It’s time: Time Sensitive Networking and Deterministic Networking

Story

August 04, 2021

It’s time: Time Sensitive Networking and Deterministic Networking — Figure 1: The Parvus DuraCOR ultra-small-form-factor computer. Curtiss-Wright image.

By Aaron Frank and Mike Southworth

An industry perspective from Curtiss-Wright Defense Solutions

Time Sensitive Networking (TSN) is a set of Ethernet networking capabilities used to transmit time-sensitive data across standard Ethernet networks. Originally, TSN grew out of the IEEE 802.1 standard working groups on Audio Video Bridging (AVB) to make sure that independent networks streams for audio and video data arrive at the correct times to ensure synchronization. AVB also requires guaranteed delivery throughput and latency to avoid problems like missing video frames. Today, TSN features new 802.1 standards and enhancements that go beyond AVB time-synchronized streaming and can support low-latency, precision data delivery over Ethernet for a broader set of industrial, automotive, aerospace, and other real-time communications applications.

Traditional defense and aerospace systems rely on older, proven technologies such as MIL-STD-1553, ARINC 429, and CANBus to support time-critical communications. However, those legacy data buses have not evolved to meet the challenge of ever-faster operating speeds. On the civil aviation side, Ethernet technologies such as ARINC 664 (AFDX) and Time Triggered Ethernet (TTE) have gained some popularity, but these require significant up-front investment and high recurring costs. What’s more, these legacy avionics solutions aren’t flexible once deployed and cannot be dynamically configured. As an open standard technology supported by most major semiconductor suppliers like Intel, TSN is designed to coexist with today’s low-cost existing Ethernet technologies. It works dynamically, enabling time-sensitive communications channels to be configured and set up flexibly as needed in real time.

In the defense and aerospace market, TSN has great potential: Integrators are envisioning Ethernet backbones for future ground vehicles and aircraft that support not only traditional data and communications traffic, but also precision control over Ethernet for weapons systems, autonomous vehicles, and other devices historically controlled via legacy data buses.

As mentioned above, TSN is not a single standard, but a collection of capabilities governed and managed by a number of separate IEEE standards. TSN uses a profiles approach that defines which specific set of features, options, configurations, and protocols are appropriate for a particular set of TSN applications. Some profiles are well defined, while others are still works in progress. For example, work is currently underway by SAE and IEEE on the Aerospace profiles (P802.1DP), which are being defined in SAE AS6675 Working Group as IEEE 802.1DP; these will focus on security, high availability and reliability, maintainability, and bounded latency for deterministic networks that range in Design Assurance Levels (DALs).

TSN technology offers several important capabilities to embedded defense systems. For example, TSN makes use of IEEE-1588v2 Precision Time Protocol (PTP) to provide clock synchronization between nodes. Using standard Ethernet messaging, nodes can synchronize their local time-of-day clocks and also maintain clock sync accuracy. Typically, one node is chosen as the master clock (or the nodes can vote on a best master clock), while the rest of the nodes sync to the master (or grandmaster). Clocks can maintain synchronization with accuracy as low as 10 ns.

Once clocks are synchronized, TSN can be used to set up a deterministic communications channel between nodes. This setup can be defined with guaranteed parameters, such as throughput, latency, jitter, and the like. All equipment in the path from node A to node B that participates in this setup will reserve buffers and bandwidths and configure queue priorities to meet the request. Once set up, the nodes can communicate with the determinism that has been established. If the channel is no longer needed, the reservation can be taken down, returning buffers and bandwidth to other nodes that may need it.

Today, many Curtiss-Wright single-board computer (SBC) modules and mission computer systems (example, Figure 1) can already take advantage, to varying degrees, of TSN communications, requiring only software support. For example, most SBCs support the time synchronization capabilities of IEEE-1588v2 and can support PTP software daemons.

As TSN matures for the defense and aerospace industry, leading commercial off-the-shelf (COTS) vendors will start incorporating TSN capabilities into their products. COTS suppliers are starting to get up to speed on the potential benefits of TSN and investigating how to support it more fully as the next generation of Ethernet switching products takes shape.