Xmodem

A popular file-transfer protocol developed in 1977 by Ward Christansen. The protocol works by sending blocks of 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 was Xmodem 1k, a more powerful version of Xmodem that allows larger files to be sent greater distances with the same reliable error checking. Information is transmitted in 1KB blocks as opposed to Xmodem, which transmits information in 128-byte blocks.

Also see: Modem definitions