Automotive Ethernet: The Future of Autonomous Vehicles



Understanding how the new Automotive PHYs differ from other BASE-T PHYs matters. Here’s why:

Introduction
From a broad market perspective, Automotive Ethernet is a joint effort of the Automotive and Networking industries to modernize, simplify, and expand the capabilities of vehicles by improving data communications inside vehicles. The key factors that led the Automotive industry to start this effort are the need for higher bandwidth, shifting of architectures to a centralized backbone, and guaranteed latency. Reducing the complexity of today’s vehicle network infrastructure, which commonly has up to eight different networks, is also a key driver. The complexity of handling so many different networks has a huge impact on the cost of today’s vehicles. Not only does the need for specialized skill sets for each networking type add expense, so does the difficulty of managing legacy software/firmware, with resulting incompatibility and inability to reuse parts. Because these improvements required a dedicated physical layer network, the industry widely recognized the strength of Ethernet as an ultra-compatible and flexible network that could handle the requirements of the Automotive industry. In 2014, the IEEE and the Automotive industry began efforts to make Automotive Ethernet a reality.

What Makes it ‘Automotive’?
‘Ethernet’ is one of the most ubiquitous LAN communication technologies in the world, but how an Ethernet network is implemented and physically represented can vary depending on the application. The IEEE 802.3 Ethernet standard has more than 100 clauses, each defining different protocols or physical layers (PHY) designed to cater to the many industries that have embraced Ethernet for the last 40+ years. When asked to describe ‘Ethernet,’ most people probably associate the plug on the back of their desktop PC and the Cat 5e network cable that connects to it. This form of Ethernet is one of several BASE-T PHYs that have been installed in office buildings for the better part of 30 years. These technologies all use the RJ-45 form factor (computer plug), designed to operate on Category cabling up to 100 meters, and can transfer data up to 10Gbps (but most installations are 1Gbps or slower). There are also the high-speed Ethernet implementations used in server farms that can transfer up to 100Gbps but have a reach of less than 10 meters over twinaxial based copper cable and implement different connectors. However, all Ethernet system models share the same media access control (MAC) definition (only the physical layer and transmission medium differ). So, all upper layer functionality is agnostic to the specific application being implemented. Another way to say this is that all Ethernet regardless of the physical medium use the same frame format. This is one of the key benefits of Ethernet.

‘Automotive Ethernet’ typically refers to one of two IEEE PHY definitions. Either IEEE 802.3bw, the 100Mbps PHY, or 802.3bp, the 1Gbps PHY, specified in Clause 96 and Clause 97 respectively. While these PHY types are commonly referred to as 100Mbps and 1Gbps Automotive Ethernet, their official IEEE PHY name is 100BASE-T1 and 1000BASE-T1. ‘BASE-T’ meaning a baseband technology that operates over a copper twisted pair medium, and ‘1’ specifying the number of differential pairs needed within the copper link segment. Both were developed within the IEEE at nearly the same time, and the use case for each is quite similar so they share several design features. These include a 15meter reach, full duplex operation over a point-to-point single unshielded twisted pair (UTP) architecture, and threelevel pulse amplitude modulation (PAM3) line coding

But why not just reuse the existing Ethernet definitions? Each protocol and PHY defined in the IEEE 802.3 standard is developed such that its implementation can be flexible to avoid limiting the technology to a specific application space. That being said, there also needs to be a starting point with use cases proving broad market potential, a distinct identity, and technical feasibility. The Automotive industry determined that existing Ethernet technologies didn’t meet all its needs as a costcompetitive option. First, cars are not 100 meters long, so the traditional BASE-T PHYs that are built into our laptops were over-designed in terms of reach for this specific PHY. Second, the environmentally conscientious climate of the world demands fuelefficient vehicles, and one of the simplest ways to improve fuel economy is to reduce a vehicle’s weight. Much to the IEEE community’s surprise, the third heaviest component in a car is the cable harness (heaviest is the engine, and second heaviest is the chassis). The Cat 5e cable used with BASE-T PHYs have four twisted pairs (eight wires), so one of the requests was to define the Ethernet network over a single twisted pair, leading to a weight (and therefore cost) reduction compared to Cat 5e cabling (Figure 1). Lastly, the environment inside a vehicle is drastically different than the office or home. Not only do the electronics and cabling need to withstand dirt, oil, grease, freezing temperatures, and blistering heat, but there are also electromagnetic interference (EMI) concerns so that the other circuitry in the vehicle isn’t compromised from radio frequency (RF) radiation of the Ethernet network. This is very different than the typical office and data-center environment considered by earlier Ethernet standards.

Figure 1: Example of automotive wiring system inside a car. (Image credit: Chris DiMinico, MC Communications)

Conformance Testing for Automotive Ethernet PHYs
As if the differences between the Automotive market and other Ethernet markets isn’t apparent by now, there’s also literally no room for error. In the office, it’s a slight annoyance if a data packet is received in error; in a self-driving car it could mean the difference between stopping at a traffic light or running a red light into oncoming traffic. With self-driving fully autonomous vehicles all but a reality, Automotive OEMs have countless safety regulations they need to prove can be maintained while a person is not under direct control of the vehicle. So, conformance test specifications have been defined for every aspect of the vehicle’s performance, including the Ethernet components that are installed.

