ADTRAN ISU 512e Manual page 152

Glossary
XMODEM
An error-correcting file transfer, data transmission protocol used
to transmit files between PCs. The XMODEM protocol sends in-
formation 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 once again. If the block checks out, the com-
puter sends ACK (an acknowledgment). In this way, relatively er-
ror-free transmission can be accomplished.
Glossary-10 ISU 512e User Manual 61203086L1-1