Modern cars contain sensors, actuators and compute units that are connected with each other via bus systems. Since the Mercedes W140 those are connected via a bus system called Controller Area Network (CAN). Later also LIN, Flexray, and MOST were introduced due to cost (LIN) and performance (Flexray, MOST) reasons. Automotive Ethernet is now trying to replace all of those bus systems.
Today, you can find a mixture of block based, frame based and packet based Bus Systems. LIN and CAN are block based, Flexray is frame based and Ethernet and MIPI are packed based. MOST is something in between.
To provide all ECUs with required bus data, the data must be gatewayd between the different bus systems. This is a big challenge.
Understand the Differences
Block and frame based bus systems like LIN, CAN and Flexray implement ISO/OSI Layer 1 and 2 in one sub system. The block based protocols have a static length. The frame based protocols have a variable frame length. Both concepts have a checksum (CRC) at the end.
In comparison to the block and frame based bus system, ethernet is operating packet based with a strict separation of each ISO/OSI layer for one specific function. Ethernet uses a strict address concept with global unique address identifiers (GUID) for each node burned in by the manufacturer.
Ethernet one the one hand and LIN, CAN and Flexray on the other hand are intended with contradictory paradigms which both have its advantages and disadvantages.
Packages used by ethernet are variable housings around one piece of data. They are used to:
- Address the source and target
- Define a package type
- Define the size
- Check the package integrity
Blocks and Frames are more like buckets that you can fill with different kind of data. These bucket are used to:
- Define the priority
- Define the size (only Flexray)
- Check the package integrity
You can understand both, when you underdand where they are coming from.
- Packages were used mainly for asynchronous and event based communication. One package is completely different from the next and when it is sent, it is not known yet, when the next package is required.
- Blocks and frames in comparison to packages have their origin in signal transfer in vehicles. Data sources and sinks are always the same and the data is, one time defined, continuously transfered. This setup makes the overhead of the ISO/OSI layers completely redundant.
Understand the Challenges
The different paradigms of LIN, CAN, Flexray on the one hand and Ethernet on the other hand lead to big challenges, when used in the same environment.
Especially the concept of addressing vs broadcasting by default shows that no reversible conversion is possible. Conversion from LIN, CAN and Flexray to Ethernet is possible. Frames can be carried as payload inside an ethernet frame.
But Ethernet frames cannot be converted to LIN, CAN and Flexray without information loss. One Ethernet package is much longer than each of the LIN, CAN and Flexray frames. Theoretically the packages can be splitted, but there are difficult topics to be handled seperately.
- Frame Segmentation and Integration
- Block collisions on LIN and CAN
- Ethernet Jumbo Frames
- Security vulnarabilities
How to solve the Challenges
Ethernet, as an package based protocol, has much more overhead than LIN, CAN and Flexray, but all use cases can be handled with Ethernet that could be handled before.
Therefore BMW and Hyundai are pushing Ethernet connecttions to the edge with 10BASE-T1S. Due to the multidrop option 10BASE-T1S does not need switched networks and intends to be the cheapest implementation for ethernet in vehicles.
Due to the clear and transparent layer technology, all other concepts from the T1 standards like WOL and TSN can be further used.
Time will tell, if this approach will make the race.
The Automotive Ethernet Blog 