Time-sensitive networking

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Time-Sensitive Networking (TSN) describes a series of standards on which the Time-Sensitive Networking Task Group ( IEEE 802.1 ) is working. The TSN Task Group emerged from the renaming of the Audio / Video Bridging Task Group , which existed until November 2012, and continues its work. The renaming resulted from the expansion of the work area of ​​the standardization group. The standards currently being standardized define mechanisms for the transmission of data via Ethernet networks. Most of the projects define extensions to the IEEE 802.1Q bridging standard . These extensions primarily address the transmission with very low transmission latency and high availability. Possible areas of application are convergent networks with real-time audio / video streams and, in particular, real-time control streams that e.g. B. used in automobiles, in modern aircraft or in industrial plants for control.

Key components

The various sub-standards and thus the key components of TSN technology can be divided into three basic categories. Each of the sub-standards from the various categories can also be used individually, but a TSN network can only achieve the highest possible performance in the overall network and by using all mechanisms. These three categories are:

  1. Time synchronization: All participating devices need a common understanding of the time
  2. Scheduling and traffic shaping: All participating devices work according to the same rules when processing and forwarding network packets
  3. Selection of communication paths, reservations and fault tolerance: All participating devices work according to the same rules when selecting and reserving bandwidth and communication paths

The TSN key components in detail

Time synchronization

The name Time-Sensitive Networking says it all: In contrast to standard Ethernet according to IEEE 802.3 and Ethernet bridging according to IEEE 802.1Q , time plays an important role with TSN. In order for a TSN network to function with clocked end-to-end transmission of communication streams with strict real-time requirements and thus fixed, immovable upper time limits, each participant in the network must have its own internal clock and thus a basic understanding of time. Furthermore, the clocks of all participants, both end devices and Ethernet switches , must be synchronized. The synchronization ensures that all participants always follow the same communication cycle and carry out the right actions at the right time, coordinated with one another.

Time synchronization in TSN networks can be implemented using different methods. Theoretically it is possible to equip every switch and every end device with a radio or GPS clock. However, this is cost-intensive and it cannot always be ensured that a radio or GPS signal is available, for example in a network in an automobile, in a factory hall or in a tunnel. For this reason, TSN usually uses the Precision Time Protocol according to IEEE 1588 for the synchronization of the network, which distributes time information via packets over the data network itself. In addition to the general IEEE-1588 specification, the Time-Sensitive Networking Task Group of the IEEE 802.1 has adopted an IEEE-1588 profile as the IEEE 802.1AS-2011 standard. This profile is specifically designed to limit the wide variety of options offered by the IEEE 1588 protocol to a manageable set of capabilities suitable for use in home, automation, and automotive networks.

Scheduling and traffic shaping

The scheduling and traffic shaping allows the coexistence of different traffic classes with different requirements for bandwidth and time loyalty to the same network. Standard bridging according to IEEE 802.1Q uses eight priorities that are strictly ordered. At the protocol level, these priorities are visible in the 802.1Q VLAN tag of an Ethernet frame . Although these priorities allow network traffic to be divided into eight different traffic classes, they do not guarantee a guaranteed maximum end-to-end delay even for the highest priority class. The reason for this are buffer effects in the Ethernet switches. Even an Ethernet frame with the highest priority can be forced to wait in the buffer of a switch for transmission if the switch port is already occupied by another frame that is intended for transmission.

Different time windows for different traffic classes

TSN expands the standard Ethernet communication by predictable transmission characteristics with hard and soft real-time requirements . The eight priorities specified by the Ethernet frame format are retained. Depending on the needs of the application that has to communicate over the network, additional scheduling mechanisms can be defined for each of the eight priorities. A typical application for TSN with real-time requirements is, for example, the communication of a programmable logic controller (PLC) with an industrial robot . For this communication one of the eight available traffic classes can be assigned to the time-aware scheduler according to IEEE 802.1Qbv. This scheduler enables communication on the data network to be divided into fixed, repetitive cycles. Within these cycles, the eight different priorities can be served according to a fixed grid. The basic concept corresponds to a time division multiple access (TDMA). In this way, system-critical communication flows, such as communication between a robot and a controller, can be separated from the rest of the network communication and time guarantees can be met. The fixed allocation of time windows to the Ethernet priorities avoids the conflict between time-critical and non-time-critical Ethernet frames in the buffers of an Ethernet switch, since both types of traffic communicate separately from one another. An example of such a scheduler configuration is shown in Figure 1 below:

