Glossary definition of Xmodem

Also known as: X-Modem

X-Modem is an error-correction protocol used in the transfer of serial data files between modems.

Modems with the X-Modem protocol will send data in 128-byte blocks or packets, and if the transfer of a block is successful the receiving modem then returns a positive acknowledgement (ACK). Any errors detected in the sending of the block will result in a negative acknowledgement (NAK) being returned, which prompts the transmitting modem to resend the data. Depending on the version of X-Modem being employed (there were several variants), it will use either checksum or cyclic redundancy check (CRC) methods of error detection.