Analysis of IEEE802.1DG, the automotive Ethernet TSN standard

After 2010, IEEE started to upgrade Ethernet to TSN, a process that may take 20 years. TSN stands for Time-Sensitive Networking, which has a wide range of applications, covering six areas:

1) Ethernet Audio and Video Bridge, EAVB, 802.1BA standard;

2) 5G RF fronthaul FrontHaul, namely 802.1CM/de standard;

3) Industrial automation, namely IEC/IEEE 60802 standard;

4) In-vehicle TSN is the 802.1DG standard;

5) Service Provider, namely 802.1DF standard;

6) Aerospace Onboard is the IEEE P802.1DP/SAE AS6675 standard.

Among them, the last four are unfinished, and a "P" will be added before the standard. For example, the current automotive TSN standard is P802.1DG, which is expected to be completed in 2024. The most closely related to TSN is the automotive Ethernet switch chip. Without exception, any automotive Ethernet switch chip in the world supports TSN and must support it. Ethernet switch chips are also essential chips for smart driving and smart cockpits. Taking the Mercedes-Benz S-Class as an example, each car has at least 5 Ethernet switch chips.

Automotive Ethernet OSI Model

Image source: Marvell

The automotive Ethernet standard is divided into two categories: the physical layer PHY and the link layer. The TSN standard is mainly in the second layer link layer. The automotive Ethernet PHY standard is mainly to develop a single-pair twisted pair standard. The biggest difference between traditional Ethernet and automotive Ethernet is that traditional Ethernet requires 2-4 pairs of wires, while automotive Ethernet only requires one pair, and they are unshielded. This alone can reduce 70%-80% of the connector cost and 30% of the weight. This is the main reason for the birth of automotive Ethernet, and it is also to meet the EMI electromagnetic interference in the car.

Automotive Ethernet PHY Standard Distribution

Image source: Marvell

Automotive Ethernet is ambitious, and 10Base-T1S is intended to replace the traditional CAN network. 10Base-T1S is IEEE 802.3cg, which is OPEN Alliance's TC14, 100Base-T1 is IEEE 802.3bw, which is OPEN Alliance's TC1, 100/1000Base-T1 ECU test standard is OPEN Alliance's TC8, 1000Base-T1 is IEEE 802.3bp, which is OPEN Alliance's TC12, and 2.5/5/10GBase-T1 is IEEE 802.3ch, which is OPEN Alliance's TC15 standard. The 802.3cy standard with bandwidth exceeding 10G is also being developed.

TSN Tools at a Glance

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TSN itself is not a brand new technology. IEEE released the IEEE 1588 precision clock synchronization protocol in 2002. In 2005, IEEE 802.1 established the IEEE 802.1AVB working group and began to develop an audio/video transmission protocol set based on the Ethernet architecture to solve the standards for real-time, low latency and traffic shaping of data in Ethernet, while ensuring compatibility with Ethernet. AVB has attracted the attention of technical organizations and companies in the automotive and industrial fields. IEEE established the TSN working group and developed a series of standard sets for clock synchronization, traffic scheduling, and network configuration. In this process, organizations such as AVnu, IIC, and the OPC UA Foundation have jointly actively promoted the standards of TSN technology.

Companies in the industrial field (including B&R, TTTech, SEW, Schneider, etc.) set out to develop shapers for strict time tasks in the industrial field, established a shaper working group, and held the first shaper working group meeting in Vienna in September 2016. Later, more companies and organizations (including Germany's Industry 4.0 organization LNI, the US Industrial Internet Organization IIC, China's Edge Computing Industry Alliance ECC, Industrial Internet Industry Alliance AII, etc.) joined the research of TSN technology and built multiple test beds. In 2019, IEC and IEEE cooperated to establish the IEC 60802 working group and held the first working group meeting in Japan so that TSN development in the industrial field can achieve underlying interoperability. At the same time, the OPC UA Foundation also established a (Field Level Communication, FLC) working group to integrate TSN technology with OPC UA specifications to provide a high-bandwidth, low-latency, and semantically interoperable industrial communication architecture suitable for smart manufacturing and industrial Internet fields.

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Compared with traditional Ethernet, the biggest feature of TSN is deterministic service.

TSN signal transmission process

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The network has filtering databases, transmission port status monitoring, and queue management. These are used to solve network resource allocation and scheduling problems. The Stream Reservation Protocol (SRP) mechanism adopted by IEEE 802.1Qat is a standard for configuring TSN. SRP allows the talker in the network to send data to the listener with appropriate network resources and propagate this information in the network. The bridge between the terminal nodes maintains a record of a talker's demand for resources such as path bandwidth for the same data stream registered by one or more listeners.

