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Controller Area Network (CAN) is a serial bus standard originally developed by Robert Bosch GmbH for connecting electronic control units (ECUs). Although initially created for automotive (as a vehicle bus) nowadays it is used in many embedded control applications. The CAN data link layer protocol is standarized in ISO 11898-1 (2003). This standard describes mainly the data link layer - comprised by logical link control (LLC) sublayer and the Media Access Control (MAC) sublayer - and some aspects of the physical layer of the ISO/OSI Reference Model. All the other protocol layers are left to the network designer's choice.
In the CAN model, all devices are connected to a single shared bus and they are all allowed to start a transmission. Therefore, a Carrier Sense Multiple Access/Collision Avoidance (CSMA/CA) scheme is implemented, i.e. if two or more devices start transmitting at same time, there is a priority based arbitration scheme to decide which one will be granted permission to continue transmitting.
It is a very robust protocol, with error detection and signalling, self-checking and fault confinement. Faulty CAN data and remote frames are re-transmitted automatically.
Bitrates up to 1 Mbit/s are possible at networks length below 40 m. Decreasing the bit rate allows longer network distances (e.g. 250 kbit/s at 250 m).
There are two message formats:
- The standard format: with 11 identifier bits
- The extended format: with 29 identifier bits
The CAN standard requires the implementation must accept the standard format and may accept the extended format.
There are several CAN physical layer standards:
- ISO 11898-2: CAN high-speed
- ISO 11898-3: CAN fault-tolerant (low-speed)
- ISO 11992-1: CAN fault-tolerant for truck/trailer communication
- SAE J2411: Single-wire CAN (SWC)
ISO 11898-2 uses a two-wire balanced signaling scheme. It is the most used physical layer in car powertrain applications and industrial control networks.
The ISO 11898-4 standard defines the time-triggered communication on CAN (TTCAN). It is based on the CAN data link layer protocol providing a system clock for the scheduling of messages.
Also, as the CAN standard does not include tasks of application layer protocols, such as flow control, device addressing and transportation of data blocks larger than one message, there were created many implementations of higher layer protocols. Among these are DeviceNet, CANopen, SDS and CAN Kingdom.
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