There are three functions in the IEEE BASE-T1 PHY architecture (Figure 2) that have distinct capabilities and encompass all of the mandatory conformance requirements: physical medium attachment (PMA), physical coding sublayer (PCS), and PHY Control (function within the PMA).

Figure 2: 100BASE-T1 PHY Architecture (Image Credit: IEEE 802.3bw-2015 Standard)

Physical Medium Attachment Conformance Testing
The PMA is the PHY sublayer that drives the actual data signal onto the UTP cabling, directly manipulating the transmitted voltage. For this reason, most of the conformance requirements specified in the PMA test specification are related to voltage amplitude, transmit jitter, return loss, etc. The most difficult test to accurately set up is the Transmitter Distortion test. This test metric first appeared in the original Gigabit Ethernet (IEEE 802.3 Clause 40) standard as a time domain approach to quantify the linearity of an Ethernet transmitter. Since the PHY architecture is point-to-point full duplex operation, both 100BASE-T1 nodes that are linked together are transmitting and receiving on the same wires simultaneously. Therefore, it is important that each transmit output amplifier is operating in a linear state and can suppress any non-linear by-products of the two signals being summed on the copper UTP. To measure this, the test method described in Clause 96 defines a sine wave of specific amplitude and frequency to be injected into the transmit path of the device under test (DUT). The DUT is simultaneously transmitting a pseudorandom test pattern. The test pattern and the sine wave are summed and measured on an oscilloscope by probing the transmit path. The actual transmitter distortion value to determine conformance is a product of an IEEE-defined Matlab script that downloads the oscilloscope capture and analyzes the measured waveform.

The test equipment needed for the transmitter distortion test setup (Figure 3) consists of a real-time oscilloscope, differential probe, and waveform generator. All of these are common in test houses and used for most time domain test cases. However, the injected sine wave needs to be synchronized with both the oscilloscope sampling clock and DUT transmit clock to guarantee that the clock domains aren’t misaligned. Not doing so will unintentionally add distortion into the system and result in inaccurate distortion values. There are two commonly used methods to properly frequency lock all the clocks within the test setup: (1) synchronize the test equipment to the DUT’s 66.67 MHz transmit clock (TX_TCLK), or (2) provide the DUT an external reference clock that is generated from the test equipment. In either scenario the tester will need access to the TX_TCLK and the ability to probe it, or the option to provide an external clock source to the silicon. So, this requires the silicon vendor to provide such hardware features to properly characterize a PHYs transmitter distortion. This isn’t always the case. The IEEE standard does not define a test method for this scenario. To resolve this, recently some T&M vendors have implemented clock and data recovery (CDR) functionality into their oscilloscopes, which removes the need to have direct access to the DUT’s TX_TCLK.

Figure 3: 100BASE-T1 PMA Transmitter Distortion Test Setup

Physical Coding Sublayer and PHY Control Conformance Testing
The PCS and PHY Control test specifications are less straightforward. These sublayers are the digital logic within the PHY and are typically governed by transmitter and receiver state machines that define specific state behavior and timing requirements for each operation. Rather than calling for oscilloscope measurement of standardized test patterns transmitted by the PHY, these test cases require that the DUT successfully achieve a link with a test station that behaves as if it is a PHY. Meeting this requirement means that the test station must be able to encode a specific test sequence into the PAM3 signalling. What’s more, the test station must also decode the PAM3 signal the DUT transmits. To achieve these goals, the University of New Hampshire InterOperability Laboratory (UNH-IOL) created an FPGA-based test tool (Figure 4) that performs the necessary conversion to PAM3 signalling as well as bit-level error injection to fully stress the DUT’s receiver logic.

Figure 4: UNH-IOL 100BASE-T1 PCS and PHY Control Conformance Test Tool

Ethernet PHY receiver definitions specify many requirements, but how the functionality is implemented is left to the designer. So, test sequences necessary to test conformance can vary among silicon companies. One of the least consistent PHY parameters is the time necessary to achieve a link. Typically, the number of received idle symbols needed for the DUT to reliably recover the clock of the remote PHY governs the time needed to achieve a link—making test tool flexibility to accommodate any DUT crucial. Additionally, many test cases within these conformance test specifications perform negative test conditions, meaning intentionally injecting errors within the data stream to observe how the PHY behaves. Because of this, some test cases require bit errors, erroneous PAM3 symbols, or Ethernet packets with incorrect CRC values. The test setup used by UNH-IOL (Figure 5) uses a PC with custom software to dynamically control the test sequences used to accommodate any silicon design.

Figure 5: 100BASE-T1 PCS and PHY Control Test Setup

Conclusion
While the IEEE 802.3bw and IEEE 802.3bp PHY definitions may seem similar to other BASE-T PHYs, many environmental considerations and specific use-case data was used to create these unique Ethernet technologies within the IEEE standard. Test specifications were created specifically for these new Automotive PHYs, which require specialized test tools and attention to test setup details not considered in previous Ethernet conformance testing.


Curtis Donahue is the Senior Manager of Ethernet Technologies and manages the Automotive Ethernet Test Group at the University of New Hampshire InterOperability Laboratory (UNH-IOL). His main focus has been the development of test setups for physical layer conformance testing, and their respective test procedures, for High Speed Ethernet and Automotive Ethernet applications.

Donahue holds a Bachelor of Science in Electrical Engineering from the University of New Hampshire (UNH), Durham and is currently pursuing his Masters in Electrical Engineering at UNH.

 

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