A popular file-transfer protocol developed in 1977 by Ward Christansen that works by sending data in 128-byte blocks from PC to PC. Included with this data is an error detection system called a checksum. When the data is received, the error detection system ensures that the entire message reached its destination. If not, the receiving computer sends a request for retransmission of the data.
Later came Xmodem 1 k, a more powerful version of Xmodem that allowed 1 KB blocks of data to be sent greater distances with the same reliable error checking. There are also enhanced versions of Xmodem called Ymodem and Zmodem.