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Car systems: MOST, Ethernet or both at once?

Posted: 03 Oct 2014     Print Version  Bookmark and Share

Keywords:electronic system  car  Ethernet  MOST  connectivity 

Editor's Note: Interaction among various domains within car electronic systems is best achieved with a network that can serve as a backbone between various systems, but which technology? Microchip's Henry Muyshondt looks at both MOST and Ethernet technologies and the ongoing battle between them, and explains why creating a system that takes advantage of each one is the best choice to make.

Car electronic systems are some of the most complex systems in production, consisting of many sub-systems or domains. As more functions are added to each vehicle, these domains must communicate with each other, and sometimes there needs to be coordination between multiple domains.

For example, an Advanced Driver Assistance System (ADAS) needs to process information from multiple cameras and other sensors. The result of this processing needs to be presented to the user through screens and audio playback devices, typically part of the infotainment system.

The information needs to be seamlessly combined with other information flowing through the infotainment system, which is really the interface to the driver and the passengers. Data from the ADAS also must make its way to the body control, brake control, steering control and engine control domains, if specific automatic actions are to be taken by the car. A telematics control system communicates to the world outside the vehicle, but must also interface to the occupants of the car and with other domains, such as the infotainment, ADAS, and navigation systems.

All this interaction among various domains is best achieved if there is a network that can serve as a communication backbone between various systems. Much is being written about the most appropriate network for doing this. The situation is often presented as a battle between Media Oriented Systems Transport (MOST technology), the de facto standard used as the infotainment backbone in vehicles, and Ethernet, the standard used for the IT infrastructure in the consumer/commercial world.

No need to make hard choices

The reality is that there is no need to make a hard choice between these technologies, and their combination results in a system that takes the advantages of each one without the limitations each can have.

A car is made up of sub-systems that may communicate with one another but have very different requirements for the data transports that they use. For example, a simple button to lower and raise a window has very low data-communication requirements and is extremely cost sensitive. You won't be running a full TCP/IP stack or even a Controller Area Network (CAN) controller to keep track of the window.

Instead, many cars use an inexpensive serial bus called Local Interconnect Network (LIN) that uses a single wire for communication. CAN also provides many useful functions in the vehicle and, though more complex than LIN, is easy to implement in a variety of automotive microcontrollers. Many diagnostic functions, as well as body and engine control communications, flow over CAN, which uses a message-based protocol and is optimised to send relatively small amounts of information.

As the amount of data that needs to be transported increases, faster and more complex networks are needed. These networks require significantly more processing power than the ones previously described and also use orders of magnitude more bandwidth, due to the amount of data that needs to be moved from one place to another.

In the late 1990s, the automotive industry developed MOST technology to fulfil its audio and video communications needs. The network was conceived, from the beginning, to send continuously flowing, time-sensitive data to various devices, with the primary intention of simplifying automotive information and entertainment systems.

There are now more than 165 car models with MOST networks in them, according to the MOST Cooperation, the association of car makers and their suppliers that is the caretaker of the standard. There are also more than 150 million devices deployed to implement the technology, with some high-end cars including up to 15 devices each.

The Internet Protocol (IP) has also become important as the car connects to the outside world. Most of the traffic in the IT world uses various protocols geared around Ethernet and IP packets, and many applications rely on this standard to process the information that can be flowing in and out of the car to reach all the way around the world.

Despite often being portrayed as adversaries where only one should prevail, MOST technology and Ethernet each have certain advantages, and there is a lot of synergy between them. In fact, the MOST150 standard natively supports a dedicated Ethernet-packet channel that can carry unmodified Ethernet frames.


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