Time-Sensitive Networking


Time-Sensitive Networking is a set of standards under development by the Time-Sensitive Networking task group of the IEEE 802.1 working group. The TSN task group was formed in November 2012 by renaming the existing Audio Video Bridging Task Group and continuing its work. The name changed as a result of the extension of the working area of the standardization group. The standards define mechanisms for the time-sensitive transmission of data over deterministic Ethernet networks.
The majority of projects define extensions to the IEEE 802.1Q Bridges and Bridged Networks, which describes virtual LANs and network switches. These extensions in particular address transmission with very low latency and high availability. Applications include converged networks with real-time audio/video streaming and real-time control streams which are used in automotive applications and industrial control facilities.

Background

Standard information technology network equipment has no concept of "time" and cannot provide synchronization and precision timing. Delivering data reliably is more important than delivering within a specific time, so there are no constraints on delay or synchronization precision. Even if the average hop delay is very low, individual delays can be unacceptably high. Network congestion is handled by throttling and retransmitting dropped packets at the transport layer, but there are no means to prevent congestion at the link layer. Data can be lost when the buffers are too small or the bandwidth is insufficient, but excessive buffering adds to the delay, which is unacceptable when low deterministic delays are required.
The different AVB/TSN standards documents specified by IEEE 802.1 can be grouped into three basic key component categories that are required for a complete real-time communication solution based on switched Ethernet networks with deterministic quality of service for point-to-point connections. Each and every standard specification can be used on its own and is mostly self-sufficient. However, only when used together in a concerted way, can TSN achieve its full potential as a communication system. The three basic components are:
  1. Time synchronization: All devices that are participating in real-time communication need to have a common understanding of time
  2. Scheduling and traffic shaping: All devices that are participating in real-time communication adhere to the same rules in processing and forwarding communication packets
  3. Selection of communication paths, path reservations and fault-tolerance: All devices that are participating in real-time communication adhere to the same rules in selecting communication paths and in reserving bandwidth and time slots, possibly utilizing more than one simultaneous path to achieve fault-tolerance
Applications which need a deterministic network that behaves in a predictable fashion include audio and video, initially defined in Audio Video Bridging ; control networks that accept inputs from sensors, perform control loop processing, and initiate actions; safety-critical networks that implement packet and link redundancy; and mixed media networks that handle data with varying levels of timing sensitivity and priority, such as vehicle networks that support climate control, infotainment, body electronics, and driver assistance. The IEEE AVB/TSN suite serves as the foundation for deterministic networking to satisfy the common requirements of these applications.
AVB/TSN can handle rate-constrained traffic, where each stream has a bandwidth limit defined by minimum inter-frame intervals and maximal frame size, and time-trigger traffic with an exact accurate time to be sent. Low-priority traffic is passed on best-effort base, with no timing and delivery guarantees.

Time Synchronization

In contrast to standard Ethernet according to IEEE 802.3 and Ethernet bridging according to IEEE 802.1Q, time is very important in TSN networks. For real-time communication with hard, non-negotiable time boundaries for end-to-end transmission latencies, all devices in this network need to have a common time reference and therefore, need to synchronize their clocks among each other. This is not only true for the end devices of a communication stream, such as an industrial controller and a manufacturing robot, but also true for network components, such as Ethernet switches. Only through synchronized clocks, it is possible for all network devices to operate in unison and execute the required operation at exactly the required point in time.
Although time synchronization in TSN networks can be achieved with GPS clock, this is costly and there is no guarantee that the endpoint device has access to the radio or satellite signal at all times. Due to these constraints, time in TSN networks is usually distributed from one central time source directly through the network itself using the IEEE 1588 Precision Time Protocol, which utilizes Ethernet frames to distribute time synchronization information. IEEE 802.1AS is a tightly constrained subset of IEEE 1588 with sub-microsecond precision and extensions to support synchronisation over WiFi radio. The idea behind this profile is to narrow the huge list of different IEEE 1588 options down to a manageable few critical options that are applicable to home networks or networks in automotive or industrial automation environments.

