Fundamentals of Automotive Ethernet

Figure 1: Automotive Ethernet is designed to support
increasingly complex vehicle electronic systems.

When we speak of “Automotive Ethernet”, we’re referring to a group of Ethernet interfaces intended for in-vehicle use, customized to meet the needs of the automotive industry. The first Automotive Ethernet standard was defined by Broadcom in 2011 with BroadR-Reach. Since then, IEEE has released standards for 100Base-T1, 1000Base-T1, 10Base-T1S and most recently MultiGBase-T1. Together, these standards define the general technology known as Automotive Ethernet.

Probably the first question you ask is, “Why not just use standard Ethernet?” A summary of the fundamental features of Automotive Ethernet will show how much better Automotive Ethernet is than standard Ethernet at meeting the industry’s demand for a higher speed, robust, lightweight and lower cost data interface, one that can ultimately replace the many other protocols currently used throughout the vehicle.

T1 Cabling

Standard Ethernet was not designed for a vehicle’s harsh environmental conditions, with temperature range of -40°C to 125°C and the shock and vibration of up-to-4G acceleration. Nor was it designed to meet the automotive industry’s stringent EMI/EMC requirements. The 100-meter reach of standard Ethernet is not needed in a vehicle, and with fuel economy a growing concern, the four twisted pairs in a CAT 5 Ethernet cable are prohibitively weighty and expensive.

Automotive Ethernet systems are all based on the use of a single twisted pair, signified by the T1 in the protocol names, which can be unshielded. It has no defined connector type, so you are not limited to using the RJ45 connector in your design. It’s been estimated (by Broadcom) that T1 systems reduce vehicle cable weight by about 30% and connectivity costs by about 80%.

Full-Duplex Data Communication

Figure 2: Automotive Ethernet most often employs
full duplex communication over a single twisted pair.

Data communication systems may be full duplex or half duplex. A full-duplex system is capable of communicating in both directions simultaneously, while a half-duplex system can only communicate in a single direction at one time. The telephone is a full-duplex system. A handheld “walkie-talkie” transceiver is a half-duplex device.

Standard Ethernet systems may be either half or full duplex (Figure 2), with half-duplex systems characterized by the use of upstream and downstream lanes transmitted over different pairs of wire to form a full link. Automotive Ethernet systems are usually full duplex over the single T1 cable, reducing the amount of cable required—and with it weight and cost—while increasing speed by removing latency.

One challenge to debugging full-duplex systems, however, is separating the bi-directional traffic, which cannot be done using an oscilloscope alone. Teledyne LeCroy’s Automotive Ethernet toolkits, decoders and compliance test packages can do this automatically with the use of the TF-AUTO-ENET breakout fixture and SMA boards.

Point-to-Point Network Topologies

Standard Ethernet systems address packets to different nodes on the network. The use of addressing adds to the complexity of the packet structure, which reduces throughput and adds to the potential for error.

Automotive Ethernet systems most commonly employ a point-to-point topology where two nodes are directly connected by the single T1 cable. One node is designated a “Master,” which will send commands and queries to a “Slave” whose only job is to answer to the Master over the same cable and follow orders.

10Base-T1S systems may optionally utilize Multidrop, a bus-like topology similar to that used by CAN where multiple nodes take turns communicating, although they must still support the point-to-point topology. Multidrop is half duplex (and slower) by nature, since only one device can transmit at a time.

Multi-level Data Encoding

Higher speed data communications are required to support the growing complexity and volume of data used in today’s vehicles. This is especially true with regards to driver assistance and autonomous driving systems. Automotive Ethernet protocols offers data rates several orders of magnitude greater than protocols like CAN, LIN, FlexRay and MOST that have traditionally been used in vehicles, up to 10 Gb/s for MultiGBase-T1 compared to 150 Mb/s for MOST.

Figure 3: DME signal with embedded clock and data.

Differential Manchester encoding is used for the lower speed 10Base-T1S. It transmits an embedded clock along with the data and guarantees very low DC baseline wander due to highs and lows being of equal duration (Figure 3). A low-to-high transition represents a logical 0, while a high-to-low transition represents a logical 1. The embedded clock and data are easy to recover, even in noisy environments like an automotive vehicle.

Figure 4: PAM3 signal with three levels
designated as logical -1, 0 or +1.

Higher speed variants of Automotive Ethernet all use a type of multi-level pulse amplitude modulation (PAM) to encode data. 100Base-T1 and 1000Base-T1 use PAM3 multi-level signaling (Figure 4). At any given time, a PAM3 signal is in one of three states designated as -1, 0, or +1. The levels are detected as the signal crosses voltage thresholds set by the receiver, shown as dashed lines in the figure. Significant combinations of -1, 0 and +1 are known as ternary symbols. PAM3 signaling transfers 1.5 bits per clock cycle and increases the throughput for the same baud rate as bi-level signals. So, for a given clock rate, the symbol rate is proportionally lower. This reduces signal transition times, which reduces EMI, another plus for automotive applications.

MultiGBase-T1 uses PAM4 signaling with four logical levels, which further increases throughput but decreases noise tolerance.

Figure 5 summarizes the salient characteristics of the various Automotive Ethernet technologies. Our next blog post will expand upon the differences between Automotive Ethernet variants and show how each fits into a particular automotive application.

But a distinct benefit of Automotive Ethernet is that, regardless of physical layer differences, at the upper levels of the code stack, all Automotive Ethernet protocols are still Ethernet, a well known protocol with many available developers. Automotive Ethernet code can be more easily and cheaply adapted to new Automotive Ethernet protocols and new applications, with a consistent message structure operating throughout the vehicle, instead of the many translation layers that are now required to link systems programmed for different protocols.

Figure 5: A summary of Automotive Ethernet technologies.

Watch Bob Mart explain the fundamentals of Automotive Ethernet in the on-demand webinar, How to Become an Expert in Automotive Ethernet Testing, Part 1.

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08 June 2021