What is the ideal Ethernet choice for automotive applications?

Ethernet is gaining popularity in several automotive applications. It can handle high data rates with lower latency and lesser weight than controller area network (CAN) and local interconnect network (LIN) buses. However, conventional Ethernet developed for information technology (IT) infrastructure applications is too sensitive to interference and too noisy for most automotive applications.

This article will:

Compare conventional IT Ethernet with the specific Ethernet standards for automotive applications
Review automotive Ethernet implementation options and technological applications
Presents the BroadR-Reach protocol and the use of eye diagrams for testing automotive Ethernet
Consider the future of multi-Gigabit automotive Ethernet standards

Automotive Ethernet is a physical layer standard built on the foundation of IEEE 802.3. The basic automotive Ethernet standards are 802.3bw (100BASE-T1) and 802.3bp (1000BASE-T1), which operate differently than the IT-version Ethernet (100BASE-TX) (Table 1).

The 15-meter maximum length of automotive Ethernet cables is one standard difference compared to the 100 meters for IT Ethernet. More recently, the IEEE added 802.3ch, which provides for multi-Gigabit Ethernet at standard rates of 2.5G bit/sec, 5G bit/sec, and 10G bit/sec over the same 15-meter cable length.

**Table 1:** Like conventional Ethernet, automotive Ethernet is built on the IEEE 802.3 standard, but uses different sub-standards. (Table: Keysight)

Cables
The cables are a fundamental difference between IT and automotive implementations of Ethernet. Weight is a concern in automotive applications (the lighter, the better). The single, two-wire unshielded twisted pair (UTP) cable is lightweight and cost-effective compared to an IT Ethernet cable.

The 100BASE-TX IT Ethernet runs over two-wire pairs inside a cable that’s a Category 5 or higher. In an automotive Ethernet cable, the UTP cable is used to transmit and receive activities at the same time.

An automotive Ethernet cable’s ends require a hybrid transceiver to distinguish between sent and received data. If a UTP cable is used, it must be short in length, given the harsh electromagnetic conditions. Also, the impedance tolerances and losses must be tightly controlled in automotive Ethernet cabling.

Unfortunately, the rollout of multi-gigabit automotive Ethernet will not allow for a simple retrofit and will likely require new cabling. For example, shielded twisted pairs will replace the UTP cables. In some instances, Cat 5 cables might be required. Today, Cat 5 cables are already used in some noise-sensitive automotive Ethernet applications.

Signal encoding
PAM-3 4B/3B coding is used in 100 MB and 1Gb automotive Ethernet. A modulation type and signal encoding optimize the spectral efficiencies, enabling automotive Ethernet to transmit a given signal stream using the least bandwidth necessary. It can transmit up to 75 MHz. In IT Ethernet, 4B5B, MLT-3, and NRZ-I coding are combined to deliver data rates of up to 125 MHz.

Connector
An RJ45 connector is considered the interface for IT Ethernet. The connector type is undefined for automotive Ethernet. Although this improves implementation flexibility, it also results in testing challenges because the subsystems are built using different connectors.

Technologies and applications
Several advanced driver-assistance systems (ADAS) employ automotive Ethernet. While low-voltage differential signaling (LVDS) and other interfaces are used for some applications, such as the transfer of camera data, automotive Ethernet is used to transfer LiDAR and sensor data and for the controls (Figure 1).

**Figure 1:** Automotive Ethernet is used to transfer LiDAR and other sensor data in ADAS systems. (Image: Murata)

One-Pair-Ether-Net
The One-Pair Ether-Net (OPEN) alliance is a group of automotive technology providers working to encourage research, development, and adoption of automotive Ethernet. OPEN has developed standards for testing automotive Ethernet systems in various applications.

Power-over-Ethernet
Power-over-Ethernet (PoE) has been adopted by automotive Ethernet to eliminate separate power cabling and reduce the overall vehicle weight. PoE does not interfere with signal transmission rates, contributing to more reliable automotive systems.

Energy-efficient Ethernet
Energy-efficient Ethernet is a specific protocol that helps reduce vehicle power consumption by turning off network devices or entire subnets when the engine is off. It includes an energy-optimized Ethernet implementation when the engine is running.

