The automotive industry is constantly evolving, and advancements in technology have revolutionized the way vehicles operate. One of the key innovations in this regard is the use of automotive Ethernet, a high-speed communication network that enables various components within a vehicle to exchange data seamlessly. In this article, we will delve into the physical layer of the automotive Ethernet bus, exploring its functionalities and the standards that govern its operation. Then we will touch up on the higher layer protocols running on the same.
The need for a robust and reliable communication system in vehicles led to the development of the automotive Ethernet standard. While the conventional CAN/LIN buses were initially sufficient, OEMs recognized the need for faster communication especially for multimedia transport. Broadcom invented the Broad-R Reach protocol which is a modification of conventional Ethernet to operate over a single unshielded-twisted wire also called the 100 BASE-T1 and found wide applications in infotainment systems. Later it was standardized by OPEN Alliance Special Interest Group. The IEEE Standardized it as IEEE 802.3bw. With this, the scope of automotive Ethernet has expanded significantly, and it plays a crucial role in enabling various vehicle functions, such as sensor data fusion, real-time control, and high-speed connectivity.
IEEE 802.3bw, commonly referred to as 100Base-T1, is the physical Layer of Automotive Ethernet. It specifies the method of encoding, signaling and physical media for automotive Ethernet communication. The 100Base-T1 standard supports data rates of up to 100 Mbps over a single pair twisted-pair cable, making it suitable for a wide range of automotive applications. Unlike the conventional Ethernet which has two pairs of unidirectional lines, the single line is full duplex in Automotive Ethernet.
The physical encoding of automotive Ethernet is also defined. The 100 Base -T1 employs a combination of PAM3, 4B/3B and 3B2T encoding scheme. The data coming as 4Bits are grouped to a 3B data. This is then encoded to 2 Ternery Symbols and finally sent as Pulse-Amplitude Modulation (PAM) with 3 different voltage levels. The 100Mbps line operates at a signal rate of 66.67Mb/s. The later version of the 1000Base-T1, also called, IEEE802.3bp follows an 80B/81B modulation with PAM3 encoding at 750 Mb/s. Physical Layer of Automotive Ethernet supporting 2.5G bit/sec, 5G bit/sec and 10G bit/sec Gigabit speeds are also available today.
With just the change in the physical layer, it is possible to run all the typical applications on top of the Automotive Ethernet powered by the TCP/IP stack. While many applications can run in this model, inherently TCP is quite a slow protocol, that cannot meet the stringent timing requirements of many real time applications. To address this problem, various alternatives are proposed. OF them two are the most popular - Audio Video Bridge (AVB) its evolution - Ethernet Time-Sensitive Networking (TSN) and Time-Triggered Ethernet (TTEthernet). These protocols operated at Layer 2 and above to prioritize time-sensitive data streams, ensuring low latency and deterministic communication.
AVB - Audio Video Bridging refers to the broad set of standards that allows transmission of real time high-bandwidth media streams. It offers the capabilities to reserve bandwidth, prioritize traffic and perform time-synchronization across the end points in the network. Some of the standards under includes Generalized Precisions Time Protocol (gPTP; IEEE 802.1AS-2011) for time-synchronization, IEEE 802.1Q-2014 for traffic prioritization and IEEE 802.1Qat for Stream Reservation Protocol (SRP). It also employs a credit-based scheduler that allows data with higher priority over the lower one. IEEE then renamed the Audio Video Bridging to Time-Sensitive Networking and extends it to add two new capabilities - Time-scheduled traffic and Frame-pre-emption.
With these features, TSN enables the nodes – talkers and listeners – to achieve high throughput and low latency. While these AVB and TSN Ethernet protocols run directly on top of the Ethernet and applications can leverage them directly, it is possible for TCP/IP to co-exist in the same system.
Time-Triggered Ethernet (TTE) is another Layer 2 technology that finds applications in automotive Ethernet. It is defined by the SAE International and standardized as SAE AS6802. It provides per packet traffic management features including time-triggered traffic scheduling, per-flow policing of packet timing and traffic partitioning. Three traffic classes - Synchronization Traffic, Time-triggered traffic and Time-triggered traffic and defined and the rest of the un-guaranteed bandwidth are available for other regular protocols.
Both TSN and TTE offers reduced latency and high bandwidth, in general the TSN finds wide application in multimedia, infotainment and other soft real time uses while the Time-Triggered Ethernet is being used in hard real-time applications such as chassis, powertrain etc.
The network architecture for automotive Ethernet is a critical aspect that determines the scalability, reliability, and performance of the communication system. A well-designed network architecture ensures efficient data flow, minimizes latency, and provides redundancy for fault tolerance.
One typical approach to designing the automotive network architecture is the use of a heterogeneous networking topology where data flows across multiple buses. The Ethernet is organized as a hierarchical network with partitioning done based on the application use cases. Also, legacy options such as VLAN (Virtual LAN) IDs are used to further partition the traffic only to the relevant zones.
By understanding the evolution of the automotive Ethernet, the Physical Layer of Automotive Ethernet, and the various technologies that enhance its capabilities, automotive engineers can make informed decisions when designing their communication systems.
The future of automotive Ethernet looks very promising as it is poised to become the preferred choice for the vehicle bus. On the technology front also, it is evolving with speed increasing beyond 10 Gbps and standards like IEEE 802.3cg standard (known as 10BASE-T1S or 10SPE (10 Mbps single pair Ethernet)) that utilizes physical layer collision avoidance (PLCA) feature, is being explored to achieve a deterministic bandwidth.
Deploying an automotive Ethernet is more challenging as it highly involves network specific planning and configuration and robust testing. But there is little doubt that with Time-Triggered Ethernet, AVB and TSN Ethernet, it is going to be the next CAN in vehicle communication.
Electrical/electronic architecture, also known as EE architecture, is the intricate system that manages the flow of electrical and electronic signals within a vehicle.