EMBEDDED TECHNOLOGY FEATURE
systems are easier to scale, test, build, deploy, and maintain. The DDS data-centric design makes it
seem like all data is local, so application aren’t by default coupled at all. DDS controls interactions with QoS policies including deadlines, latency budgets, update frequencies, reliability, durability, ownership, ordering, and filtering. It transparently connects hundreds of platforms, dozens of networks, languages, versions, operating systems, and chip architectures. Coupling is only by design. By contrast, OPC UA applications talk
directly to each other. Client/server connects clients through servers. In PubSub, every subscriber gets exactly the same data from the publisher at the same rate. DDS automatically discovers named
“topics” across the system regardless of where the application lives. Applications don’t have to do anything or have any knowledge to find the data they need. OPC UA subscribers query a server to
get a configuration that includes the publisher of the data they need. They can also introspect a publisher to see what it can publish. Both are active queries; OPC UA doesn’t do systemwide automatic discovery.
SECURITY OPC UA secures the underlying transports. There is no generic way to indicate which data is allowed to flow to which client. DDS instead provides dataflow security
regardless of network, operating system, or location. Secure DDS uses a signed “permissions” document that specifies directional access (e.g. read but not write) for every topic and application. This requires no code; security can be added after the system is running. DDS systems can grow to many
thousands of applications with transparent routing between subsystems. Top to bottom, across subnets, or with any pattern (pubsub, request-reply, or queuing), DDS offers a unified data model, a single security model, and consistent access to data. With MQTT or AMQP, a publisher can
talk to a cloud server, but not to other OPC UA client/server or PubSub subscribers. OPC UA client/server can roll up workcells into a larger factory. OPC UA PubSub can support multiple devices on a single network. OPC UA does not offer unified system data access in the same way DDS does. OPC UA subscribers can select a
DataSet with a filter, but only to ensure reception of the correct data. They can also limit access to a single publisher.
Figure 3c / ELECTRONICS Figure 3a
Figure 2 (source: RTI): Autonomous vehicles are revolutionising the automotive industry. With a data-centric approach, DDS handles both on-vehicle and control-room use cases. Data routing provides a consistent data model throughout the system, thus building a reliable, large-scale infrastructure. To adopt software drive, the IA industry needs a similar system approach
Figure 3: a. The DDS-OPC UA gateway standard makes the OPC UA information model available to DDS software applications
b. The primary use case is to convert OPC UA devices into DDS devices. This allows intelligent software applications to access any OPC UA device
c. Together, these standards can provide attractive software development and easily-integrated devices. It lets AI work at every system level
DDS data centricity fundamentally
works by filtering flows. QoS matching allows subscribers to receive information only from capable sources. Time-based filtering decouples the speed of producers from uses of information. Content filtering analyses the content and delivers only data that fits the specification. Together, these filters ensure delivery of the right data to the right place at the right time with minimal wasted bandwidth.
USE OF TSN TSN, or Time-Sensitive Networking (IEEE 802.1), is a developing set of Ethernet standards to deliver data in bounded time. TSN is limited to smaller, single- subnet systems. With TSN, OPC UA PubSub can deliver
better real-time performance and reliability on a subnet. DDS uses QoS settings to optimise use
of the underlying network, supporting everything from slow, lossy networks like satellite links to isochronous transports like backplane buses. Combining DDS with TSN combines high-level data access with real-time subnets. OPC UA’s main use case is to help
Figure 3b
manufacturing engineers build workcells with minimal software. In stark contrast, most DDS users are software engineers building closely-coordinated systems. Smart manufacturing systems may need both sophisticated software and interoperable devices. The OMG recently passed a standard for an OPC UA/DDS gateway that makes the OPC UA information model available within DDS. The transition to smart machines will
not be smooth; product lines, companies, and entire national economies are at stake. The defining technology of the next 20 years is exponential computing growth. Architectures must enable software that can use this compute power.
RTI
www.rti.com
ELECTRONICS | SEPTEMBER 2020 27
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