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Industrial Ethernet Book 104

Standard Description 802. 1Qbv Time-aware shaping (per-queue based) 802.1ASrev Timing and synchronization (mechanisms for faster fail-over of clock grandmasters) 802.1CB Redundancy (frame replication and elimination 802.1Qca Path control and reservation (based on IEEE802.1aq; IS-IS) 802.1Qbu Frame pre-emption 802.1Qcc Enhancements and improvements for stream reservation 802.1Qch Cyclic queuing and forwarding 802.1Qci Per-stream filtering and policing maintenance, centralized data analytics and machine to machine coordination. In general, IEEE TSN standardization extends the functionality of standard Ethernet to now ensure that: • Message latency is guaranteed through switched networks • Critical and non-critical traffic can be converged in one network without risk of impact on delivery of the critical traffic by collisions with the non-critical traffic • Higher layer protocols can share the network infrastructure with real-time control traffic • Components can be added to real-time control systems without network or equipment alterations • Network faults can be diagnosed and repaired faster because of more precise information on their source Key TSN Standards 802.1Qbv At the core of Time-Sensitive Networking is a time-triggered communication principle. In TSN this concept is known as the “timeaware shaper” (TAS), which deterministically schedules traffic in queues through switched networks. It is being standardized as IEEE 802.1Qbv. With the time-aware shaper concept it is possible to control the flow of queued traffic from a TSN-enabled switch. Ethernet frames are identified and assigned to queues based on the priority field of the VLAN tag. Each queue is defined within a schedule, and the transmission of messages in these queues is then executed at the egress ports during the scheduled time windows. Other queues will typically be blocked from transmission during these time windows, therefore removing the chance of scheduled traffic being impeded by non-scheduled traffic. This means that the delay through each switch is deterministic and that message latency through a network of TSN-enabled components can be guaranteed. The TAS introduces the concept of transmission gates. A gate has two states, ‘open’ and ‘closed’. The transmission selection process, which selects the next message for transmission at the egress, will only select messages from those queues whose gates are in the ‘open’ state. The state of the gates is also defined by the network schedule. Closing gates to non-scheduled traffic queues is another method of providing immunity to time-critical messages to guarantee bounded maximum latency through the network. While the TAS guarantees that critical messages are protected against interference from other network traffic, it does not necessarily result in optimal bandwidth usage or minimal communication latency. Where these factors are important, a pre-emption mechanism can be used. 802.1Qbu IEEE 802.1Qbu works together with IEEE 802.3br (Interspersing Express Traffic Task Force) on a standardized pre-emption mechanism. This standard addresses the fact that the TAS described in IEEE 802.1Qbv avoids transmission jitter by blocking lower priority queues (for the duration of one maximum interfering frame) in advance of the transmission point of the critical frame. In cases where minimal latency for scheduled messages is desired, the TAS mechanism may not be the optimal solution. Therefore on links where pre-emption as defined by IEEE 802.1Qbu is supported, the transmission of standard Ethernet or jumbo frames can be interrupted in order to allow the transmission of high-priority frames, and then resumed afterwards without discarding the previously transmitted piece of the interrupted message. There are use cases where preempting an ongoing transmission is beneficial, e.g. to allow immediate transmission of a scheduled message and ensure minimum communication latency, or to facilitate maximum bandwidth usage on network links with a large amount of scheduled traffic. 802.1ASrev Clock synchronization is a vital mechanism for achieving deterministic communication with bounded message latency in TSN. A robust mechanism for providing global time lays the foundation for the scheduling of traffic queues through each participating network component. The IEEE 802.1ASrev project is working to create a profile of the IEEE 1588 PTP synchronization protocol for TSN. This profile will enable clock synchronization compatibility between different TSN devices. The work is happening in parallel with a project in the IEEE 1588 working group in order to harmonize the two standards, such that IEEE 802.1AS eventually becomes a profile of IEEE 1588.802.1ASrev also addresses support for fault tolerance and multiple active synchronization masters. Fault Tolerance IEEE 802.1ASrev standardizes the use of multiple grandmaster clocks as well as the possibility to make multiple connections to these grandmaster clocks. Replication of grandmaster clocks results in shorter fail-over times in cases when a grandmaster becomes Technology 34 industrial ethernet book 2.2018


Industrial Ethernet Book 104
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