Time-Sensitive Networking Status Update

A growing list of high-profile automation technology suppliers are bringing Time-Sensitive Networking and its promise of real-time determinism on standard Ethernet closer to real-world application.

Though industrial technology marketing issues are addressed, ODVA’s industry conference and annual meeting serves largely as a gathering for technologists to discuss the fine-grained details of burgeoning and developed technologies in the industrial networking space. It was at this conference in 2015 that I first heard the Time-Sensitive Networking (TSN) concept explained at length. At this year’s ODVA event, the TSN topic held a larger role on the agenda, as an increasing number of automation technology suppliers get involved with the IEEE 802.1 Time-Sensitive Networking Task Group.

TSN standards and associated technologies now represent 20 new projects that contribute to the IEEE 802.1 TSN group, said Steve Zuponcic, technology manager at Rockwell Automation. He noted that, as the testing of TSN continues to prove its viability for industrial use, the remaining challenge in bringing TSN to market is to blend TSN with existing industrial technologies.

One aspect of this blending of technologies currently being worked on involves how an industrial network handles scheduled traffic. “Scheduled traffic is given highest priority on the network,” Zuponcic said. “This is fine on a lightly loaded system, but when scheduled traffic gets large, traditional CIP (Common Industrial Protocol) traffic will get less bandwidth.”

To address this issue, Zuponcic explained that the TSN task group is exploring the use of programmable logic controllers (PLCs) as hosts for TSN’s centralized network configuration engine. “The core precept of TSN is that all network communications are managed so that there is a guarantee for performance and data delivery,” Zuponcic said. “If all streams are defined and payloads are known, RPIs (route processor interfaces) are then determined, maximum data delivery latencies are provided and system network calculations can determine if successful operation of the network is achievable. All devices on the system participate in this traffic-planning process by publishing [their information] to a centralized network configuration engine. This notifies the engine of the traffic requirements for the connections involved. Currently a separate box [controller] is used for the centralized network configuration engine, but it should reside on a PLC.”

Zuponcic added that a centralized user configuration tool could be part of a PLC’s programming software, noting that significant work on IEEE’s 802.1Qbv—scheduled Ethernet (traffic timing)—is being done to allow for modeling and simulation of systems before buying hardware.

According to the IEEE, the 802.1 Qbv amendment specifies time-aware queue-draining procedures, managed objects and extensions to existing protocols that enable bridges and end stations to schedule the transmission of frames based on timing derived from IEEE 802.1AS. Virtual Local Area Network (VLAN) tag-encoded priority values are allocated allowing simultaneous support of scheduled traffic, credit-based shaper traffic and other bridged traffic over Local Area Networks (LANs). Bridges are increasingly used to interconnect devices that support scheduled applications (e.g., industrial automation and process control). This amendment will provide performance assurances of latency and delivery variation to enable these applications in an engineered LAN while maintaining existing guarantees for credit-based shaper and best-effort traffic, [thereby allowing] those two kinds of networks (engineered and non-engineered) to be consolidated into a single network with a significant cost reduction to the user.

“The big issue for TSN when it comes to industrial control is that there are many standards and no motivation toward convergence,” said Zuponcic. “TSN demands a holistic view—it drives everyone towards the same wire to share data.”

This means that all vendors must participate “on a shared wire,” said Zuponcic. “A lack of participation results in a lack of service.”

Though not all automation technology vendors are onboard with TSN yet, their numbers have been growing dramatically, as evidenced by all the activity taking place around the TSN testbeds.

Paul Didier of Cisco provided an update at the ODVA meeting on the TSN testbeds being conducted by the Industrial Internet Consortium (IIC) and National Instruments’ IoT Lab.

“The IIC testbed is focused on flexible manufacturing,” said Didier, “with the goal of delivering real-time control and synchronization of high-performance machines over standard Ethernet. To do this, participants are working to combine different traffic flows on a single network based on TSN.”

With the help of National Instruments (NI), four TSN plugfests have been conducted at their facility. The first one was held in June 2016 with NI, Cisco, TTTech, Schneider Electric, Analog Devices, KUKA, Intel, GE and Ixia participating. The goal of that plugfest was to get devices from all participating suppliers synchronizing over 802.1AS.

The plugfest held this past October at NI added Bosch Rexroth, B&R Automation and Belden to the mix of suppliers and resulted in the TSN testbed demo showcased at the IOT Solutions World Congress last fall.

Didier said that the testing process for participants in a plugfest starts with time synchronization and is followed by traffic scheduling. “We don't have protocols running on this yet, but are looking to include linear motion apps at the March plugfest in Frankfurt.”

Looking ahead to the work planned for the March plugfest and beyond, Didier said, “We’re confident that centralized configuration of time-sensitive features will be achieved, making it possible to coordinate motion control, I/O, controller-to-controller and machine-to-machine communications seamlessly across the same networking backbone. Also, data from all of these communications can be made available to users via the network or cloud.”

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