The data link layer is a fundamental part of the Controller Area Network (CAN) protocol stack, playing a vital role in ensuring reliable message communication between devices.
Core Responsibility
Based on the reference provided, the primary responsibility of the CAN data link layer is:
- Message Transfer: The data-link layer is responsible for transferring messages (or frame) from a given node to all other nodes in the CAN network. This means that when a node sends a message, it is broadcast across the entire bus for all connected nodes to potentially receive.
Key Functions
Beyond simple transfer, this layer handles several critical tasks to ensure data integrity and manage communication on the shared bus:
- Error Handling: It incorporates mechanisms to detect and manage errors that may occur during transmission.
- Bit Stuffing: A technique used to ensure there are never too many consecutive bits of the same value, which helps maintain synchronization between nodes and assists in error detection.
- Checksums: Used to verify the integrity of the transmitted data. The receiver calculates a checksum and compares it to the one sent by the transmitter to detect errors.
- Acknowledgment: After sending a message, the data link layer waits for acknowledgment from the receivers. This confirms that at least one other node on the network successfully received the message without reporting a CRC error.
- Message Framing: It structures the data into defined message frames (like data frames, remote frames, error frames, and overload frames) that adhere to the CAN standard format for transmission on the bus.
- Arbitration: Although the reference doesn't explicitly detail it, the data link layer manages bus access through a non-destructive arbitration method, allowing multiple nodes to attempt transmission simultaneously without data collision in the traditional sense. Messages with higher priority (lower identifier value) win access to the bus.
Subdivision
The data link layer itself is further organized into sub-layers. As noted in the reference:
- It is subdivided into two further layers: (a) ... (the specific names of these sub-layers are not provided in this reference).
In essence, the CAN data link layer acts as the bridge between the higher application layers (which handle what data is sent) and the physical layer (which handles how the bits are physically put onto the wire). It ensures that messages are correctly formatted, broadcast reliably to all nodes, checked for errors, and acknowledged.
