The checksum used in the original protocol was extremely simple, and errors within the packet could go unnoticed. This led to the introduction of XMODEM-CRC by John Byrns, which used a 16-bit CRC in place of the 8-bit checksum. CRC's encode not only the data in the packet, but its location as well, allowing it to notice the bit-replacement errors that a checksum would miss. Statistically, this made the chance of detecting an error less than 16 bits long 99.9969%, and even higher for longer error bit strings.