XMODEM

The "Christensen" file transfer protocol, probably the most

widely available protocol used for file transfer over seriallines (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)