The "Christensen" file transfer protocol , probably the most widely available protocol used for file transfer over serial line s (e.g. between modem s). XMODEM uses 128-byte packet s 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 modem s. 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)