Xmodem is an error-correcting protocol for modem that was created in 1978 by Ward Christensen and became a de facto standard. Modems that agree on using the Xmodem protocol send data in 128-byte blocks. If a block is received successfully, a positive (ACK) acknowledgement is returned. If an error is detected, a negative (NAK) acknowledgement is returned and the block is resent. Xmodem uses the checksum method of error checking.
See modem error-correcting protocols for a comparison with other error-correcting protocols.