Editing XMODEM (section)

==XMODEM-CRC==
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,{{sfn|Christensen|1982}}{{sfn|Forsberg|1986}} which used a 16-bit [[Cyclic redundancy check|CRC]] in place of the 8-bit checksum.{{r|meeks198902}} CRCs 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.{{sfn|Boswell|1986}}

XMODEM-CRC was designed to be backwardly compatible with XMODEM. To do this, the receiver sent a <kbd>C</kbd> (capital C) character instead of a <kbd><NAK></kbd> to start the transfer. If the sender responded by sending a packet, it was assumed the sender "knew" XMODEM-CRC, and the receiver continued sending <kbd>C</kbd>'s. If no packet was forthcoming, the receiver assumed the sender did not know the protocol, and sent an <kbd><NAK></kbd> to start a "traditional" XMODEM transfer.{{sfn|Boswell|1986}}

Unfortunately, this attempt at backward compatibility had a downside. Since it was possible that the initial <kbd>C</kbd> character would be lost or corrupted, it could not be assumed that the receiver did not support XMODEM-CRC if the first attempt to trigger the transfer failed. The receiver thus tried to start the transfer three times with <kbd>C</kbd>, waiting three seconds between each attempt. This meant that if the user selected XMODEM-CRC while attempting to talk to ''any'' XMODEM, as it was intended, there was a potential 10 second delay before the transfer started.{{sfn|Boswell|1986}}

To avoid the delay, the sender and receiver would generally list XMODEM-CRC separately from XMODEM, allowing the user to select "basic" XMODEM if the sender didn't explicitly list it. To the average user, XMODEM-CRC was essentially a "second protocol", and treated as such. This was not true of FidoNet mailers, however, where CRC was defined as the standard for all TeLink transfers.{{sfn|Bush|1995|p=G.1}}