Figure 1 - Example: Traffic Schedule according to IEEE 802.1Qbv

In each cycle, data traffic with VLAN priority 3 is processed during the time window. Since the scheduler requires time synchronization according to IEEE 802.1Qbv, all network participants (switches and end devices) know at what point in time which priority can be sent and processed in the network. The remaining priorities are processed within time window 2. Within this time window, the processing of the priorities according to IEEE 801.Q applies again.

The coexistence of the different traffic classes can be further improved by combining further scheduling and traffic shaping mechanisms with the IEEE 802.1Qbv method. The IEEE 802.1Qav traffic shaper specified in the AVB standards can also be assigned VLAN priority 4 in time window 2, for example. The following coexistence of network traffic could thus be realized:

  • Communication with hard real-time conditions in time window 1: data traffic between a controller and an industrial robot
  • Communication with soft real-time conditions in time window 2: Transmission of a data stream from a video camera over the network using IEEE 802.1Qav
  • Communication without time guarantees in time window 2: background transmission of data, collection of status information

The prerequisite is that all devices in the network support all different sub-standards (IEEE 802.1Qbv, IEEE 802.1Qav, ...).

Time windows and protective tapes

Once an Ethernet network interface has started to transmit an Ethernet frame to the medium, this transmission must be completed and completed, including the creation and transmission of the CRC32 check value, which is important for error detection . This means that if a frame is transmitted for too long, a time window with hard real-time conditions is violated. This can be seen in Figure 2 below:

Figure 2 - Example: Frame sent late violates the next time window.

Shortly before the end of the second time window in cycle n, the transmission of a new frame is started. Unfortunately, the transmission of this frame takes so long that the end time is already within time window 1 of cycle n + 1. As a result, time-critical frames, for which this time window was actually intended, are delayed further, and the time guarantees actually given by TSN may no longer be met. The TSN scheduler according to IEEE 802.1Qbv must therefore contain a mechanism that prevents this behavior.

The IEEE 802.1Qbv scheduler must ensure that the Ethernet network interface does not, under any circumstances, send a frame at the precise point in time when changing from one time window to the next. This is achieved by placing a guard band before the start of each time window, in which no Ethernet frame may be sent. The length / duration of this guard band corresponds to the time required to transmit a maximum Ethernet frame. For an Ethernet frame according to IEEE 802.3 with a single VLAN tag according to IEEE 802.1Q, the length, including interframe spacing , corresponds to 1522 bytes + 12 bytes = 1534 bytes. The length or duration of the frame depends on the transmission speed of the Ethernet connection. With 100 Mbit / s Ethernet the following duration results:

In this case, the protective band must therefore have a duration of at least 122.72 µs. This guard band reduces the actually usable bandwidth in the individual time windows. This can be seen in Figure 3 below:

Figure 3 - Example: TSN schedule with guard bands

Caution: For the sake of illustration, a smaller value was taken for the size of the protective band in Figure 3 than is necessary for protection against a frame, as shown in Figure 2. It is also assumed that time window 1, by definition and configuration of the time-aware scheduler, contains data with a higher priority than data in time window 2. Thus, protective bands must be used to prevent frames from time window 2 from violating time window 1.

The protective bands in front of the transitions between the time windows prevent a frame from "protruding" into the next time window. In this way, the fixed upper limits can be adhered to within the time window. However, the protective tapes also have some disadvantages:

  • The time a guard tape consumes cannot be used for data transfers. This reduces the effectively usable bandwidth of the Ethernet connection.
  • A single time window can never sensibly be chosen smaller than the size of the guard band. This has negative effects on the minimum cycle time, especially with slower Ethernet connections.

