Time-Sensitive Networking (TSN) 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 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 (QoS) 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:
Time synchronization: All devices that are participating in real-time communication need to have a common understanding of time Scheduling and traffic shaping: All devices that are participating in real-time communication adhere to the same rules in processing and forwarding communication packets 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 (AVB); 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 a best-effort basis, 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 is it 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 (IEEE 802.11). 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 (gPTP) 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 (the time required for receiving, processing, queuing and transmission of timing information from the ingress to egress ports), and the link latency of each hop (a propagation delay between two adjacent bridges). These calculated delays are then referenced to the GrandMaster (GM) clock in a bridge elected by the Best Master Clock Algorithm, a clock Spanning Tree Protocol to which all Clock Master (CM) and endpoint devices attempt to synchronize. Any device that does not synchronize to timing messages is outside of the timing domain boundaries (Figure 2).
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