Feature: Industrial
a shared, secure “global dataspace”, DDS provides a “single source of truth”, enabling a high degree of cohesion (how related the functions within a single module are), whilst guaranteeing loose coupling (the interdependencies between modules). Distributed systems express coupling in
four dimensions: time, space, flow and types:
Time: No dependency on startup or join sequence. Participants may come and go; adding or removing applications or flow paths doesn’t affect others.
Space: Data can come from any physical location and from any producer. Producers and consumers may reside in devices or applications anywhere. In a larger system, applications can transparently live at the edge, in the fog, or in the cloud.
Flow: Data flow rates or reliability specifications between endpoints do not interact. Each application can request data at a different update rate, over any network, and with or without reliability guarantees.
Type: Automatically converts dissimilar types for a data flow if they “match”, allowing systems to evolve over time.
DDS makes it seem like all the data in
the system is local. Applications read and write to a “global dataspace” that looks like local memory, and the data-centric DDS middleware ensures it contains the right data. Unlike OPC UA, OPC UA pub/sub and MQTT, there are no clients, servers or brokers. Te global data space sits between every participant, providing secure, deterministic access to information without tight coupling. Te most important capability of DDS
is the Quality of Service. DDS allows every application to request 21 different parameters such as deadlines, latency budgets, update frequencies, history, liveliness detection, reliability, ordering, filtering, and more. Tese QoS parameters allow system designers to construct a
distributed application based on the requirements for, and availability of, each specific piece of data. Examples include: • Durability allows late-joiners to get data that was produced before they started.
• Deadline and separation specify minimum and maximum data update rates for each subscriber.
• Liveliness ensures that each dataflow is healthy.
• Latency budget, transport priority and reliability decouple flow on a per-stream basis.
Convergence Ethernet (standardised under the IEEE 802.3 protocol) is one of the original
networking technologies. Because of its simple deployment and its ability to evolve without losing backward compatibility, it has become the de facto standard in IT networking. Despite being around for nearly half a century, it was only in the last decade that the operational technology side started to incorporate Ethernet into its solutions. Industrial applications usually have strict
temporal and deterministic requirements. By definition, Ethernet doesn’t guarantee deterministic message delivery or real- time behaviour. However, extremely high performance enables it to serve most such applications, provided there is a way to manage network traffic.
Figure 1: Data link in the OSI reference model
Figure 2: DDS/TSN real-time, deterministic, virtualised, data-centric architecture – applications subscribe to actionable information, not data
20 September 2023
www.electronicsworld.co.uk
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