This article is part of TechXchange: Time for Time-Sensitive Networking
What you will learn:
- The key elements that make up time-sensitive networking.
- How is TSN used?
Ethernet has established itself as a reliable wired solution for computer and automation networks. The open standard allows terminals to be quickly and easily connected and scaled to exchange data with relatively inexpensive hardware.
However, Ethernet was not originally designed to meet the demands posed by automation technology, especially for big data and real-time communication. Due to these restrictions, various bus systems in automation have evolved using Ethernet at the physical level while implementing proprietary real-time protocols on the high end.
Such systems often lead to exclusive use of network infrastructure and vendor dependencies. Networks currently responsible for handling time-critical data are separated from networks that direct less critical data traffic to eliminate mutual negative interference.
In the future, Industry 4.0 applications will increasingly require more consistent and robust Ethernet networks. These networks can only be produced at significant cost with the current infrastructure. To that end, Time-Sensitive Networking (TSN) offers a solution to change these current conditions and provide increased throughput for many industries.
What is TSN?
TSN is a set of standards being developed by the Time-Sensitive Networking Task Group, part of the IEEE 802.1 working group, an offshoot of the IEEE 802 project created by the IEEE Standards Association. (see picture). TSN is focused on creating convergence between information technology (IT) and industrial operational technology (OT) by extending and adapting existing Ethernet standards.
Although it may seem, TSN is an Ethernet standard and not an Internet Protocol (IPS) standard. However, the extensions address in particular transmission with very low transmission latency and high availability.
Now that we are aware of lineage, standards define mechanisms for the time-sensitive transmission of data over deterministic Ethernet networks. Most projects define extensions to IEEE 802.1Q bridges and bridged networks (aka VLAN support), which describe VLANs and network switches.
TSN technology aims to standardize functionality on OSI-Layer 2 so that different protocols can share the same infrastructure. The challenge here is to configure critical and non-critical data traffic so that neither real-time characteristics nor performance are impaired.
Distribution of TSN
Typically, traditional Ethernet networks involving automated sectors, such as those found in manufacturing, are based on the hierarchical automation pyramid, separating information technology (IT) from operational technology (OT). Computing includes classic office communication with typical end devices, such as computers and network attached storage systems (NAS). The OT end includes systems, machines, and software used for process control and automation.
These domains are fundamentally different in the way they communicate, with IT relying on bandwidth and OT in charge of high availability. Data traffic at the IT level is often classified as non-critical, while data traffic is designated as priority at the OT side. Because of this separation, each level uses a particular communication standard.
While the Ethernet bus system with TCP/IP is widespread at the IT level, various bus systems (or field bus systems) that meet guaranteed latency requirements are widespread at the OT level. Each control vendor usually promotes a specific fieldbus system.
For the end user, this means that controller selection also determines bus selection. As a result, the user often became dependent on the manufacturer since the different bus systems were incompatible. This is no longer the case, as the continuous transmission of data is a fundamental necessity for digitized companies, whatever their sector of activity.
Industrial automation is already going through a restructuring phase based on the implementation of flexible and intelligent manufacturing, typically described or already implemented within the framework of industry 4.0, intelligent production and IoT. This is detailed in the automation pyramid, which integrates TSN and divides the structure in two, with the upper part (strategic direction, plant management, supervision) dedicated to IT and the lower part (control, terrain, etc.) implemented with OT.
The separation of control and field levels is blurring, creating the need for a uniform, converged network where critical data traffic can be transmitted concurrently with non-critical data without adverse effects. Thus, the existing Ethernet must be adapted to meet these requirements. This is where the TNS working group comes in, as sub-standards to enable the convergence of critical and non-critical data traffic over a shared Ethernet infrastructure are being defined.
time is the key
All network equipment needs the same understanding of time to meet this aforementioned convergence, which means that all switches and terminals in a network must be synchronized. To accomplish this task, two different approaches are currently in play.
One is the IEEE 1588-2008 standard, which calls for providing the clock with the most accurate time to designate to act as a grandmaster clock or centralized time element. The working group has also created a unique profile that describes the use of the IEEE 1588 specifications in conjunction with IEEE 802.1Q, especially applications that do not require full throughput.
Another important feature is the transmission of critical and non-critical data traffic within a converged network. Critical data traffic is given priority for delivery at a scheduled time, while non-critical data traffic is designated as a lower priority. Eight traffic classes are already established according to IEEE 802.1Q, and they are used to prioritize different types of data traffic.
That said, the standard’s QoS (Quality of Service) was not designed to send critical and non-critical data traffic in parallel. Due to the buffering mechanisms of Ethernet switches, a low priority Ethernet data packet can delay data streams even if they have a high priority. New prioritization mechanisms have been introduced to allow and regulate this coexistence.
Enter IEEE 802.1Qav, which is designed so that data streams with real-time requirements are prioritized over best effort traffic. A great example of this prioritization includes the Credit-Based Shaper (CBS), which was created by the IEEE 802.1 working group to manage the priority transmission of TSN Audio/Video Bridging (AVB) technology.
The shaper assigns send credits to data streams. Data packets with reserved bandwidth are preferably transmitted as long as the credit remains in the positive range. These credits are spent during transmission until they become negative. Therefore, once a preferred transmission reaches a negative value, the next best-effort data packets in line are transmitted. If this delays the transfer of designated data packets with reserved bandwidth, the credit is increased to allow priority Ethernet frames to be transmitted successively.
Due to the wide range of TSN functions, integration is best handled by programmable microchips, such as field-programmable gate arrays (FPGAs). Compared to traditional integrated circuits, where the functions are predetermined, FPGAs can be programmed and configured to generate complex digital functions based on the application.
The standardization process is not complete, and the implementation of various standards is an ongoing process, as new technologies are continuously introduced to the market every year. Since TSN standards are currently still being reviewed and modified, the ability to expand and reprogram is a critical factor in implementation and deployment in an industrial environment.
Nevertheless, TSN provides the foundation to meet these changing requirements and the spectrum helps meet ever-changing latency, jitter and reliability requirements to meet the latest application requirements.
Read more articles in the TechXchange: Time for Time-Sensitive Networking