Time synchronization
Time-sensitive applications — such as electronic control units (ECUs) for real-time audio, video, ADAS, and other applications — require deterministic communications. The IEEE 802.1AS standard supports time synchronization in automotive Ethernet networks.

AV bridging
In an automotive Ethernet network, audio and video signals are transported using audio-video bridging based on IEEE802.1 specifications. These enable local Ethernet networks to stream signaling, transport, and synchronize the audio and video streams. In many automotive applications, the ECU that originates the audio or video stream uses a wireless connection to the drivers for dispersed AV elements, such as the speakers and subwoofers. AV bridging connects the AV elements with their driver.

Diagnostics Over IP
Diagnostics over IP (DoIP) supports vehicle diagnostics using the ISO 13400 standard. DoIP analyzes ECU performance and provides firmware updates. It’s implemented with dedicated Ethernet connections and runs over TCP/IP to enable remote vehicle diagnostics, communicating with an external piece of test equipment and the different ECUs.

BroadR-Reach
BroadR-Reach, developed by Broadcom, is another adaption of Ethernet for automotive applications. It’s a combination of 100Base-T and 1000Base-T protocols and is folded into the activities of the OPEN alliance. For example, the 1000Base-T uses a PAM-5 signal and 100Base-T uses a PAM-3 signal. (BroadR-Reach is a PAM-3 implementation.)

Another difference: the 1000Base-T and 100Base-T have data rates of 125 Msymbols/s, while the data rate for BroadR-Reach is 66.67 Msymbols/s. The bandwidth is reduced by a factor of two, enabling the use of lower-quality and lower-cost cabling while supporting the filtering required to meet automotive signal-integrity requirements.

Like the automotive Ethernet, BroadR-Reach can reach up to 15 meters using UTP cabling. BroadR-Reach is also specified for use with shielded twisted pair cabling that reaches up to 40 meters.

Testing
Lower-speed automotive network standards such as CAN, LIN, and FlexRay do not have specific compliance testing standards. Various IEEE and OPEN alliance documents have defined compliance testing standards for automotive Ethernet. Design validation testing is critical to ensure interoperability between system elements from different manufacturers.

A few key performance testing considerations include:

Transmitter distortion
Return loss
Jitter and stability of transmit clock frequency
Signal voltage droop
Power spectral density (PSD)
Media delivery index (MDI) jitter of the data signals

In contrast with IT Ethernet, automotive Ethernet uses PAM3/PAM4 modulation to deliver high data rates and reliability. Eye diagrams are an effective technique for the PAM signaling in automotive Ethernet (Figure 2).

**Figure 2:** Eye diagrams are the fastest way to evaluate PAM signaling used in automotive Ethernet. (Image: Tektronix)

Coming soon: Multi-gigabit optical automotive Ethernet
The IEEE P802.3cz “Multi-Gigabit Optical Automotive Ethernet” is under development and expected to deliver up to 100 Gb/s multi-plexing. The advantages for optical Ethernet include enhanced electromagnetic compatibility (due to the inherent galvanic isolation of optical fibers), low weight, and low cost.

Like copper-based automotive Ethernet, multi-gigibit optical automotive Ethernet adoption will be driven by ADAS and the connected architectures of next-generation automobiles. Copper-based and optical automotive Ethernet implementations are expected to coexist for the indefinite future (Figure 3).

**Figure 3:** In the future, multi-gigabit optical automotive ethernet (green) is expected to coexist with copper-based automotive ethernet (red). (Image: KDPOF)

Summary
Ethernet is an important networking protocol in modern automobiles. Automotive Ethernet has fundamental differences compared to IT Ethernet, including the cabling, signaling protocols, and connectors.

One key difference between Ethernet and other automotive networking buses such as CAN, LIN, and FlexRay is compliance testing. Ethernet testing is defined by the IEEE and OPEN alliance. The other standards have no required compliance testing. The emergence of multi-gigabit optical, automotive Ethernet will enhance the utility of connectivity in ADAS and other automotive systems.

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