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Industrial Ethernet Book Issue 98 / 12
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Time Sensitive Networking drives diverse network traffic

Time Sensitive Networking (TSN) will bring future standardized and universally interoperable Ethernet networks that can provide calculable, guaranteed end-to-end latencies, limited latency fluctuations and extremely low packet loss. TSN as the future of industrial networking is closer than many may realize.

PICTURE YOURSELF ON A FACTORY FLOOR and all around you machinery, conveyor systems and workpieces are constantly communicating with each other to enhance automation flexibility and efficiency. Intelligent industrial production scenarios like this may sound futuristic but the first step toward connected factories is rapidly becoming the norm.

In what is commonly known as the industrial Internet of Things (IIoT), each application scenario relies on the ability of the supporting communication network to send information to a destination and receive a response in a reliable and pre-calculable timeframe.

As organizations adopt connected devices and rely on intelligent factory scenarios, the potential for missed connections, from physical collisions between moving parts on the factory floor, and network transmission delays will increase. In a motion control scenario, even a millisecond delay in data signaling between devices could drastically damage a production line and cost the business millions of dollars.

The Time-Aware Scheduler implements time-based prioritization via tTime-Aware Gates that sit between the CoS queues and the selection of the packets to be sent.

Abundant data streams

Speed, real-time communication and true determinism are critical for the success of industrial applications today. With the rise of the IIoT and the influx of data, traffic and bandwidth issues becoming more pronounced.

Real-time requirements have always been present in automation networks, but with large numbers of field sensors, available bandwidth and coexistence of different traffic types becomes an issue in the network upstream towards the factory backbone. Standard Ethernet is not able to provide real-time guarantees when time-critical and background traffic share network infrastructure.

Currently in development at IEEE, time sensitive networking (TSN) is a technology designed to improve the reliability of Ethernet networks, both through fault-tolerance mechanisms and methods for co-existence of time-critical and background traffic.

TSN offers an entirely new level of determinism for standard IEEE 802.1 and 802.3 Ethernet networks. Automation network designers working in IIoT applications can achieve easier process scalability and better real-time information and the capability to transmit and fully utilize the bandwidth of their Ethernet network, without the fear of breaking time-critical communication.

Good time synchronization is a prerequisite for the TSN Time-Aware Scheduler.

The ins and outs of TSN

TSN consists of a series of standards and mechanisms that serve the various requirements of deterministic data transmission. A standard form of configuration is required to implement these different mechanisms jointly in a network and over various network devices.

IEEE has presented three different models of configuration for TSN, which are currently in the standardization process. Each has nuances in the way requirements to the network infrastructure are conveyed and processed:

Centralized model: Talkers and listeners communicate over a direct end-to-end connection with a logical, central configuration instance. The Centralized Network Configuration (CNC) calculates the time slot for a new data stream based on the information that is present on the network topology and the already assigned source reservation and then configures the involved network participants accordingly.

Decentralized model: In direct contrast to the centralized approach, this model distributes end device requirements in the network. The common configuration of TSN mechanisms is based on the local information present in each device. The end device presents its requirements to the first network device (switch), from where the information is distributed to the rest of the network.

Hybrid approach: Blending the centralized and decentralized approaches, this model retains the concept of the end devices presenting the requirements to the first Ethernet switch. The actual TSN configuration takes place in a centralized manner, as the first switch forwards the requirements to the CNC. The advantage with this method is that the end devices only need to support one single configuration protocol, but the network can be managed as centralized or decentralized. Common to all three models is an automated configuration that ensures the handling of the TSN network remains manageable.

The guard band in TSN prevents Best Effort frames from extending into a time slot that is reserved for real-time data, but it decreases the available bandwidth.

TSN configuration process

To configure the network, TSN mechanisms are identified and activated as necessary. The sending device, or "talker," announces information about the data stream it wants to transmit. This information includes identified characteristics, such as the stream multicast media access control (MAC) address and Class of Service priorities. An end device, known as the "listener" that is interested in a data stream, registers and receives the data packets associated with the data stream and with the aid of the announced information. Which TSN mechanisms need to be configured depends on the requirements of the stream to be transmitted and the capabilities of all Ethernet switches on the transmission path.

