An error-correcting file transfer, data transmission protocol used to transmit files between PCs. The XMODEM protocol sends information in 128 byte blocks of data. Some sums (check sums) are done on each block and the result is sent along with the block. If the result does not check out at the other end, the computer at the other end sends a request (a NAK - negative acknowledgment) to retransmit that block again). If the block checks out, the computer sends ACK (an acknowledgment). In this way, rela- tively error-free transmissions can be accomplished.
