A commonly used error detection mechanism that validates the integrity of a data set, formatted in a block or frame, through the use of a statistical sampling process and a unique mathematical polynomial. In a data communications application, the transmitting device statistically samples the data in the block or frame and applies a 17-bit generator polynomial based on a Euclidean algorithm to generate a description of the text field, or cyclic checksum, which is appended to the block or frame or text as either a 16- or 32-bit value prior to transmission.The receiving device executes the identical process and compares the results of its process to the CRC value appended to the data block. If the two values match, the data block almost certainly was unerrored in transmission.The integrity factor is 10 -14 , which means that the possibility of an undetected error is 1 in 100 trillion. If the receiving device determines that the block or frame is unerrored, it returns a positive acknowledgement (ACK). If, however, it determines that the block or frame is errored, it returns a negative acknowledgement (NAK), which prompts the transmitting device to retransmit that specific block or frame, which has been stored in a buffer.When that block or frame has been positively acknowledged, the sending device erases it from buffer memory and transmits the next. In this example, CRC is part of an error control mode known as recognition and retransmission, and is used by communications protocols such as Kermit and XMODEM. CRC also is used by MS-DOS when writing data to a hard drive or floppy disk, and by file compression utilities such as PKZIP. See also block, buffer, cyclic checksum, error control, frame, Kermit, MS-DOS, recognition and retransmission, text field, and XMODEM.
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