The time-aware scheduler according to IEEE 802.1Qbv contains the length-aware scheduling mechanism, which can reduce the negative effects of the guard bands. With length-aware scheduling, the scheduler compares the length of an Ethernet frame and the remaining time before changing to the next time window before it begins to send. If the frame to be sent is smaller or if it corresponds to the time until the next change, the scheduler allows sending - despite the guard band - since the following time window is not endangered. However, this assumes that the length of the frame to be sent is known in advance. This is only the case with store-forward switching , so the cut-through switching , which is advantageous for lower end-to-end latency, can not be used in this case. The length-aware scheduling also has no influence on the necessary size of the guard band and thus on the minimum cycle time. Length-aware scheduling cannot therefore alleviate all the disadvantages of the guard band.

Frame pre-emption and minimization of the guard band

In order to reduce the negative effects caused by the guard bands, the IEEE working groups 802.1 and 802.3 have jointly specified the frame pre-emption method, which enables the transmission of a frame to be interrupted and subsequently continued. This requires changes to be made to both the 802.3 Ethernet and 802.1 bridging standards. For this reason, the frame pre-emption technology is described in two separate standards, IEEE 802.1Qbu for the bridging component and IEEE 802.3br for the Ethernet ( MAC ) component.

Figure 4 - Example: Frame Pre-Emption minimizes the guard band

Figure 4 shows the basic operation of the frame pre-emption. The transmission of the Ethernet frame is interrupted shortly before the transition to the next time window and resumed as soon as the priority to which the frame is assigned becomes active again. In the example in Figure 4, this is the case directly in the next time window. The original Ethernet frame is thus transmitted in two parts from one Ethernet interface to the next interface. Like every normal Ethernet frame, both parts are terminated by a CRC32 checksum. However, the last 16 bits of the sum are inverted so that these frames can only be understood and transmitted by devices that support frame pre-emption. A different start of frame delimiter (SFD) is also used for the identification of partial frames .

The support of frame pre-emption must be reported to the neighboring devices by a switch or end device via the LLDP . If a neighboring device receives such a message and supports frame pre-emption itself, the capability is enabled on this Ethernet port. There is no explicit negotiation between the devices, and each individual connection between switches and end devices must be checked individually by the devices.

Frame pre-emption only works on direct connections between Ethernet switches and end devices. A split frame is always reassembled in the directly adjacent device. Unlike the fragmentation of the Internet Protocol (IP), end-to-end fragmentation is not supported.

By being able to interrupt a frame even after the transmission process has started, the guard band can be reduced significantly: The length of the guard band is now dependent on the accuracy with which the frame pre-emption works. IEEE 802.3br defines the best accuracy of the frame pre-emption unit at 64 bytes, since this represents the minimum length of a still valid Ethernet frame. In this case, the protective tape only has to protect against frames with a size of 64 bytes + 63 bytes = 127 bytes, since all larger frames can be interrupted one more time and transmitted as a partial frame.

This minimizes the loss of bandwidth and enables short cycle times even at transmission rates of 100 Mbit / s. Since the frame is interrupted directly in the MAC layer of the Ethernet interface during the transmission process, cut-through switching can also be supported, as the size of the frame does not have to be known in advance. The MAC interface only checks at the intervals determined by the precision of the pre-emption whether the frame has to be interrupted or not.

Selection of communication paths, reservations and fault tolerance

TSN technology, in particular the IEEE 802.1Qbv scheduler, is used in system-critical networks: control networks for automation systems or for communication between different components within the automobile. In these networks, not only is strict adherence to time guarantees indispensable, these networks must also be protected against errors and failures, such as device defects. The TSN Task Group specifies the future standard IEEE 802.1CB. Furthermore, already specified procedures for high availability such as HSR or PRP according to IEC 62439-3 can be used for TSN .

Either path control and reservation according to IEEE 802.1Qca, a manual configuration or manufacturer-specific algorithms in network management systems can be used for the registration of fault-tolerant communication flows through the network .

In the IEEE P802.1Qcc project, the TSN Task Group deals with the specification of management interfaces and concepts for how TSN networks can be managed and configured on a larger scale in the future. In particular, a decentralized and a centralized approach are discussed that cover different use cases, with and without centralized network management. The current status of the discussion, both on the IEEE P802.1Qcc sub-project and on other TSN technology sub-projects, can be tracked via the IEEE 802.1 publicly accessible document archive.

