Xmodem.
A protocol for transferring files between computers (often across phone lines) and catching errors that occur during transfer. This ensures that the file sent is identical to the file received. XMODEM was actually the name of a program that included the XMODEM file transfer protocol. Today, it refers to the way the file is sent. Other transfer protocols, such as YMODEM and ZMODEM, improve on the XMODEM idea by allowing faster data transmission through larger packets of data and the capability to continue transmitting data when the checksum values don’t match.