 shows how this core standard becomes the connectivity bus for the system, integrating native devices and applications directly and legacy, or non-core-standard devices and applications through protocol gateways or bridges. In this way non-standard entities can be “normalized” into the core connectivity standard. This core connectivity reference architecture is central to the IIC's guidance on ensuring secure, device to device to application interoperability for IIoT systems.
Figure 2. Using a Core Connectivity Standard provides for interoperability and streamlined integration within and across IIoT systems.
Evaluating Connectivity Standards.
To reduce the integration and interoperability challenge across different IIoT systems, a key goal of the IIC, the document provides a method and template for evaluating connectivity technologies and standards for the IIoT. It includes assessments of most IIoT standards like DDS, OPC-UA, HTTP/WebServices, OneM2M, MQTT and CoAP. Many of these standards turn out to address different levels of the connectivity stack as you can see in figure 3. Further details on each standard are provided in the document.
Figure 3. IIoT connectivity standards and their location on the connectivity stack.
DDS as a Core Connectivity Standard.
From figure 3, you can see that the document assesses 4 connectivity framework standards including DDS. In addition, the Connectivity Framework document provides guidance on requirements for choosing core connectivity framework standards. A core connectivity framework must:
- Provide syntactic interoperability
- Provide way to model data, a type-system (e.g. DDS, OPCUA)
- Can't be just a “simple” messaging protocol (MQTT, COAP, etc)
- Be an open Standard with strong governance:
- from SDOs like IEEE, OASIS, OMG, W3C, IETF
- Be horizontal & neutral
- Be stable and deployed in many industries
- Have standards-defined core gateways to all other connectivity core standards
- Provide core functions like publish-subscribe, request-reply, discovert, etc.
- Meet non-functional requirements: performance, scalability, security, ...
- Meet business criteria: not require single components from single vendors, have supported SDKs, have open source implementations, etc.
In figure 4, you can see the 4 potential core connectivity framework standards assessed against these requirements. DDS supports all the requirements and is a promising standard for IIoT systems across all industries.
Figure 4. IIoT Connectivity Core Standards Criteria applied to key connectivity framework standards.
In particular, if you compare DDS with another promising connectivity framework standard, OPC-UA, from figure 5 below, you can see that they address very different system use cases. If your primary challenge is integrating software applications across an IIoT system, then DDS is a good choice. If you challenge is to provide an interface for your edge device to allow system integrators to later integrate it something like a manufacturing workcell, then OPC-UA is a good choice.
Figure 5. Non-overlapping system aspects addressed by the core connectivity framework standards.
As you can see, this IIC document provides a lot of important guidance and clarifying concepts for IIoT connectivity. You can use it's IIoT connectivity standards assessment profile to assess other standards you may be interested in for your system, or use its guidance to choose among the leading standards. For more detail download the document for yourself.