Basic Concepts of TSN

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Let's take a look at TSN through specific products. The most relevant to TSN is the automotive Ethernet switch. Automotive Ethernet switch manufacturers can be divided into two echelons. The first echelon is Marvell and Broadcom, both of which have products that support up to 802.3ch. Broadcom is slightly stronger and has a higher price, while Marvell's price is slightly lower. Broadcom's product line is too long, and Marvell focuses more on the automotive and enterprise network fields, so Marvell has a higher market share in the automotive market. The second echelon includes Microchip, NXP, and Realtek. It only supports up to 802.3cg. NXP provides an overall solution, focuses more on the automotive field, and has a relatively high market share. Realtek focuses on high cost performance, and Volkswagen is its largest customer. These five companies basically monopolize the market.

Currently, the world's most advanced automotive Ethernet switch chip is Broadcom's BCM89586M, which not only supports 802.3ch up to 10G, but also the latest 802.3cg. In terms of TSN, it supports IEEE 802.1AS 2020, IEEE802.1CB, IEEE 802.1Qbv and IEEE 802.1Qci, and is currently the chip with the highest TSN support. IEEE 802.1AS 2020 and IEEE 802.1Qbv are essential elements of TSN and must be supported by any automotive Ethernet switch.

IEEE 802.1AS 2020

All communication issues are based on clocks. Ensuring the accuracy of clock synchronization is the most basic issue. The TSN working group developed a clock based on IEEE1588 and formulated a new standard IEEE802.1AS-Rev. It is used to achieve high-precision clock synchronization. For TSN, the most important thing is not "fastest transmission" and "average delay", but "delay in the worst state" - this is like the "barrel theory", the system's capacity depends on the shortest board, that is, for deterministic networks, the worst delay is the delay definition of the system.

IEEE1588 protocol, also known as PTP (Precise Time Protocol), can achieve sub-microsecond time synchronization accuracy. Version 1 was released in 2002 and version 2 was released in 2008. Its main principle is to periodically calibrate and synchronize the clocks of all nodes in the network through a synchronization signal, so that Ethernet-based distributed systems can achieve precise synchronization. IEEE 1588PTP clock synchronization technology can also be applied to any multicast network.

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The synchronization principle of the IEEE1588 protocol and the proposed Delay Request-Response Mechanism are shown in the figure above. IEEE1588 divides the messages involved into event messages and general messages. The classification is based on whether it is necessary to record accurate timestamps when sending and receiving messages. According to the several messages described above, they can be classified as follows:

Event message (event message) : sync/Delay_Req/Pdelay_Req/Pdelay_Resp.

General message (general message) : Announce/Follow_up/Delay_resp/Pdelay_Resp_Follow_Up/Magnament/Signaling.

Pdelay_Req/Pdelay_Resp/Pdelay_Resp_Follow_Up measures the link delay between two clock ports through the peer delay mechanism. The link delay is used to correct the time information in the Sync and Follow_Up messages.

IEEE802.1AS-Rev is an extension of the 1588 specification defined for Ethernet Layer 2. Its extensions include common peer delay services for all domains, support for fine time measurement FTM, support for link aggregation (802.1AX), improved scope of use - including 1-step timestamp standardization and support for long links and rings, better responsiveness, including faster master station interaction and reduced BMCA convergence time. In addition, IEEE802.1AS-Rev supports synchronous information transmission in multiple domains and redundancy support capabilities (configurable redundant paths and redundant master stations).

IEEE802.1CB

The high reliability of TSN is mainly based on the 802.1CB standard. This is also the main reason why autonomous driving must use TSN. Only TSN can enable the entire system to reach the highest level of functional safety, ASIL D. Similarly, the degree of binding with adaptive AUTOSAR is also relatively high.

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As we all know, L4 autonomous driving requires a redundant processor, but how to establish the communication mechanism between the main processing system and the redundant processing system? This is where 802.1CB comes in. 802.1CB is the redundancy between two systems, and the redundancy between chips still uses the multi-host fail-operational mechanism of PCIe switches. The two are similar.

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For very important data, 802.1CB will send an extra data backup, which will be transmitted along the path farthest from the intersection of the main data path. If both data are received, the redundant frame will be eliminated at the receiving end. If only one frame of data is received, then the backup mode will be entered. ISO/IEC 62439-3 has defined two types of redundancy, PRP and HSR. This type of redundancy is global and has a higher cost. 802.1CB only provides redundancy for key frames, which reduces costs. The formulation of the 802.1CB standard mainly relies on Cisco and Broadcom.