XMODEM

old protocol for data transfer between two computers which are connected by a modem (Computers)

XMODEM is a simple file transfer protocol developed as a quick hack by Ward Christensen for use in his 1977 MODEM.ASM terminal program. XMODEM became extremely popular in the early bulletin board system (BBS) market, largely because it was so simple to implement. It was also fairly inefficient, and as modem speeds increased this problem led to the development of a number of modified versions of XMODEM to improve performance or address other problems with the protocol. Chuck Forsberg collected a number of these into his YMODEM protocol, but poor implementation led to a further fracturing. His later ZMODEM protocol was so superior that it led to the demise of all of these XMODEM derivatives in the early 1990s.
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XMODEM, old protocol for data transfer between two computers which are connected by a modem

The "Christensen" file transfer protocol, probably the most widely available protocol used for file transfer over serial lines (e.g. between modems). XMODEM uses 128-byte packets with error detection, allowing the receiver to request retransmission of a corrupted packet. XModem is fairly slow but reliable.
Several variations have been proposed with increasing packet sizes (e.g. XMODEM-1K) and different error detection (CRC instead of checksum) to take advantage of faster modems. Sending and receiving programs can negotiate to establish the best protocol they both support.
Standard XMODEM specifies a one-second timeout during the reception of characters in the data block portion of a packet.
Chuck Forsberg improved upon XMODEM by developing YMODEM and ZMODEM.
[Chuck Forsberg, "XMODEM/YMODEM Protocol Reference"].
(1995-02-02)

XMODEM