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In Car Embedded Networks
When thinking about the future of infotainment at the system’s level, it is natural to consider the possibility that all data (control, audio, video, etc.) could be placed on one high speed network. Given the trend since the early 1970’s of migration from point to point communications between a stand-alone electronic control units (ECU) and a sensor to so-called multiplexed networks, this would seem to be a logical outcome. However, this has not happened in practice for a variety of reasons, and therefore it is worth reviewing the state of the art in automotive embedded networks briefly.
In 1994, the Society for Automotive Engineers (SAE) defined a classification for automotive communication protocols based on data transmission speed and functions that are distributed over the network. Table 2-3 provides an overview of these classifications.
| SAE
Designation
|
Description |
Data Rate |
Examples |
| Class A |
Simple control data, low-cost; typically integrated in seat control, door lock, lighting, trunk release, rain sensor, etc. |
< 10kbps |
LIN, TTP/A |
| Class B |
Support data exchange between ECU’s to reduce required no. of sensors |
10kbps to 125kbps |
J1850, low-speed CAN |
| Class C |
High speed real time |
125kbps to 1Mbps |
High Speed CAN |
| Class D |
Very High speed data such as Multimedia data |
> 1Mbps |
MOST, IDB1394,
FlexRay |
|
Table 2-3: Automotive Networks
Because of the differences in requirements of functions embedded in cars, the real world trend as been towards increasing hybrid networks, allowing for differentiation in performance, safety, and cost for each application. For this reason, it is common in today's vehicles that the electronic architecture include multiple different types of networks interconnected by gateways. For example, the Volvo XC90 embeds up to 40 ECUs interconnected by a LIN bus, a MOST bus, a low-speed CAN, and a high-speed CAN. In the near future, it is likely that a bus dedicated to occupant safety systems (e.g., airbag deployment, crash sensing) will be added.
Thus, if it was desired to map all of the video distribution interfaces onto an automotive bus and given the fact that video will require much more than 1Mbps realized throughput, then a significantly enhanced Class D network would be required supporting data throughputs of at least 20 Mbps. Both the MOST and IDB1394 networks meet this requirement, while FlexRay is currently limited to 10Mbps and so is not a viable candidate.
MOST Network
MOST (Media Oriented System Transport) is a multimedia network development which was initiated in 1998 by the MOST Cooperation, a consortium of carmakers and component suppliers. MOST is a fiber-optic point-to-point network that is implemented in a ring, star, or daisy chain topology over plastic optical fibers (POF) at a data rate of 25 and 50 Mbps, and is based on synchronous communication. This supports end-user applications like radios, global positioning system (GPS) navigation, video displays, and entertainment systems. MOST's POF physical layer provides a much better resilience to EMI and potentially higher transmission rates than classical copper wires. BMW and DaimlerChrysler are two car manufacturers who have production models that employ a MOST network.
IDB-1394 Network
IDB-1394 is an automotive version of IEEE 1394 for in-vehicle multimedia and telematic applications jointly developed by the IDB Forum and the 1394 Trade Association. The system architecture of IDB-1394 permits existing IEEE 1394 consumer electronics devices to interoperate with embedded automotive grade devices. IDB-1394 supports a data rate of 100 Mbps (extendable to 400Mbps per IEEE 1394b-2002) over a twisted pair or POF, with a maximum number of embedded devices which are limited to 63 nodes. IDB-1394 is optimized for both isochronous and asynchronous data transfer. From the point of view of transmission rate and interoperability with existing IEEE 1394 consumer electronic devices, IDB-1394 is a flexible and feature rich multimedia data network.
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