For industrial and automotive markets real-time communication is a crucial element for future proven systems. Therefore a new set of standards collectively referred to as time-sensitive networking standards (TSN) is created by the TSN task group. Those sub-standards are all part of the IEEE 802.1 working group and aim to solve diverse challenges of modern real-time communication over Ethernet. Companies can customise their setups through various combinations of those standards. In order to understand what can be achieved with TSN we will take a look at each single standard first and further assess several examples for combinations of standards.
The currently planned (and partially completed) standards are:
- IEEE 802.1Qbv: Time-aware shaping (per-queue based)
- IEEE 802.1ASrev: Timing and synchronisation (mechanisms for faster fail-over of clock grandmasters)
- IEEE 802.1Qbu: Frame pre-emption
- IEEE 802.1CB: Redundancy (frame replication and elimination)
- IEEE 802.1Qcc: Enhancements and improvements for stream reservation
- IEEE 802.1Qca: Path control and reservation (based on IEEE802.1aq; IS-IS)
- IEEE 802.1Qch: Cyclic queuing and forwarding
- IEEE 802.1Qci: Per-stream filtering and policing
- IEEE 802.1CM: Time-sensitive networking for fronthaul
The IEEE 802.1Qbv standard is at the core of TSN and defines up to 8 queues per port for forwarding traffic. Frames are assigned to queues based on Quality of Service (QoS) priority. The time-aware shaper (TAS) mechanism blocks all ports except one based on an time schedule in order to prevent delays during scheduled transmission. In other words there is a gate in front of each queue which opens at a specific point of time which is reserved for this queue guaranteeing network paths for time critical traffic over standard Ethernet.
IEEE 802.1ASrev is a prerequisite to enable TAS and lays the foundation for the scheduling of traffic queues through each participating network component. The timing and synchronisation protocol standardises the use of multiple grandmaster clocks as well as the possibility to make multiple connections to these grandmaster clocks. Multiple clock-masters for redundancy enable high availability of TSN networks as in cases when a grandmaster becomes faulty system elements such as end nodes and bridges are still able to remain synchronised by taking the time from the redundant grandmasters.
802.1ASrev is a profile of the IEEE 1588 PTP synchronisation protocol.
IEEE 802.1Qbu solves an issue of 802.1Qbv, which guarantees that critical messages are protected against interference from other network traffic but does not necessarily result in optimal bandwidth usage or minimal communication latency. If these factors matter frame pre-emption is used in order to interrupt the transmission of long frames in favour of high-priority frames. As soon as the high-priority traffic is through the remaining part of the interrupted frame can be transmitted. This procedure allows for optimal bandwidth utilisation of background traffic when used with TAS and enables low-latency communication in non-scheduled networks.
IEEE 802.1CB describes a standard for a redundancy management mechanism (similar to HSR and PRP). Messages are copied and are communicated in parallel over disjoint paths through the network. At the receiver end redundant duplicates are eliminated to create one seamless stream of information. This method ensures high availability for data sent over TSN networks.
With IEEE 802.1Qcc there is also a standard which improves existing reservation protocols such as SRP (Stream Reservation Protocol) in order to meet requirements of professional, industrial, consumer and automotive markets. This includes support for more streams, configurable SR (stream reservation) classes and streams, improved description of stream characteristics, support for Layer 3 streaming, deterministic stream reservation convergence as well as UNI (User Network Interface) for routing and reservations. This method further enables central configuration models for dynamic scheduling of TSN networks and allows standard, consistent setting of TSN schedules in switches from various vendors.
IEEE 802.1Qca is important in order to create flexibility especially if seamless redundancy is used in the network. This standard discovers the network by collecting topology information from nodes in order to find redundant paths through the network and to ensure redundancy in the future. This gives the user greater control over network paths and redundancy.
IEEE 802.1Qch collects packets according to their traffic class and forwards them in one cycle. This cyclic enqueuing and queue draining procedure gives a defined (but not optimal) upper boundary for latency and enables time-controlled communication in conformity to other 802.1 standards. Basically this is a simple way to use TSN if controlled timing is desired but reducing latency isn’t highly important.
The IEEE 802.1 Qci sub-standard provides a possibility to filter frames on ingress ports based on arrival times, rates and bandwidth. This ensures protection against excess bandwidth usage, burst sizes as well as against faulty or malicious endpoints. Further faults are isolated to specific regions in the network to prevent impacts on the system.
IEEE 802.1CM enables the transport of time sensitive fronthaul streams in Ethernet bridged networks. The standard defines profiles that select features, options, configurations, defaults, protocols and procedures of bridges, stations and LANs that are necessary to build networks that are capable of transporting fronthaul streams which are time sensitive.
All of those TSN standards combine to a very powerful toolset enabling greater flexibility and ease of use without sacrificing deterministic performance within a wide range of application areas requiring real-time communication.
For applications which only require guaranteed delivery for the traffic it would be sufficient to limit TSN implementation to the 802.1Qci (ingress policing) standard. If timed messaging and specified latency are desired you would additionally use 802.1Qbv (TAS) and 802.1ASrev (global time sync). To make the system fail-proof you would further use 802.1CB (seamless redundancy). For adaptive network and self-healing features you could extend the use of TSN standards by also implementing 802.1Qca (path control and reservation; go even further by also using 802.1Qcc).
The benefits of TSN are as diverse as the combinations and will impact a huge variety of markets like the renewable energy sector where the technology can help to cut downtime and increase efficiency, the transportation sector where autonomous driving features can be realised at affordable costs and above all factory automation where data accessibility will be improved significantly, gateways can be eliminated and more.
Note: Stay tuned for an in-depth Q&A session with Erich Brockard, EBV Director Application and author of this article, on the matter.
For more information read also “Getting Started With TSN – An Overview” here.