Why in-vehicle network is critical to the advancement of autonomous and connected vehicles

By Alan Varghese, Keysight Technologies

Cars in the 1960s such as the Hillman Hunter built by the Hillman Motor Car Company based in Coventry, England, had only about 50 wires totaling about 100 feet in length in their harness. 

Compare that to today’s cars that have more than 1,500 wires that can total close to 1.5 miles in length and weigh more than 100 pounds. 

The harness may be getting heavier; automotive manufacturers state that the weight has jumped by about 30 percent in just one model evolution, with the integration of autonomous technologies. 

Bandwidth requirements for the future IVN

The requirements for the in-vehicle network (IVN) include high bandwidth, low latency, and high reliability to operate in the harsh operating environment of the automobile. 

Over the years, there has been multiple technologies such as analog, controller area network (CAN), FlexRay, local interconnect network (LIN), low voltage differential signaling (LVDS), and media oriented systems transport (MOST) that have been used for the IVN (Figure 1). 

When we look at next-generation applications, these legacy technologies cannot support the bandwidth requirements; moreover, some are proprietary and high in cost. 

To get a better understanding of the bandwidth requirements, remember the approximate bit rate of a video stream can be calculated as: Frame Size = Resolution x Color Depth; Bit Rate = Frame Size x Frame Rate.

So, for an Advanced Driver-Assistance Systems (ADAS) camera capturing a 1080p image with a color depth of 24-bits and transmitting at 30 fps, the bit rate to be supported equals: Frame Size = 1920 x 1080 x 24 = 49,766,400; Bit Rate = 49,766,400 x 30 = 1493 Mbps.

The table below shows typical volumes of data from the different sensors involved in autonomous driving:

Multiple competing standards for the IVN

• Automotive ethernet: Automotive Ethernet is considered a replacement for legacy IVN technologies and most autos today are equipped with 100BASE-T (100 Mbps). Different manufacturers emphasize it for different areas – for example, Hyundai for infotainment systems, while Volkswagen for ADAS connectivity. In 2019 and 2020, the standard added both lower speed (10 Mbps) and multigigabit speeds. The latest standard for data rates of 2.5, 5, and 10 Gbps called 802.3ch was completed in early 2020. In addition, a new task force, IEEE 802.3cy, began its activities in 2020 to develop an automotive PHY for 25, 50, and 100 Gbps.

• SERDES (ASA):  Another standard for the IVN is based on serializer/deserializer (SERDES) protocol. The Automotive SerDes Alliance was founded in 2019 by BMW, Broadcom, Continental, Fraunhofer, Marvell, and NXP for SERDES standardization.  It now comprises 36 members and was created to expand the ecosystem beyond the then available proprietary SERDES solutions, such as Texas Instruments’ FPD-Link, Maxim Integrated’s GMSL, and Inova Semiconductor’s Apix. The new standard can provide bandwidths from 3.6 to 13 Gbps for up to 15 meters. 

• SERDES (MIPI A-PHY): In November 2020, the MIPI Alliance released their A-PHY v1.0 automotive SERDES PHY specification. The spec allows for asymmetric data in point-to-point or daisy-chain topology, with optional power delivery. Data rate equals 16 Gbps with a roadmap to 48 Gbps on the downlink and an uplink rate of 200 Mbps; latency is low (6 us) and reach is 15 meters. The primary application is to connect sensors to the image signal processor in the electronic control unit (ECU), and the graphics signal processor in the ECU to the displays. 

Is the IVN Ethernet, SERDES, or Both? 

Some automotive manufacturers and Tier 1 suppliers feel for a few initial years; we may see both standards.  However, after that, automotive Ethernet with data rates up to 100 Gbps will subsume all others.

Kirsten Matheus, engineer at BMW, might have a slightly different point of view. She has suggested that SERDES is necessary and the right technology for ADAS sensor connections that carry asymmetric data point-to-point; whereas Ethernet is a networking technology good for other automotive applications. Considering that Kirsten played a key role in the standardization of automotive Ethernet, her views should hold some weight (Source: Automotive SerDes Alliance kick-off, May 2019, Salt Lake City). 

Automotive OEMs that are trying to finetune their IVN roadmaps, could adopt one of two strategies:

1. A hedging strategy i.e. implement both Standards for the IVN – until ADAS requirements for Levels 3-5 driving become much clearer. The negative with this approach is gateways may be necessary to translate data across the different domains/zones, and this will add cost as well as weight.

2. A technical strategy i.e. design around the need for high-speed point-to-point links by putting increased processing and data compression at each sensor. The negative with this approach is that the cost of the sensor suite will go up, and the increased processing will require heat dissipation.

Testing of the IVN

As far as testing of the IVN is concerned, it is important to test transmitter, receiver, and channel performance. With hundreds of tests to be performed, automated compliance test software with interpretation of specifications, repeatable results, setup wizards with user-friendly GUIs, and report generation are just as important for automotive engineers as technical specs such as bandwidths, sampling speeds, and signal resolution. 

Transmitter testing is completed mostly with an oscilloscope to ensure signals being sent are not the cause of impurities; while receiver testing is completed to check accurate detection of input signals – using signal stimulus or arbitrary waveform generators. Impedance and return loss measurements are important in time and frequency domains to ensure reliable system performance and to diagnose signal integrity issues. 

Conclusion

The automotive industry has come a long way since the days of the Hillman Hunter. Advances towards autonomous and connected vehicles bring challenges that must be addressed by the in-vehicle network. The multiplicity of sensors, controls, and interfaces required for ADAS and new infotainment features require high-bandwidth connections – traditional networks such as CAN, MOST, and FlexRay will not suffice. With the advent of new standards such as automotive Ethernet and SERDES, faster data communications are possible and the needs of future connected vehicles can be met.