Automotive Ethernet may become the key to realize ADAS and autonomous driving

Ethernet technology is low-cost, lightweight, has high data transmission rates, and is not proprietary, making it the key to enabling advanced driver assistance systems (ADAS) and autonomous driving. In order to achieve the safety and precise latency requirements required to control vehicles, the industry is currently developing a variety of open standards that help improve the reliability, timing, redundancy, and fault detection capabilities of Ethernet-based applications, so that this technology can be used in all functions of the vehicle.

The common goal of the automotive industry and Ethernet is to use its advanced network architecture to run increasingly sophisticated applications, from infotainment to advanced driver assistance systems and even control of mission-critical systems. Currently, Ethernet is used for on-board diagnostics (OBD), firmware updates, and image transmission from surround cameras. The next stage is to use Ethernet to connect sensors to embedded central processing units (CPUs) to achieve sensor fusion required for advanced driver assistance system applications such as active cruise control, lane keeping assist, traffic sign recognition, pedestrian detection, and collision avoidance.

In the future, Ethernet networks will be used for direct control of the powertrain and chassis, providing mission-critical functions such as braking, steering, transmission, and engine control, and will eventually bring together all the functions required for autonomous driving.

Improve communication protocol requirements to increase reliability and reduce latency

Increased functionality means that connectivity between vehicle components needs to be enhanced, especially in terms of bandwidth, safety and reliability, and latency. As the number of processors, cameras, and sensing components such as radars continues to increase, not only will more bandwidth be required, but also more emphasis will be placed on reliable transmission of time-sensitive data. The reliability of transmission depends on precise and short delays, especially in mission-critical control. For example, if packet loss occurs in applications such as collision avoidance, the brakes may not be applied in time, causing irreparable disasters. Similarly, if the system bandwidth cannot accommodate the aggregation of data from multiple cameras and sensors, autonomous driving will not be possible.

From the Ethernet perspective, bandwidth is mainly applied to the physical layer (PHY Layer), while reliability and latency are mainly applied to the media access control (MAC) layer.

TSN standards are being developed to meet security and low latency requirements

The open standards currently under development are aimed at meeting the safety, reliability and latency requirements of future advanced driver assistance system functions and autonomous systems, ensuring reliable and timely interoperation between all automotive components. In order to achieve the safety and low latency requirements required for controlling vehicles, IEEE 802.1, the statutory standard setting unit for Ethernet switching and data transmission control, is currently developing a new set of open standards, collectively known as Time Sensitive Networking (TSN) (Table 1), to achieve low-latency, accurate and reliable synchronous packet transmission.

Table 1 Main TSN standards

TSN is a superset of the Audio Video Bridging (AVB) standard, which describes how to guarantee bandwidth and perform data synchronization on Ethernet networks to ensure quality of service (QoS). The accuracy of its specifications is within one microsecond (μs), and the required implementation exceeds the requirements of many mission-critical functions. The main TSN standards are described as follows:

IEEE 802.1AS-Rev: Timing and Synchronization for Time-Sensitive Applications

The IEEE 802.1AS-Rev standard introduces redundancy capabilities in the master clock and fault detection to improve the reliability of real-time clock settings in electronic control units (ECUs) and meet ISO 26262 functional safety requirements.

IEEE 802.1Qbv: Scheduled Traffic Enhancement

The IEEE 802.1Qbv standard adds a time-aware queue emptying procedure based on the timing taken from the IEEE 802.1AS standard, while supporting scheduled traffic, credit-based scheduling traffic, and other bridged traffic.

This standard adds transmission gates to eight priority queues, which can close low-priority queues at specific points in time to allow high-priority queues to use the network immediately, thereby protecting access to high-priority, low-latency control frames. This is similar to what the Society of Automotive Engineers (SAE) called time-triggered Ethernet in 2011-AS6802, which can achieve periodic audio and video (AV) traffic transmission such as IEEE 1722 voiced Class A streams that transmit a frame every 125 microseconds.

IEEE 802.1Qbv introduces the concept of guard band, which is to prevent traffic from starting transmission during a specific period to ensure that control frames can be sent at the scheduled time. To support scheduled traffic, time-aware shaping must be used to reduce communication delay and jitter. The method is to insert time-aware high-priority Class A data TA1-TA3 between two O1 frame segments to shorten the delay. During the guard band period, the transmission of non-time-sensitive data will be blocked to prevent the new time-aware (high priority) data packets from being delayed by other data O. The transmitted data has both priority and deterministic properties, which is the key to the control system.

IEEE 802.1Qbu/IEEE 802.3br: Considering frame priority

Prioritization as defined by IEEE 802.1Qbu and IEEE 802.3br avoids excessive guard bands and allows for the fragmentation of frames after they have begun transmission. In other words, a high priority frame can interrupt the transmission of a low priority frame. The IEEE 802.3br standard defines the required frame fragment data encapsulation and error detection value, with a minimum of 64 bytes. The priority and control of the transmit queues is related to the traffic transmitted on the output port. Figure 2 illustrates the implementation of IEEE 802.1Qbu/IEEE 802.3br frame preemption for TA1, TA2, and TA3.

Figure 2 Timing table sample

IEEE 802.1Qci: Stream-by-stream filtering and management

The IEEE 802.1Qci standard for inbound receive traffic is still in its early stages and is intended to mitigate the effects of erroneous operating nodes. It defines management and filtering functions, including detecting and mitigating disruptive transmissions caused by other systems in the network, to improve the robustness of the network.

IEEE 802.1CB: Frame duplication and elimination to improve reliability

The IEEE 802.1CB standard, also in its early stages, introduces seamless redundancy and fault detection to improve functional safety and frame duplication and elimination. For example, network nodes can be connected in a ring to perform bidirectional traffic transmission, which requires two different paths to the destination. The IEEE 802.1CB standard specifies how to eliminate duplicate frames at the receiving node. To ensure functional safety, high availability, redundant fault-tolerant design and fast fault detection capabilities are required. Ring topology is a way to introduce redundancy in the network. IEEE 802.1CB and IEEE 802.1AS-Rev standards introduce fault detection methods.

MAC merging sublayer module supports multiple TSN standards

Many manufacturers have been committed to providing AVB and TSN Ethernet MAC products for automotive applications for many years. Among them, Cadence Ethernet MAC not only supports transmission queues, but also recently added support for IEEE 802.1AS-Rev and IEEE 802.1Qbv standards, using gate opening timers and gate closing timers in each transmission queue.

The company has implemented a MAC Merge Layer (MMSL) module with two Ethernet MAC options, one is Prioritized MAC (pMAC) and the other is Fast MAC (eMAC) (Figure 3). The eMAC is only used to support a single transmit queue. When priority is disabled, MMSL will mediate between eMAC and pMAC on a frame-by-frame basis. The eMAC still has the highest priority, but the frames sent from the pMAC are sent unmodified.

Figure 3 MAC merging sublayer

In addition, the company also provides solutions that support the IEEE 802.1Qbu priority hardware requirements, and will provide corresponding support once the IEEE 802.1CB and IEEE 802.1Qci standards are more clearly defined.

Emerging TSN standards have good reliability and accelerate the deployment of automotive Ethernet networks

Emerging TSN standards, such as the new generation AVB transport protocol, provide features that fully meet ISO 26262 requirements and extend the deployment of automotive Ethernet to safety-critical systems. The TSN standard aims to improve the robustness, reliability, redundancy, and fault detection capabilities of Ethernet networks to facilitate the use of Ethernet networks in real-time control and safety-critical applications.