End-to-end safing includes a combination of
techniques like information redundancy (adding CRC checks to the payload, for
example), redundant transmissions, time diversity in transmissions, and so forth.
Most commonly, checksums are added to the payload section of a transmission to
verify the correctness of a transmission. The checksums, sequence counter, and
timeout expectation (or time stamp) are applied, in addition to any protocol-level
parity and checksums. As these parameters are generated and evaluated by the
software, at either end of the communication, the whole communication path is safed,
resulting in end-to-end safing.
Note: Medium coverage using CRC can be claimed under following circumstances:
- Medium overall coverage of failure modes in data transmission; a Hamming distance of three or more.
- A CRC value for message information is embedded in message; with a CRC size of 8 bits and a 0x97 polynomial results in a Hamming distance of four for a data length of less than 119 bits. The transmitter includes the CRC value mentioned and the receiver confirms the data after calculating and comparing the CRC value (typically used in a LIN bus).
- A CRC value for message information and message ID is embedded in the message; with a CRC size of 10 bits and a 0x319 polynomial results in a Hamming distance of four for a data length of less than 501 bits. The transmitter includes the CRC value mentioned and the receiver confirms the data after calculating and comparing the CRC value.
- A CRC value for message information and message ID is embedded in the message; with a CRC size of 15 bits and a 0x4599 polynomial results in a Hamming distance of five for a data length of less than 127 bits. Additionally, burst errors of a length up to 15 can be detected. The transmitter includes the CRC value mentioned and the receiver confirms the data after calculating and comparing the CRC value (as used in CAN).
- A CRC value for message information is embedded in the message, with a CRC size of 24 bits and a 0x5D6DCB polynomial results in a Hamming distance of the CRC of six for a data length of less than, or equal to, 248 bytes and a Hamming distance of the CRC of four for a data length of greater than 248 bytes. The transmitter includes the CRC value mentioned and the receiver confirms the data after calculating and comparing the CRC value (as used in FlexRay™ for the frame CRC).
- A CRC value for message header (including the message ID) is embedded in the message; with a CRC size of 11 bits and a 0x385 polynomial results in a Hamming distance of six for a data length of less than, or equal to, 20 bits. The transmitter includes the CRC value mentioned and the receiver confirms the data after calculating and comparing the CRC value (as used in FlexRay™ for the header CRC).