by Todd Walter, National Instruments, AVnu Alliance Industrial Segment Chair
IoT adoption promises increased amounts of data over widely distributed networks. This change will require new standards for sharing and transferring critical information. One approach will be Time-Sensitive Networks, a new standard from the IEEE 802 committee.
The Internet of Things (IoT) promises a world of smarter, hyper-connected devices and infrastructure where manufacturing machines, transportation systems and the electrical grid will be outfitted with embedded sensing, processing, control and analysis capabilities. Once networked together, they’ll create a smart system of systems that shares data between devices across the enterprise and in the cloud.
These systems will generate incredible amounts of data, such as the condition monitoring solution for the Victoria Line of the London Underground rail system, which yields 32 TB of data every day. This Big Analog Data will be analyzed and processed to drive informed business decisions that will ultimately improve safety, uptime and operational efficiency.
Though much of this raw, unprocessed data is not time critical and can be passed between network layers and subsystems with little regard for latency or synchronization, an entire class of mission-critical, time-sensitive data must be transferred and shared within strict bounds of latency and reliability. Data such as critical control and fault detection information that must be processed, shared, and acted upon immediately, regardless of other network traffic.
Much of today’s network infrastructure is not equipped to handle such time-sensitive data. Many industrial systems and networks were designed according to the Purdue model for control hierarchy in which multiple, rigid bus layers are created and optimized to meet the requirements for specific tasks. Each layer has varying levels of latency, bandwidth and Quality of Service, making interoperability challenging and flexibly changing data connections virtually impossible. In addition, today’s proprietary Ethernet derivatives have limited bandwidth and require modified hardware.
To support the new capabilities of IoT-enabled infrastructure, designers and end users alike need reliable, remote, and secure access to smart edge devices. Network technologies must evolve to satisfy the requirements of these next-generation industrial systems and radically advance the way we operate our machines, electrical grids, and transportation systems.
Time-sensitive networks: the time is now
Existing IT networks are defined by IEEE 802 standards, which specify requirements for different Ethernet layers and functions and ensure interoperability between devices. Today, industrial suppliers, IT vendors and silicon providers are collaborating within IEEE 802 and the recently formed AVnu Alliance to update standard Ethernet protocols and provide bounded, low-latency data transfer for time-critical data in IoT applications.
This next-generation standard, called Time-Sensitive Networking or TSN, addresses the shortcomings of existing networks. The AVnu Alliance, working with member companies Broadcom, Cisco, Intel and NI, will drive the creation of an interoperable ecosystem through certification; similar to the way the WiFi Alliance certifies products and devices to be compatible with the IEEE 802.11 standard.
The new TSN standard will offer many benefits over today’s standard and specialty Ethernet protocols, including:
Large data sets from advanced sensing applications such as machine vision, 3D scanning, and power analysis can put a strain on network bandwidth. Proprietary Ethernet derivatives commonly used for industrial control today are limited to 100 MB of bandwidth and half-duplex communication. TSN will support full-duplex standard Ethernet with higher bandwidth options such as 1 GB, 10 GB, and in the future even the 400 GB version in work in IEEE 802.3.
Most of the lower-level field buses used today achieve security through air gap and obscurity. They are influenced by the automotive industry, for which air-gapped and closed CAN networks carry all the control and operational data. But recent security breaches have exposed the need to fully extend security into the critical lower levels of control infrastructure. TSN protects critical control traffic and incorporates top-tier IT security provisions. Segmentation, performance protection and temporal “composability” can add multiple levels of defense to the security framework.
By using standard Ethernet components, TSN can integrate seamlessly with existing brownfield applications and standard IT traffic to improve ease of use. In addition, TSN inherits many features of existing Ethernet, such as HTTP interfaces and web services, which enable the remote diagnostics, visualization and repair features common in IoT systems. As an added benefit, leveraging standard Ethernet chip sets drives component cost down by virtue of high-volume, commercial silicon, especially compared with specialty Ethernet variants that are centered on lower-volume, ASIC-based implementations.
Latency and synchronization
TSN prioritizes the low-latency communication required for fast system response and closed-loop control applications. It can achieve deterministic transfer times on the order of tens of microseconds and time synchronization between nodes down to tens of nanoseconds. To ensure reliable delivery of this time-critical traffic, TSN provides automated configurations for high-reliability data paths, where packets are duplicated and merged to provide “lossless compression” path redundancy.
Introducing IEEE 802.1 Time-Sensitive Networking
The TSN standard leverages the existing 802.1 AVB standard to extend the use cases beyond Audio-Video to control systems. To achieve this, the standard provides improvements to the specifications to the network-wide precision clock reference, limiting network delays to specific values rather than best-effort, redundant control and data paths with “instantaneous” switchover, definitions of worst-case for work cell and factory, and coexistence of other bulk traffic. The primary challenge that these specifications must address is guaranteed (worst-case) delivery of packets in the presence of interfering traffic. One can therefore think of TSN as a Quality of Service (QoS) set of specifications for the IoT.
To support a converged network with both time-critical traffic with guaranteed worst-case delivery latency and standard IT traffic, interference between network traffic must be avoided. Three techniques will eliminate interference: time synchronization, scheduling and having time-aware traffic shaping.
The traffic shaping will use the schedule to control logical gating on the switches. The non-time sensitive traffic is blocked during specific time windows to ensure that the egress port is ready when the time-sensitive traffic is expected. The time synchronization to control the schedule and traffic shaping will be handled using IEEE 802.1AS, essentially a profile of the IEEE 1588v2 standard.
The future will arrive on time
The IEEE 802.1 TSN Task Group and the AVnu Alliance are working to finalize the standards. Portions of the standard and working references from vendors will begin to appear in 2016. You can learn more about the details of this work at:
www.ieee802.org and www.avnu.org.
As IoT adoption continues, increased amounts of data and widely distributed networks will require new standards for sharing and transferring critical information. Just as an ambulance or fire engine receives priority among other traffic during an emergency, the TSN standard ensures that critical, time-sensitive data are delivered on time over standard network infrastructure. Welcome to life in the fast lane with the IoT.
Filed Under: Design World articles, Networks • connectivity • fieldbuses
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