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Automotive communication backbone: MOST vs Ethernet

Posted: 15 Oct 2014 ?? ?Print Version ?Bookmark and Share

Keywords:Automotive? electronic systems? Advanced Driver Assistance System? ADAS? Media Oriented Systems Transport?

Packets of information had to be sent over long distances and over a path that could change as links between the locations changed. A very simple physical connection was needed.

Each packet had to be encapsulated with addressing and control information, so that it could be rerouted at will through computer systems that could join and leave the network at any time. This is akin to putting a letter in an envelope and sending it to a recipient in another country. The exact routing can change, but the important thing is that the information gets to the destinationeven if you can't predict the exact time it will get there, and even if that time varies from transmission to transmission.

Any data exchanged with systems outside of the vehicle, whether over a diagnostics interface or over a cellular modem, will likely be put into individual packets with a source and destination address, along with other information designed to control how the data travels from point A to point B.

The external connection is unreliable and requires this approach. This type of transmission is very useful for things such as email, web browsing, and moving data that is not time critical or time sensitive. In addition, the widespread use of Ethernet has resulted in a wide variety of applications that use IP packets.

These packets are the mainstay of this type of communication, whether it is the wired IEEE 802.3 Ethernet physical layer or other mechanisms, such as IEEE 802.11 wireless LAN or cellular communications. Having an Ethernet-based medium in the car makes it easy to use these IP applications with little change from their non-automotive instantiations.

The fundamental architecture of Ethernet is based on Carrier Sense Multiple Access with Collision Detect (CSMA/CD). This means that all participants sense whether the transmission medium is busy, and they try to transmit when it is not. If multiple transmitters start to transmit, a collision occurs and is detected. All transmitters then back off and try again after a random timeout.

The problem with this architecture is that it is impossible to predict when there will be a collision. As more devices participate, the more time (and thus bandwidth) is wasted backing off and trying again. The system is nondeterministic, and there can be wide variations in latency, especially if one device has to communicate with several other devices.

In fact, some studies have shown that bandwidth utilisation can be less than 50% in heavily loaded networks, with the rest of the time used to arbitrate and get control of the physical medium. The nodes that compete with one another are said to be in the same collision domain.

This problem is not an issue for data without hard real-time requirements.

If some part of a web page is painted a few hundred milliseconds faster or slower than another part, the user won't even notice. Email can take a few seconds more to make its way around the world without any catastrophic effects. The simplicity and low cost of the overall worldwide system make it attractive.

The problem manifests itself with audio, video, and any other application where a continuously flowing stream of data can't afford to be interrupted, or if a control message has to get there within a predetermined timeframe. Buffering can help, but it introduces delays that are unacceptable for applications such as cameras or other ADAS functions where latency is a safety issue.

Enter switches. Switches keep track of where the sources and destinations of various packets are located. They learn the network as traffic flows through it. The effect is that they separate collision domains and eliminate many of the bottlenecks that exist with the basic CSMA/CD technology of Ethernet. Traffic is only sent out to the port with the path to the destination, and other devices never see it.

The trade-off is that you need additional hardware and buffering. Determinism is only a statistic, not a hard real-time feature. Switches are added to the basic network interfaces located in each device that uses the network. These switches can be centralized, or a three-port switch can be added to each device so they can all be daisy chained.

However, all network interfaces need to connect to a switch if they are to avoid colliding with other devices, adding hardware and cost beyond the actual Ethernet transceiver in each device. If devices share the physical medium without a switch, they go back to the CSMA/CD mechanisms.

Switches used in the automobile also need to be more specialised than the typical switch in an office, because they need to include hardware features for bridging audio and video and to provide time sensitive networking (TSN), previously referred to as AVB. Ethernet TSN includes extra hardware to distribute clocks, provide timestamps for each packet that is transmitted, and provide mechanisms for bandwidth reservation and packet prioritisation.

The bottom line, though, is that Ethernet is widely used, and just about all data entering and leaving a vehicle is likely to make its way ultimately to an Ethernet network of some kind. Ethernet refers to particular packet formats, as well as to a specific physical layer.

The case for MOST technology: Stream transmission
The addressing information used in IP packets is needed when packets are travelling long distances and over uncertain and varying paths.

However, it results in significant amounts of overhead when the main purpose is to get large amounts of data between a well-determined source and a well-determined sink that are in relatively close proximity and in a relatively fixed configuration, such as within the vehicle.

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