The Last Word: Making Ethernet Deterministic

Summary

At one of my previous jobs, the company tried to implement a 5BASE-T Ethernet network to collect and aggregate data from automated test systems. Yes, this was over coaxial cable. And no, it was not deterministic. Anything but. The result was multiple network crashes per hour. If only today’s robust Ethernet technologies were available at that time.
 

That was then; this is now

“Over time, industrial Ethernet has replaced fieldbus networks due to its ability to offer a standardized physical layer combined with greater bandwidth,” wrote Thomas Burke, strategic global advisor at CC Link Partner Association, in a May 2021 Automation.com article titled The Road to Deterministic Ethernet for Industrial Automation. “This allows multiple devices to be connected over one network while ensuring high-speed data transmission. However, standard Ethernet, as defined by IEEE 802.3 specifications, is intrinsically a non-deterministic technology, as it cannot prevent message collisions.

“The different solutions to achieve determinism often consisted of custom, ‘proprietary’ industrial Ethernet protocols aimed at addressing specific tasks or domains. Thus, their scalability was limited, as each was tailored for a specific application area. Furthermore, while many of them could be considered open, they would not be compatible with each other.”

Enter: Time sensitive networking

Standard information technology (IT) network equipment can’t provide synchronization and/or precision timing. It is not time sensitive. For straight-ahead IT data processing, it is more important to deliver data reliably than to be constrained by time. Therefore, there are no constraints on delay or synchronization precision. Network congestion is handled by throttling and retransmitting dropped packets at the transport layer. However, there is no method of preventing congestion at the link layer. Data can be lost when the buffers are too small or the bandwidth is insufficient, but excessive buffering adds to the delay, which is unacceptable when low deterministic delays are required.

The time sensitive network (TSN) standards specified by IEEE 802.1 can be grouped into three key component categories required for a complete real-time communication solution based on switched Ethernet networks with deterministic quality of service (QoS) for point-to-point connections. Each standard specification can be used on its own and is mostly self-sufficient. However, only when used together in a concerted way, TSN as a communication system can achieve its full potential. The components are:

• Time synchronization: All devices that are participating in real-time communication need to have a common understanding of time.
• Scheduling and traffic shaping: All devices that are participating in real-time communication adhere to the same rules in processing and forwarding communication packets.
• Selection of communication paths, path reservations, and fault-tolerance: All devices that are participating in real-time communication adhere to the same rules in selecting communication paths and in reserving bandwidth and time slots, possibly using more than one simultaneous path to achieve fault tolerance.

Applications that need a deterministic network that behaves in a predictable fashion include control networks that accept inputs from sensors, perform control loop processing, and initiate actions; safety-critical networks that implement packet and link redundancy; and mixed media networks that handle data with varying levels of timing sensitivity and priority.

Burke wrote in a Sept. 2023 Automation.com article titled Why TSN is Good for Digital Transformation in Manufacturing: “The most common backbone for IT communications is Ethernet. The improvement in Ethernet performance is greatly outpacing that of the niche industrial transports based on the needs of a much larger IT market. In 2016, Ethernet specifications were improved to include TSN. This was the missing link to enable Ethernet as the backbone for machine automation, combining IT networks and operational technology (OT) networks, and the combining of control and information communications within a machine. With new products on the market with support for TSN, machine builders can now deliver equipment with a common Ethernet TSN backbone, enabling direct data access with the entire machine, from controller to sensors and actuators.”
 

Looking ahead

Burke added that industry 4.0 technologies are unleashing a new wave of innovation that is sure to increase performance, improve quality, and reduce costs. With new technology such as Ethernet TSN, we will see machines shift from their original designed for purpose and fixed performance to those that can offer quantified performance, quality and cost metrics, and that will improve over time, based on OEM analytics and feedback.
Fortunately, we’ve come a long way since 5BASE-T.

_{This column originally appeared in the April 2024 issue of InTech magazine 2024.}