A popular file-transfer protocol developed in 1977 by Ward Christiansen. 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. Compare with Ymodem, Zmodem.