Ethernet Gets Deterministic for Automotive Applications

Automotive systems are adopting deterministic Ethernet communications to handle a wide range of functions that require time-sensitive responses.

The traditional point-to-point wiring harness used in most older cars and still used in many current model cars has its days numbered as a deterministic Ethernet-based digital serial bus replaces the old analog wiring harnesses.  The wiring harness is also the third heaviest component (behind the chassis and engine) in a typical car, and any technology that reduces this weight directly contributes to fuel economy. Due to the complex cabling of the point-to-point wiring harness, the harness is also the third highest cost component in a car (also after the engine and the chassis). Since harnesses typically have to be built one at a time they can account for close to 50 percent of cost of labor for assembling the entire car.

The serial bus greatly reduces the weight, complexity and cost of the wiring harness, while allowing all the latest digital subsystems such as the infotainment electronics and many other items such as gauges, lights, mirrors, seats, door locks, and other accessories to easily link to each other and to the host controller in the car (Figure 1). Additionally, the Ethernet interconnect will also allow easier interfacing to advanced driver assistance systems as such systems become commonplace in the next few years.

 Figure 1: The architectural change from a point-to-point wiring approach to a networked bus architecture as described by Ixia simplifies the wiring harness and considerably reduces weight and cost.

Figure 1: The architectural change from a point-to-point wiring approach to a networked bus architecture as described by Ixia simplifies the wiring harness and considerably reduces weight and cost.

The automotive and IEEE 802.3 standards organizations have defined a new physical layer interface (PHY) that allows data to flow over an unshielded twisted pair connection at up to 100 Mbits/s. The simple twisted-pair interconnect will greatly reduce the harness weight and cost. The twisted-pair interconnect standard, referred to as 100BASE-T1, will soon be finalized by the IEEE 802.3 standards organization. And, as one might expect, a next-generation PHY interface capable of gigabit data transfers is in the early definition stage.

Deterministic Challenges
Also defined by the IEEE Standards Association are additions to the IEEE 802.1 and IEEE 802.3 Ethernet standards that will allow for deterministic operation within standard Ethernet media-access controllers (MACs) or Ethernet switches.  The deterministic aspect of 100BASE-T1 is critical in automotive applications, since many functions are time critical and the control system must respond in real time. Once final, the standards will provide a menu of options that the automotive vendors can select from to optimize the level of deterministic performance needed in time-critical automotive applications.

These deterministic features are referred to as the time-sensitive network (TSN) aspects of the standard. Features include time synchronization and ingress policing, which are essential requirements for almost all new automotive applications.  Another addition to the standard, referred to as scheduled traffic, will be required for the most time-critical engine control unit applications such as braking, acceleration, etc.

Non-deterministic versions of Ethernet are currently used for connecting the infotainment products, cameras used for safety (backup and side view) and for diagnostic applications, as delays ranging up to hundreds of microseconds would not cause any noticeable impact on the systems.  The addition of deterministic capabilities allows Ethernet to be used as the network backbone in the car, providing connectivity for the engine control unit, as well as braking and steering control in drive-by-wire systems. Eventually the 100BASE-T1 will be able to replace all of the multiple serial networks currently in use—CAN, FlexRay, LIN, LVDS, and MOST.

Advanced Automotive Systems Demand Higher Data Rates
Inside today’s car, most information is transferred at less than 1 Mbit/s, depending on the function. Table 1 from Ixia highlights the various functions in a car and their latency and bandwidth requirements.   With all the new functions and features being added to the cars—active safety control, more efficient engines and controls, advanced driver assistance, car-to-car communications, internet connectivity, and still other features, cars will need the equivalent of a new Super Highway where cars can move at 200 miles per hour and not worry about traffic, explains Jeff Owens, the chief technology officer at Delphi Automotive LLP. In the future, Owens continues, data transfer speeds will need to increase a thousandfold and decisions will need to be made by a well-connected system of controllers.

The data from many of the functions throughout the vehicle—like braking, acceleration, temperature, pressure, voltage, steering angle—is time critical and must be transferred error-free. One bad data packet could lead to a serious problem such as improper braking or over-acceleration. Of all the serial interfaces, Ethernet will dominate because its bandwidth capability, common language and protocols give it the ability to control passing information and avoid simultaneous transmission by two or more systems. 

table1_worddoc version

Table 1: Summary of Connectivity Requirements by Domain (courtesy Ixia).

What’s Available?

Several companies have developed the PHY technology to meet the 100BASE-T1 standard.  For example, Broadcom developed a proprietary PHY standard, BroadR-Reach, which employs PAM-3 signalling and some tricks developed for the company’s 1 Gbit copper Ethernet to achieve long signalling distances of up to 700 meters. The PHY also implements a better encoding scheme that reduces the bandwidth required on the wires. Echo cancellation technology is also used to permit bidirectional data to be transmitted over a single pair of wires.

The BroadR-Reach was a single vendor solution. However several organizations founded the OPEN sig to make the PHY a licensed open standard with sponsorship from major players in the automotive market. Currently, Broadcom, NXP, and Freescale (now part of NXP) offer the PHY.

Although the 100Mb/s data transfer speed is enough for a video transmission, it is not fast enough to serve as the network backbone in the car. Thus the automotive vendors and the IEEE Standards organization have created a task force to define a 1 Gbit/s PHY standard (802.3bp) for use on a twisted pair for links up to 15 meters for the automotive market. Referred to as 1000Base-T1 (1 stands for 1 pair), the PHY will hopefully be standardized in 2017.

The ability to send power over the Ethernet wiring can further reduce the wiring needed in a car. Another standards taskforce is trying to standardize PoE over a single pair. Referred to as IEEE 801.3bu (1-pair power over data lines, PoDL), this standard requires a minor modification to the current PoE standard. Closely related to PoE, the implementation of energy-efficient Ethernet (EEE) circuits can save a considerable amount of power, especially if some of the Ethernet circuits don’t all turn off when the engine turns off. No one wants the circuits to drain the battery when the car isn’t running, since the battery capacity is limited without the car’s generator running.

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