Let us take a look of the frame format of a simple standard CAN frame which is transfered over the bus.

Standard CAN: 11-Bit Identifier

The meaning of the bit fields of Figure 2 are:

• SOF—The single dominant start of frame (SOF) bit marks the start of a message, and is used to synchronize the nodes on a bus after being idle.
• Identifier—The Standard CAN 11-bit identifier establishes the priority of the message. The lower the binary value, the higher its priority.
• RTR—The single remote transmission request (RTR) bit is dominant when information is required from another node. All nodes receive the request, but the identifier determines the specified node. The responding data is also received by all nodes and used by any node interested. In this way all data being used in a system is uniform.
• IDE—A dominant single identifier extension (IDE) bit means that a standard CAN identifier with no extension is being transmitted.
• r0—Reserved bit (for possible use by future standard amendment).
• DLC—The 4-bit data length code (DLC) contains the number of bytes of data being transmitted.
• CRC—The 16-bit (15 bits plus delimiter) cyclic redundancy check (CRC) contains the checksum (number of bits transmitted) of the preceding application data for error detection.
• Data—Up to 64 bits of application data may be transmitted.
• ACK—Every node receiving an accurate message overwrites this recessive bit in the original message with a dominate bit, indicating an error-free message has been sent. Should a receiving node detect an error and leave this bit recessive, it discards the message and the sending node repeats the message after rearbitration. In this way each node acknowledges (ACK) the integrity of its data. ACK is 2 bits, one is the acknowledgement bit and the second is a delimiter.
• EOF—This end-of-frame (EOF) 7-bit field marks the end of a CAN frame (message) and disables bit–stuffing, indicating a stuffing error when dominant. When 5 bits of the same logic level occur in succession during normal operation, a bit of the opposite logic level is stuffed into the data.
• IFS—This 7-bit inter-frame space (IFS) contains the amount of time required by the controller to move a correctly received frame to its proper position in a message buffer area.