Current status (as of October 19, 2019)

TSN basic standards

TSN consists of many individual standards. Not all of them are absolutely necessary for a TSN network. These are the basic standards:

default title status date pages
IEEE 802.1Q-2018 Bridges and Bridged Networks Approved draft 2.2 May 7, 2018 2000
IEEE 802.1AB-2016 Station and Media Access Control Connectivity Discovery (specifies the Link Layer Discovery Protocol (LLDP)) Ratified and published March 11, 2016 146
IEEE 802.1AS-2011 Timing and Synchronization for Time-Sensitive Applications in Bridged Local Area Networks Ratified and published March 30, 2011

Corrigendum 1: 2013

Corrigendum 2: 2015

292

128

13

IEEE 802.1AX-2014 Link aggregation Ratified and published December 24, 2014

Corrigendum 1: 1-2017

344

113

IEEE 802.1BA-2011 Audio Video Bridging (AVB) systems Ratified and published September 30, 2011

Corrigendum 1: 2016

163

60

IEEE 802.1CB-2017 Frame Replication and Elimination for Reliability (Seamless Redundancy) (FRER) Ratified and published 28th September 2017 102
IEEE 802.1CM-2018 Time-Sensitive Networking for Fronthaul Approved draft May 7, 2018 62

New stand-alone TSN base standard specifications

default title status date
IEC / IEEE 60802 TSN Profile for Industrial Automation Draft 1.0 17th January 2019
IEEE 802.1CS Link-local Registration Protocol Draft 2.1 March 8, 2019
IEEE 802.1DC Quality of Service Provision by Network Systems Draft 0.1 16th January 2019
IEEE 802.1DF TSN Profile for Service Provider Networks Draft 0.0 March 9, 2019
IEEE 802.1DG TSN Profile for Automotive In-Vehicle Ethernet Communications Draft 0.1 April 27, 2019

Amendments added to IEEE 802.1Q-2018

The following previous IEEE 802.1Q-2014 amendments were included in IEEE 802.1Q-2018:

default Amendment title status date In IEEE 802.1Q-2018
IEEE 802.1Qbu-2016 Amendment 26 Frame preemption Ratified and published August 30, 2016
IEEE 802.1Qbv-2015 Amendment 25 Enhancements for Scheduled Traffic Ratified and published March 18, 2016
IEEE 802.1Qca-2015 Amendment 24 Path Control and Reservation (PCR) Ratified and published March 11, 2016 Section 45
IEEE 802.1Qch-2017 Amendment 29 Cyclic Queuing and Forwarding Ratified and published June 28, 2017
IEEE 802.1Qci-2017 Amendment 28 Per-Stream Filtering and Policing Ratified and published 28th September 2017

802.1Q-2018 Amendments

The following IEEE 802.1Q-2018 amendments are currently being specified:

default Amendment title status date
IEEE 802.1Qcp-2018 Amendment 30 Bridges and Bridged Networks Amendment: YANG Data Model Ratified and published September 14, 2018
IEEE 802.1Qcc-2018 Amendment 31 Stream Reservation Protocol (SRP) Enhancements and Performance Improvements Ratified and published October 31, 2018
IEEE 802.1Qcj Automatic Attachment to Provider Backbone Bridging (PBB) services Draft 0.3 September 12, 2018
IEEE 802.1Qcr Bridges and Bridged Networks Amendment: Asynchronous Traffic Shaping Draft 0.5 June 12, 2018
IEEE 802.1Qcy Bridges and Bridged Networks Amendment: VDP Extension to Support NVO3 Draft 1.2
IEEE 802.1Qcw YANG Data Models for Scheduled Traffic, Frame Preemption, and Per-Stream Filtering and Policing Draft 0.1 September 2018
IEEE 802.1Qcx YANG Data Model for Connectivity Fault Management Draft 0.3 6th July 2018

Current basic standards in revision

The following basic standards are currently in the revision process:

default title status date
IEEE 802.1AS-Rev Timing and Synchronization for Time-Sensitive Applications Draft 7.3 2nd August 2018
IEEE 802.1AX-Rev Link aggregation revision Draft 0.3 23 March 2018

802.1AB-2016 Amendments

The following IEEE 802.1AB-2016 amendments are currently being specified:

default Amendment title status date
IEEE 802.1ABcu LLDP YANG Data Model Draft 0.0 15th December 2017

