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Time-Sensitive Networking, Eventually

POSTED 08/28/2020

Time-sensitive networking (TSN) --a set of extensions to Ethernet standard IEEE802.1 defining protocols and requirements for deterministic, real-time industrial communication-- is on the way.

TSN could be considered the next stage in Ethernet evolution, introducing deterministic features missing from traditional Ethernet technology and promising to bring IT and operational technology (OT) together. 

TSN ensures punctual and reliable transmission of data using the following elements:

  • 802.1AS (guarantees high precision time synchronization of all devices in a TSN network)
  • 802.1Qbv (allows scheduling communication streams to achieve minimum jitter and latency)
  • 802.1Qbu (provides methods to assign priority to communication streams) 

But don't start preparing your goodbyes for existing industrial communications and Ethernet protocols just yet.

Existing data exchange standards and protocols with TSN extensions are well suited to the short-to-medium-term demands of industrial automation. Furthermore, TSN itself is still in development, as are TSN-ready chips and switches. These factors, combined with a relatively slow-moving control systems market mean that it is likely to be several years before TSN is widely deployed.

OPC UA, for example, the widely accepted Industry 4.0 standard for communication between machine control level and manufacturing control level. TSN extends the usage of OPC UA into the machine control level, enabling users, for the first time, to use a single communication protocol for all kinds of industrial communication from the factory floor to the cloud.

OPC UA over TSN was developed to remove vendor dependencies, streamline automation deployments and lead to improved, secure, synchronized automation performance. In part, it achieves this by ensuring unprecedented levels of timely interoperability between machines, sensors, motion controllers, robots and other devices.

OPC UA over TSN is open, fast and provides consistent and secure communications, says Varad Darji, product manager at B&R Industrial Automation Corporation.

“In the industry right now, it's not common to find a protocol with all of these qualities. Open protocols are not always secure, secure and open protocols are not as fast as they could be. Fast protocols often lack consistency and are not deterministic. OPC UA over TSN provides all of these features.”

Darji describes an industry that's fragmented between different vendor offerings, while at the same time striving to meet the data requirements of Industry 4.0.

“When I started in this field as a machine builder everyone was talking about different protocols. It was an absolute nightmare. Line speed was a problem too. And machines couldn't talk to each other. Over time, there has been a great effort in the industry to synchronize all that and with OPC UA over TSN I can finally see light at the end of the tunnel.”

The time-synchronization elements of OPC UA over TSN could be used to enable sophisticated synchronization between devices like conveyors, sensors and a robot arm, Darji explains, with a corresponding rise in throughput.

“When these elements are synchronized you can pick 300 pieces per minute instead of 100 pieces per minute. And, with smarter electricity consumption you get much higher throughput. You have the same facility and the same footprint, but now your throughput is 3-5 times higher. That turns heads,” says Darji, adding that he is seeing particular interest in OPC UA over TSN from the packaging industry, which wants to be able to synchronize robots with other machines.

In combination with OPC UA, TSN will bring benefits for factory automation as well as for process automation, including cross-vendor interoperability and real-time capability of standard network infrastructure components, says Thomas Brandl, Program Manager, Bosch Rexroth.  

“For process automation, time synchronization, a common network for time-critical control data and high-volume data like video stream are of high importance. Factory automation needs minimum jitter and latency.”

Traditionally, Brandl notes, OPC communication has been used to connect HMI devices (OPC UA client) to industrial controllers (OPC UA server). In the last few years, however, it has become more typical to integrate an OPC UA client into the control to start with OPC UA-based control for communication management.

“For now, this is limited to non-real-time communication, but in the future, TSN will remove this limitation and speed up the usage of OPC UA instead of the wide variety of Ethernet-based field buses [we see today]. Rexroth strongly supports this trend because OPC UA is an open standard that is accepted by all leading automation players and fulfills the rapidly increasing demand for cyber security,” explains Brandl.

TSN holds great promise, but it won't replace protocols that have already proven themselves in scenarios that require synchronized, deterministic motion control, says Bob Trask, North American representative, EtherCAT Technology Group.

“TSN still requires switches to operate and switches are 'gooey,' meaning they introduce lag to the system. There are also potential issues around TSN network configuration to be resolved before TSN shows its full potential,” says Trask, noting that such developments could be a decade away. In the meantime, Trask notes, EtherCAT will be still be used when real-time data communication is required.

OPC UA over TSN will “eventually” take over from some existing protocols, says B&R's Darji, but not in the next 2 or 3 years, due to the slow-moving nature of the industry. 

“It took almost 10-15 years before controllers got an Ethernet port on them. At the same time, our sales cycles are longer, often taking several years to develop, produce and market complex machines. There is no way around OPC UA over TSN now that we have a lot of major players on board but we're looking at a 4-6 year timescale before it really starts to dominate.”  

TSN by itself does not provide interoperability in industrial communication, notes Bosch Rexroth's Brandl. Rather, TSN is a large set of standard elements from which elements must be selected in order to achieve interoperability. This selection process is ongoing in IEC/IEEE 60802 and OPC FLC standardization.

“In addition to the communication profile, a common application layer is required. It is like a phone call, where you need a connection between two phones, as well as a common language and wording. The common language and wording is provided by OPC Field Level Communication which will include profiles for motion devices and industrial safety.”