IEEE 802.1AS Timing and Synchronization for Time-Sensitive Applications

IEEE 802.1AS-2011 defines the Generalized Precision Time Protocol profile which, like all profiles of IEEE 1588, selects among the options of 1588, but also generalizes the architecture to allow PTP to apply beyond wired Ethernet networks.
To account for data path delays, the gPTP protocol measures the frame residence time within each bridge, and the link latency of each hop. These calculated delays are then referenced to the GrandMaster clock in a bridge elected by the Best Master Clock Algorithm, a clock Spanning Tree Protocol to which all Clock Master and endpoint devices attempt to synchronize. Any device which does not synchronize to timing messages is outside of the timing domain boundaries.
Synchronization accuracy depends on precise measurements of link delay and frame residence time. 802.1AS uses 'logical syntonization', where a ratio between local clock and GM clock frequencies is used to calculate synchronized time, and a ratio between local and GM clock frequencies to calculate propagation delay.
IEEE802.1AS-2020 introduces improved time measurement accuracy and support for multiple time domains for redundancy.

Scheduling and traffic shaping

Scheduling and traffic shaping allows for the coexistence of different traffic classes with different priorities on the same network - each with different requirements to available bandwidth and end-to-end latency.
Traffic shaping refers to the process of distributing frames/packets evenly in time to smooth out the traffic. Without traffic shaping at sources and bridges, the packets will "bunch", i.e. agglomerate into bursts of traffic, overwhelming the buffers in subsequent bridges/switches along the path.
Standard bridging according to IEEE 802.1Q uses a strict priority scheme with eight distinct priorities. On the protocol level, these priorities are visible in the Priority Code Point field in the 802.1Q VLAN tag of a standard Ethernet frame. These priorities already distinguish between more important and less important network traffic, but even with the highest of the eight priorities, no absolute guarantee for an end-to-end delivery time can be given. The reason for this is buffering effects inside the Ethernet switches. If a switch has started the transmission of an Ethernet frame on one of its ports, even the highest priority frame has to wait inside the switch buffer for this transmission to finish. With standard Ethernet switching, this non-determinism cannot be avoided. This is not an issue in environments where applications do not depend on the timely delivery of single Ethernet frames - such as office IT infrastructures. In these environments, file transfers, emails or other business applications have limited time sensitivity themselves and are usually protected by other mechanisms further up the protocol stack, such as the Transmission Control Protocol. In industrial automation and automotive car environments, where closed loop control or safety applications are using the Ethernet network, reliable and timely delivery is of utmost importance. AVB/TSN enhances standard Ethernet communication by adding mechanisms to provide different time slices for different traffic classes and ensure timely delivery with soft and hard real-time requirements of control system applications. The mechanism of utilizing the eight distinct VLAN priorities is retained, to ensure complete backward compatibility to non-TSN Ethernet. To achieve transmission times with guaranteed end-to-end latency, one or several of the eight Ethernet priorities can be individually assigned to already existing methods or new processing methods, such as the IEEE 802.1Qav credit-based traffic shaper, IEEE 802.1Qbv time-aware shaper, or IEEE 802.1Qcr asynchronous shaper.
Time-sensitive traffic has several priority classes. For credit-based shaper 802.1Qav, Stream Reservation Class A is the highest priority with a transmission period of ; Class B has the second-highest priority with a maximum transmission period of. Traffic classes shall not exceed their preconfigured maximum bandwidth. The maximum number of hops is 7. The worst-case latency requirement is defined as for Class A and for Class B, but has been shown to be unreliable. The per-port peer delay provided by gPTP and the network bridge residence delay are added to calculate the accumulated delays and ensure the latency requirement is met. Control traffic has the third-highest priority and includes gPTP and SRP traffic. Time-aware scheduler 802.1Qbv introduces Class CDT for realtime control data from sensors and command streams to actuators, with worst-case latency of 100 μs over 5 hops, and a maximum transmission period of 0.5 ms. Class CDT takes the highest priority over classes A, B, and control traffic.