802.1CB-2017 Amendments

The following IEEE 802.1CB-2017 amendments are currently being specified:

default Amendment title status date
IEEE 802.1CBcv FRER YANG Data Model and Management Information Base Module Draft 0.0 May 14, 2018
IEEE 802.1CBdb FRER Extended Stream Identification Functions Draft 0.0 May 14, 2018

AVB standards

Only the following protocols are required for the AVB standard. The credit-based shaper CBS (from FQTSS) + SRP offer a latency of less than 250 μs per bridge .

default Amendment title status date IEEE 802.1Q-2014/2018
IEEE 802.1BA-2011 - Audio Video Bridging (AVB) systems Ratified and published September 30, 2011

Corrigendum 1: 2016

-
IEEE 802.1Qav-2009 Amendment 12 Forwarding and Queuing Enhancements for Time-Sensitive Streams (FQTSS) Ratified and published Jan. 5, 2010 in Q-2014/2018 Section 34
IEEE 802.1Qat-2010 Amendment 14 Stream Reservation Protocol (SRP) Ratified and published Sep 30 2010 in Q-2014 Section 35
IEEE 802.1AS-2011 - Timing and Synchronization Ratified and published March 30, 2011

Corrigendum 1: 2013

Corrigendum 2: 2015

-

AVB can be expanded to include the IEEE 802.1Qcc standard, which brings the following improvements:

  • Supports more streams
  • Configurable SR (stream reservation) classes and streams
  • Better description of the stream characteristics
  • Layer 3 streaming (IP) support
  • Deterministic Stream Reservation Convergence
  • UNI (User Network Interface) for routing and reservations

Other applicable standards in combination with AVB

  • IEEE 1722-2011 - AVTP (Audio Video Transport Protocol)
  • IEEE 1722.1-2013 - AVDECC (Audio Video Discovery Enumeration)

Related projects

Extends the Ethernet standard (IEEE 802.3) .

default title status date
IEEE 802.3br Interspersing Express Traffic Ratified and published October 14, 2016

Use of TSN

TSN has various areas of application. Different profiles are currently being specified that describe how the TSN sub-standards can be used and what the profiles offer. The TSN profiles select features, options, configurations, defaults, protocols and procedures of bridges, end stations and LANs in order to build bridged networks for the given TSN application. Some TSN use cases are described below:

Audio-video bridging (AVB systems)

The IEEE 802.1BA-2011 Audio Video Bridging (AVB) Systems Standard describes how a bridged network can be set up to meet the requirements of audio-video streaming . 802.1BA is suitable for various applications such as: professional audio and video studios, as well as automotive infotainment .

Mobile fronthaul networks

Fronthaul connects cellular cellular equipment networks to the remote controller. The IEEE 802.1CM-2018 Time-Sensitive Networking for Fronthaul Draft Standard describes how a fronthaul bridged network can be implemented for stringent requirements of fronthaul flows.

Industrial automation

IEC and IEEE 802 have founded a joint working group to describe TSN application scenarios in industrial automation technology. The IEC / IEEE 60802 TSN Profile for Industrial Automation is intended to define guidelines for the selection of IEEE 802 standards and features in order to deploy convergent networks for simultaneous support operations technology traffic and other traffic. A flyer will soon give an overview of the potential of TSN for industrial automation, as TSN is an Industry 4.0 enabler technology.

Bus system in the vehicle

TSN should also enable the use of Ethernet networks as bus systems in the vehicle. Further requirements for Ethernet in automobiles are presented in a presentation.

Utility networks

A white paper explains how TSN can be used in utility operational networks.

literature

  • Werner Zimmermann and Ralf Schmidgall: Bus systems in vehicle technology - protocols, standards and software architecture. 5th edition, Springer Vieweg, Wiesbaden 2014, ISBN 978-3-658-02418-5 .
  • Wolfgang Schulte: TSN - Time-Sensitive Networking VDE Verlag 2019, ISBN 978-3-8007-5078-8 .