TSN adds a level of determinism to data communication that is able to meet even the highest demands of control networks by providing a variety of components:

Time-aware scheduler: The time-aware scheduler (TAS), according to IEEE 802.1Qbv introduces the possibility for scheduling the data transmission of Ethernet frames based on Class of Service (CoS) priority and transmission time. It guarantees data forwarding and delivery at a defined point in time.

The TAS divides time into discreet segments of equal length, called cycles, which allows dedicated time slots for the transmission of data packets with real-time requirements.

Best effort Ethernet traffic: With the aid of the TAS, the transmission of conventional best effort Ethernet traffic can be temporarily interrupted for forwarding time-sensitive data traffic within the reserved time slots for high-priority traffic. The TAS permits the prioritization of periodic real-time data in relation to conventional best effort data traffic in such a way, that only the real-time packets get access to the network media within the time slot for high priority.

The TAS uses the Class of Service priorities present in the virtual local area network tag of the Ethernet header to distinguish between high-priority and background traffic.

Gate control list: The gate control list determines which traffic queue is permitted to transmit at a specific point in time within the cycle. This component indicates the length of time for which an entry will be active and is an integral part of the TAS, and can be configured on each port of each network device.

Implicit guard bands: Introduced in conjunction with TAS, guard bands suppress transmission of packets for the duration of a maximum-size Ethernet frame before a transmission gate closes and prevents transmission of best effort Ethernet frames that would intrude into subsequent time slots and potentially violate real-time guarantees. Using Store-Forward switching, the length of the packet that is ready for transmission can be taken into account when deciding whether to start sending before a gate close or not.

Precision time protocol: Time synchronization is mandatory for TSN networks. Without a understanding of time on all network devices, scheduling mechanisms such as the TAS cannot function, as time slots need to be synchronized. TSN can use any method for time synchronization, but the Precision Time Protocol, according to IEEE 1588, is a recommended solution to distribute time across an automation network.

Traffic shapers: Traffic shapers are prioritization mechanisms that permit the reservation of the maximum bandwidth necessary for time-sensitive data transmission within a defined observation time interval. The data traffic to be conveyed is subsequently transformed by the respective traffic shaper into a type and form that guarantees certain latency limits are achieved. TSN and its predecessor technology, Audio and Video Bridging (AVB), currently describe three shapers that are either fully specified or currently undergoing standardization:

IEEE 802.1Qav: The Credit-based shaper from AVB ensures the provisioning of the maximum required bandwidth for an audio/video transmission over an observation interval, without a noticeable interruption of the best effort data traffic that is simultaneously transmitted. However, no precise timing guarantees can be given.

IEEE P802.1Qch: The Cyclic queuing and forwarding shaper exhibits significantly reduced requirements concerning the time-precision of the transmission. Its role is to collect data frames with reserved bandwidth received within a cycle and transmit as "prioritized" at the start of the next cycle. It′s well-suited for cyclic data transmissions that occur in process automation.

IEEE P802.1Qcr : The Asynchronous traffic shaper doesn′t require a time synchronization mechanism and is best suited for the prioritized transmission of data packets that are needed for the time synchronization itself.

Other new mechanisms in development, such as IEEE P802.1Qci, allows for discarding data frames that have been assigned to the wrong time slot. It also permits policing and discarding of real-time data streams that use more than their reserved bandwidth.

TSN future IIoT applications

As it develops, TSN will bring future standardized and universally interoperable Ethernet networks that can provide calculable, guaranteed end-to-end latencies, limited latency fluctuations and extremely low packet loss. Some TSN standards are in the standardization process, and TAS is already finished. Due to the modular approach by IEEE 802.1, networks that are installed and implemented with TSN mechanisms that are available today can be used in the future. It′s important for network designers working in IIoT applications to prepare now by understanding what TSN is and learning how to implement such a network, so they are ready for the future of industrial networking, which is closer than many may realize.

Dr. Oliver Kleineberg, Advance Development, Hirschmann Automation and Control GmbH.

Source: Industrial Ethernet Book Issue 98 / 12
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