Timing is everything when it comes to complex automated industrial systems like those used in, say, assembly lines and power transmission plants. Actions often must occur at precise intervals; if they’re delayed by even a millisecond, the consequences could be serious to critical.
IEEE 802.3 Ethernet standards have helped speed up time-critical communications systems that industries rely on to link components with wireless sensing and control. But Ethernet has not been able to support what’s known as real-time distributed control. This enables a device or system to respond to a command almost instantaneously, with minimum delay.
These time delays, known as latency, are of concern because they decrease bandwidth and, in the process, split communications into several different Ethernet networks. Today, Ethernet can only be used for time-critical communications when it is not mixed with reserved traffic for video streaming and traditional best-effort traffic. The real-time gap has been filled, however, by other organizations, like the International Electrotechnical Commission, which has created industrial computer network protocols such as fieldbus that tweak the IEEE 802.3 standards to provide those real-time capabilities.
However, industries also want their requirements met with IEEE Ethernet standards. Getting there is the mission of the IEEE 802.3 “Distinguished Minimum Latency Traffic in a Converged Traffic Environment” Task Force. It’s working on requirements for network latency as well as maximizing bandwidth by allowing many forms of communications traffic on a single Ethernet network.
“This standard will be broadly used in different domains such as transportation, automotive, and health care, whereas the specialized fieldbuses are just for industrial automation,” says Ludwig Winkel, chair of the new IEEE 802.3 task force and a fieldbus standards manager at Siemens Industry Automation division, in Karlsruhe, Germany.
The proposed standard would allow various types of applications to coexist without the need for a new physical layer, replace existing Ethernet, or split the different communications into different parallel links. Merging of different applications into a single platform is simplified by guaranteeing that each gets the quality of service needed.
TYPES OF TRAFFIC
Industrial communications systems handle three types of applications traffic: scheduled traffic, reserved traffic, and best-effort traffic. Scheduled traffic is used to send cyclic control data. These include exchanging analog and digital values from actuators and sensors based on synchronized processes or on sampled values of current and voltage. Reserved traffic carries audio and video streams, counts and measurements, and real-time diagnostics like inspection and tracking information. Best-effort traffic sends acyclic data like configuration and diagnostic information, and firmware updates.
“Companies need a converged environment in IEEE 802.3 networks that supports both normal traffic and interspersed express traffic,” Winkel says.
ONE NETWORK FITS ALL
Converged networks are relatively simple to maintain, manage, and diagnose, providing for connectivity everywhere. They are also more flexible, making them easier to upgrade and enhance. They could support traffic that includes business transactions, Internet access, voice over Internet Protocol, and inventory tracking. Other benefits over separate systems include fewer network failures, lower installation costs, and fewer cables and connectors.
The new standard would do several things: guarantee more bandwidth and minimum latency for scheduled traffic, provide guaranteed bandwidth for reserved traffic, and still allow for the transfer of best-effort traffic, all on the same network. It also means more capacity and lower latency for high-priority traffic, and supports critical asynchronous processes for nonscheduled traffic—all at a lower cost and consuming less power.
“For example, when the best-effort traffic goes over the link, it is stopped and checked to see whether a high-priority industrial communications frame is needed,” Winkel says. If it is, then it will be dispersed by slicing up the bit stream.
“We are chopping the bit streams from the best-effort traffic so that small frames for control can be interspersed,” he says. “A download can be made from a couple of megabytes to even a gigabyte through such a communications best effort, achieving minimum latency.”
The standard will be able to support various applications protocols over a single network, such as Ethernet IP and audio/video, according to Winkel. It will also provide for better clock synchronization and better real-time behavior for nonscheduled critical traffic. It will meet control needs while also supporting non-control/deterministic applications.
“If you are doing a control loop, where time and determinism is critical, this new technology will achieve the minimum latency,” Winkel says.
The first draft of the standard is on track to be approved in November 2014.