Web links

Individual evidence

  1. ^ IEEE 802.1 Time-Sensitive Networking Task Group
  2. ↑ Document archive of the IEEE 802.1
  3. ↑ a b Time-Sensitive Networking (TSN) Task Group |. Retrieved May 20, 2019 (American English).
  4. ↑ 802.1Q-2018 Bridges and Bridged Networks - Revision |. Retrieved June 25, 2018 (American English).
  5. ↑ IEEE 802.1AB-2016 - IEEE Standard for Local and metropolitan area networks - Station and Media Access Control Connectivity Discovery. Retrieved June 25, 2018 .
  6. ↑ 802.1AS-2011 - IEEE Standard for Local and Metropolitan Area Networks - Timing and Synchronization for Time-Sensitive Applications in Bridged Local Area Networks - IEEE Standard. Retrieved June 25, 2018 (American English).
  7. ↑ 802.1AX-2014 - Link Aggregation |. Retrieved June 25, 2018 (American English).
  8. ↑ IEEE 802.1BA-2011 - IEEE Standard for Local and metropolitan area networks - Audio Video Bridging (AVB) Systems. Retrieved June 25, 2018 .
  9. ↑ Frame Replication and Elimination for Reliability (Seamless Redundancy)
  10. ^ Time-Sensitive Networking for Fronthaul
  11. ↑ Frame Preemption
  12. ^ Enhancements for Scheduled Traffic
  13. ^ Path Control and Reservation
  14. ^ Cyclic Queuing and Forwarding
  15. ^ Per-Stream Filtering and Policing
  16. ↑ P802.1Qcp - Bridges and Bridged Networks Amendment: YANG Data Model |. Retrieved June 25, 2018 (American English).
  17. ↑ Stream Reservation Protocol (SRP) Enhancements and Performance Improvements
  18. ↑ P802.1Qcj - Automatic Attachment to Provider Backbone Bridging (PBB) services |. Retrieved June 25, 2018 (American English).
  19. ↑ P802.1Qcr - Bridges and Bridged Networks Amendment: Asynchronous Traffic Shaping |. Retrieved June 25, 2018 (American English).
  20. ↑ 802.1Qcy - VDP Extension to Support NVO3 |. Retrieved June 25, 2018 (American English).
  21. ↑ P802.1Qcw - YANG Data Models for Scheduled Traffic, Frame Preemption, and Per-Stream Filtering and Policing |. Retrieved June 25, 2018 (American English).
  22. ↑ P802.1Qcx - YANG Data Model for Connectivity Fault Management |. Retrieved October 3, 2018 (American English).
  23. ↑ Timing and Synchronization: Enhancements and Performance Improvements
  24. ↑ P802.1AX-Rev - Link Aggregation Revision |. Retrieved June 25, 2018 (American English).
  25. ↑ P802.1ABcu - LLDP YANG Data Model |. Retrieved June 25, 2018 (American English).
  26. ↑ IEEE SA - 802.1CBcv - Draft Standard for Local and metropolitan area networks - Frame Replication and Elimination for Reliability Amendment: Information Model, YANG Data Model and Management Information Base Module. Retrieved June 25, 2018 .
  27. ↑ IEEE SA - 802.1CBdb - Draft Standard for Local and metropolitan area networks - Frame Replication and Elimination for Reliability Amendment: Extended Stream Identification Functions. Retrieved June 25, 2018 .
  28. ↑ IEEE 802.1: 802.1BA - Audio Video Bridging (AVB) Systems. Retrieved June 25, 2018 .
  29. ↑ IEEE 802.1: 802.1BA-Cor-1 - Audio Video Bridging (AVB) Systems - Corrigendum 1: Technical and editorial corrections. Retrieved June 25, 2018 .
  30. ↑ IEEE 802.1Qav-2009 - IEEE Standard for Local and metropolitan area networks - Virtual Bridged Local Area Networks Amendment 12: Forwarding and Queuing Enhancements for Time-Sensitive Streams. Retrieved June 25, 2018 .
  31. ↑ IEEE 802.1: 802.1Qat - Stream Reservation Protocol. Retrieved June 25, 2018 .
  32. ↑ IEEE 802.1: 802.1AS - Timing and Synchronization. Retrieved June 25, 2018 .
  33. ↑ P802.1Qcc - Stream Reservation Protocol (SRP) Enhancements and Performance Improvements |. Retrieved June 25, 2018 (American English).
  34. ^ Interspersing Express Traffic Task Force
  35. ↑ IEEE 802 Ethernet Networks for Automotive (PDF)
  36. ↑ Whitepaper How TSN could be used in a utility operational network (Word document)